ION Selective Electrode

Ion selective electrodes are electrochemical sensors whose potential varies with the logarithm of the activity of an ion in solution.

Available types:

1. The membrane is a single compound, or a homogeneous mixture of compounds.

2. The membrane is a thin glass whose chemical composition determines the response to specific ions.

3. The support, containing an ionic species, or uncharged species, forms the membrane. The support can be solid or porous.

How do pH electrodes work?

The most common type of pH electrodes are the "glass" electrodes.

They consist of a special glass membrane that is sensitive to variations in pH, as pH variation also changes the electrical potential across the glass. In order to be able to measure this potential, a second electrode, the "reference" electrode, is required.

Both electrodes can be present in a "combined" pH electrode, or two physically-separate electrodes can be used. The glass electrode consists of a glass shaft on which a bulb of a specialglass is mounted.

The inner is usually filled with 3 Mol/Litre aqueous KCl and sealed. Electrical contact is provided by a silver wire immersed in the KCl. For "combined" electrodes, the glass electrode is surrounded by a concentricreference electrode.

The reference electrode consists of a silver wire in contact with the almost-insoluble AgCl. The electrical contact with the meteris through the silver wire. Contact with the solution being measured is viaa KCl filling solution.

To minimise mixing of the solution to be measured andthe filling solution, a porous seal, the diaphragm, is used. This is usuallya small glass sinter, however other methods which allow a slow mixing contactcan also be used, especially for samples with low ionic strength. Besides the"normal" KCl solutions, often solutions with an increased viscosity, and hence lower mixing rate are used.

A gel filling can also be used, which eliminates the necessity for slow mixing devices. In contact with different pH solutions a typical glass electrode gives, when compared to the reference electrode, a voltage of about 0 mV at pH 7, increasing by 59 mV per pH unit above 7, or decreasing by 59 mV per pH unit below 7. Both the slope, and the intercept of the curve between pH and generated potential, are temperature dependent.

The potential of the electrode is approximated by the Nernst equation : E = E0 - RT log [H+] = E0 + RT pHWhere E is the generated potential, E0 is a constant, R is universalgas constant and T is the temperature in degrees Kelvin. All pH-sensitive glasses are also susceptible to other ions, such as Na or K.

This requires a correction in the above equation, so the relationship between pH and generated voltage becomes nonlinear at high pH values. The slope tendsto diminish both as the electrode ages, and at high pH. As the electrode hasa very high impedance, typically 250 Megohms to 1 Gigohm, it is necessary touse a very high impedance measuring instrument.

The reference electrode has a fairly constant potential, but it is temperature dependent, and also varies with activity of the silver ions in the reference electrode. This occurs if a contaminant enters the referenceelectrode. Calibration From the preceding, it is obvious that frequent calibration and adjustment of pH meters are necessary.

To check the pH meter, at least two standard buffer solutions are used to cover the range of interest. The pH meter should be on for at least 30 minutes prior to calibration to ensure that all componentsare at thermal equilibrium, and calibration solutions should be immersed forat least a minute to ensure equilibrium.

First use the buffer at pH 7, and adjust the zero (or the intercept). Then, after thorough rinsing with water, use the other buffer to adjust the slope. This cycle in repeated at least once, or until no further adjustments are necessary. Many modern pH meters have an automatic calibration feature,which requires each buffer only once.

Errors People assume pH measurements are accurate, however many potential errors exist. There can be errors caused by the pH-sensitive glass, reference electrode, electrical components, as well as externally generated errors. Glass Electrode Errors The pH-sensitive glass can be damaged. Major cracks are obvious, but minordamage can be difficult to detect. If the internal liquid of the pH-measuringelectrode and the external environment are connected, a pH value close to 7 will be obtained.

It will not change when the electrode is immersed in a known solution of different pH. The electrical resistance of the glass membrane will also be low, often below 1 megohm, and it must be replaced. Similar results occur if the glass wall between the inner and outer part of a combined electrode breaks.

This may occur if the outer part is plastic.The inner part can crack without any external signs. The electrical resistivity over the glass electrode is intact, but actual measuring betweenboth electrodes reveals a low resistivity. The electrode must be replaced. The glass can wear out.

This gives slow response times, as well as a lower slope for the mV versus pH curve. To rejuvenate, immerse the electrode in a 3 Molar KCl solution at 55 degrees Celsius for 5 hours. If this does not solve the problem, try removing a thin layer of the glass by immersion for two minutes in a mixture of HCl and KF (be careful, do not breathe the fumes,and wear gloves). The electrode is then immersed for two more minutes in HCl,and rinsed thoroughly.

As an outer layer of glass has been removed, the new surface will be like a new electrode, however the thinner glass will resultin a shorter electrode life.

Frequent recalibration will be required for several days. The glass can be dirty. A deposit on the glass will slow the response time, make the response sensitive to agitation and ionic strength, and also give the pH of the film, not the sample solution. If the deposit is known, use aappropriate solvent to remove it, and rehydrate the electrode in 3M KCl.

If the deposit is not known, first immerse the electrode for a few minutes in a strongly alkaline solution, rinse thoroughly, and immerse it in a strong acid (HCl) solution for several minutes. If this does not help, try using pepsin in HCl. If still unsuccessful, use the above HCl/KF method.

Reference Electrode Errors The diaphragm of the reference can become blocked. This is seen as unstable or wrong pH measurements. If the electrical resistivity of the diaphragm ismeasured, high values are reported (Most multimeters will give an over-range error). The most common reason is that AgS formed a precipitate in the diaphragm. The diaphragm will be black in this case. The electrode should be immersed in a solution of acidic thio-urea until the diaphragm is white, andthen replace the internal filling liquid of the reference electrode There is no contact across the diaphragm, due to air bubbles. This appears as if the diaphragm were blocked, except that the diaphragm is white. Ensure that the filling solution level in the reference electrode is always wellabove the sample, so that liquid is always slowly flowing from the reference electrode towards the sample. The electrode filling solution is contaminated. This appears as unstable or wrong pH measurements. Often the 0mV pH differs considerably from pH 7.

The diaphragm has its normal colour and the electrical resistivity is normal. However, the solution often becomes contaminated due to low filling solution levels, and air bubbles may also appear in the diaphragm, which obviously affects electrical resistivity. Replacing the reference filling solution several times should solve the problem, but the electrode may have been permanently damaged. The problem can be avoided by choosing gel-filled reference electrodes, double-junction electrodes, or ensuring there is an outflow of reference filling solution towards the sample. The electrode was filled with the wrong reference solution. This appears asas displaced pH measurements. Flush and replace the reference liquid.

Electrical errors Condensation or sample contamination of the electrode connecting cable. This appears as an almost-constant measurement of about pH 7, even when the pH electrode is disconnected from the cable, or as a pH which changes less than it should, when tested with two standard solutions. If the cable is disconnected from the meter, the pH will start to drift. There is a short circuit in the cable. The symptoms are similar to the above case, except that bending the cable may create sharp, spurious readings.

In most pH cables, between the two copper conductors there are two layers which appear to be insulators. The inner layer is an insulator, whereas the outer layer is a conductor to avoid trace electrical effects. If this outer layer does contact the inner conductor, there will be a short circuit. Replace suspect cables.

The input stage of the meter is contaminated with conducting liquid. The symptoms are the same as above, except that removing the cable has noeffect. Closely examine the input stage of the meter for liquid or deposits. If present, rinse with distilled water, then ethanol, and dry thoroughly. The input stage of the meter is faulty. This gives random measurements.

Shorting both input wires does not make any difference. Repair the meter. The input stage appears faulty. Shorting both input wires gives a stablepH measurement of about 7. The meter may be faulty, but probably the problem is elsewhere in the electrical circuit. Externally-generated Errors If a significant flow of liquid passes the electrode, then there canbe a minor electrical effect.

This generates a potential on the glass membrane, which is superimposed on the actual pH measurement. This effect becomes negligible for highly-conducting liquids. It is seldom observed. If the trace electric effect does influence pH measurements, the addition of a little salt to increase the conductivity, or changing the flux of liquid around the electrode, should solve the problem. Ground loops and spurious electrical currents may generate unexpected electrical signals. Such signals can strongly influence pH measurements.

A pH reading in the range of -15 to +20 is possible, even if the pH is 7. Ground loops can be eliminated by grounding the system according to themanufacturer's instructions, and ensuring insulation is in good condition. Often these problems can be extremely difficult to detect and remedy. Low ionic strength samples can be affected by electrolyte from the electrode, and special electrodes are available.

Kerosene & Diesel

Kerosene is a hydrocarbon fraction that typically distils between 170-270C (narrow cut kerosene, or Jet A1) or 100-250C ( wide cut kerosene, or JP-4 ).

It contains around 20% of aromatics, however the aromatic content will be reduced for high quality lighting kerosenes, as the aromatics reduce the smoke point.

The major use for kerosenes today is as aviation turbine (jet) fuels. Special properties are required for that application, including high flash point for safe refuelling ( 38C for Jet A1 ), low freezing point forhigh altitude flying ( -47C for Jet A1 ), and good water separation characteristics.

Diesel is used in compression ignition engines, and is a hydrocarbon fraction that typically distils between 250-380C. Diesel engines use the Cetane (n-hexadecane) rating to assess ignition delay. Normal alkanes have a high cetane rating, ( nC16=100 ) where as aromatics (alpha methylnaphthalene = 0 ) and iso-alkanes ( 2, 2, 4, 4, 6, 8, 8 - hexamethylnonane = 15 ) have low ratings, which represent long ignition delays.

Because of the size of the hydrocarbons, the low temperature flow properties control the composition of diesel, and additives are used to prevent filter blocking in cooler temperatures. There are usually summer and winter grades. Environmental legislation is reducing the amount of aromatics and sulfur permitted in diesel, and the emission of small particulates ( diameters of <10um>

What is petroleum ether?

Petroleum ether ( aka petroleum spirits ) is a narrow alkane hydrocarbon distillate fraction from crude oil.

The names "ether" and "spirit" refer to the very volatile nature of the solvent, and petroleum ether does not have the ether ( Cx-O-Cy ) linkage, but solely consists of hydrocarbons.

Petroleum ethers are defined by their boiling range, and that is typically 20C.

Typical fractions are 20-40C, 40-60C, 60-80C, 80-100C, 100-120C etc. up to 200C.

There are specially refined grades that have any aromatic hydrocarbons removed, and there are specially named grades, eg pentane fraction (30-40C), hexane fraction (60-80C, 67-70C).

It is important to note that most "hexane" fractions are mixtures of hydrocarbons, and pure normal hexane is usually described as "n-hexane".

What is LPG or LP Gas?

What is LPG or LP Gas?
LPG or LP Gas is the abbreviation of Liquefied Petroleum Gas. This group of products includes saturated Hydrocarbons - Propane (C3H8) and Butane (C4H10), which can be stored/transported separately or as a mixture. They exist as gases at normal room temperature and atmospheric pressure.

Why is it called Liquefied Petroleum Gas?
This is because these gases liquefy under moderate pressure. They liquefy at moderate pressures, readily vaporizing upon release of pressure. It is this property that permits transportation of and storage of LP Gas in concentrated liquid form.

Where does LPG come from?
LPG comes from two sources. It can be obtained from the refining of crude oil. When produced this way it is generally in pressurized form. LPG is also extracted from natural gas or crude oil streams coming from underground reservoirs. 60% of LPG in the world today is produced this way whereas 40% of LPG is extracted from refining of crude oil.

What is commercial Propane & Butane?
Ideally products referred to as "propane" and "butane" consist very largely of these saturated hydrocarbons; but during the process of extraction/production certain allowable unsaturated hydrocarbons like ethylene, propylene, butylenes etc. may be included in the mixture along with pure propane and butane. The presence of these in moderate amounts would not affect LPG in terms of combustion but may affect other properties slightly (such as corrosiveness or gum formation).

How is LPG seen & felt?
-It is colorless and cannot be seen
-It is odorless. Hence LPG is odorized by adding an odorant (Mercaptans) prior to supply to the user, to aid the detection of any leaks.
-It is slightly heavier than air and hence if there is a leak it flows to lower lying areas.
-In liquid form, its density is half that of water and hence it floats initially before it is vaporized.
-It is non-toxic but can cause asphyxiation in very high concentrations in air.

"LPG expands upon release and 1 liter of liquid will form approximately 250 liters of vapor"

What is LPG used for?
LPG is used as a fuel for domestic (cooking), industrial, horticultural, agricultural, heating and drying processes. LPG can be used as an automotive fuel or as a propellant for aerosols, in addition to other specialist applications. LPG can also be used to provide lighting through the use of pressure lanterns.

Why are Butane and Propane used in combination?
While butane and propane are different chemical compounds, their properties are similar enough to be useful in mixtures. Butane and Propane are both saturated hydrocarbons.

They do not react with other. Butane is less volatile and boils at 0.6 deg C.

Propane is more volatile and boils at - 42 deg C.

Both products are liquids at atmospheric pressure when cooled to temperatures lower than their boiling points. Vaporization is rapid at temperatures above the boiling points.

The calorific (heat) values of both are almost equal.

Both are thus mixed together to attain the vapor pressure that is required by the end user and depending on the ambient conditions.

If the ambient temperature is very low propane is preferred to achieve higher vapor pressure at the given temperature.

What are the advantages of LPG?
The advantages of LPG are as follows :- Because of its relatively fewer components, it is easy to achieve the correct fuel to air mix ratio that allows the complete combustion of the product. This gives LPG its clean burning characteristics.

Both Propane and Butane are easily liquefied and stored in pressure containers. These properties make the fuel highly portable, and hence, can be easily transported in cylinders or tanks to end-users.

LPG is a good substitute for petrol in spark ignition engines. Its clean burning properties, in a properly tuned engine, give reduced exhaust emissions, extended lubricant and spark plug life.
As a replacement for aerosol propellants and refrigerants, LPG provides alternatives to fluorocarbons, which are known to cause deterioration of the earth's ozone layer.

The clean burning properties and portability of LPG provide a substitute for traditional fuels such as wood, coal, and other organic matter. This provides a solution to de-forestation and the reduction of particulate matter in the atmosphere (haze), caused by burning the traditional fuels.

What are LPG properties?
Property---------------------Units-----Com-Propane------Com-Butane-----Mixture 50% each
Specific gravity of Liquid

at 15 deg C (Water=1)------------------ 0.504--------------0.582-----------0.543
Specific gravity of Vapor

at 15 deg C(Air=1)----------------------1.5------------------2.01------------1.75
Vapor pressure at 38 deg C--Kg/sq.cm--13.8-----------------2.6-------------8.0
Boiling point at atm pressure-Deg C------(- 42)---------------(9)------------+ 9 to - 42
Ignition temperature in air---Deg C------495-605-----------480-535-------480-605
Latent Heat of Vaporization--Btu/lb------184----------------167------------175

Karl - Fisher Titration

There are two different approaches to KF titration:-
1. Volumetric method originally developed in 1935
2.Coulometric method (the modern way) such as that used in all Cou-Lo titrators.

In the volumetric technique the active ingredient, Iodine, is introduced by volume addition of a reagent.

In the coulometric technique the Iodine is produced by electrolysis and the amount of current is measured.

In volumetric method is usually used for high water contents and also for samples such as foods which contain high water and also require large sample amounts for representative analysis.
Typically used for water contents %.

Coulometric technique is 1000 times more sensitive and will detect down to microgram level. Much better for low water contents samples, ease of operation etc.

Volumetric titration reaction

The determination is based on the iodometric titration of sulphur dioxide in water.
I2 + SO2 + 2H2O ↔ 2HI + H2SO2
To shift the equilibrium to the right the acid is neutralized with an Imidazole-buffer.
The complete reaction involves also the solvent, methanol. So the description of the Karl Fisher titration is done by the following two reactions. Where RN is the base used in the mixture.
CH3OH + SO2 + RN → [RNH]SO3CH3
H2O + I2 + [RNH]SO3CH3 + RN → [RNH]SO4CH3 + 2[RNH]I

Coulometric titration reaction:

The main compartment of the titration cell contains the anode solution plus the analyte. The anode solution consists of an alcohol (ROH), a base (B), SO2 and I2. A typical alcohol that may be used is methanol or diethylene glycol monomethyl ether, and a common base is imidazole.
The titration cell also consists of a smaller compartment with an (anode) immersed in the anode solution of the main compartment. The two compartments are separated by an ion-permeable membrane.
The Pt anode generates I2 when current is provided through the electric circuit. The net reaction as shown below is oxidation of SO2 by I2. One mole of I2 is consumed for each mole of H2O. In other words, 2 moles of electrons are consumed per mole of water.
B·I2 + B·SO2 + B + H2O → 2BH+I− + BSO3
BSO3 + ROH → BH+ROSO3−
The end point is detected most commonly by a bipotentiometric method.

A second pair of Pt electrodes are immersed in the anode solution. The detector circuit maintains a constant current between the two detector electrodes during titration. Prior to the equivalence point, the solution contains I- but little I2. At the equivalence point, excess I2 appears and an abrupt voltage drop marks the end point.
The amount of current needed to generate I2 in order to reach the end point can then be used to calculate the amount of water in the original sample.

The advantages of coulometric KF are:-
•Fast analysis - Most results available in less than 1 minute, many in less than 30 seconds.
•Low reagent usage - Multiple samples can be analysed on just one charge of reagent.
•Simple operation -Press START, inject the sample, read the result.
•No reagent calibration - No chemical calibration required.
•Direct readout - Results displayed and printed automatically.
ALL coulometers operate the same way.

10.71 coulombs of electricity will titrate 1 milligram of water.

Flash point

All flammable liquid has a vapour pressure, which is a function of that liquid's temperature.

As the temperature increases, the vapour pressure increases.

As the vapour pressure increases, the concentration of evaporated flammable liquid in the air increases.

Hence, temperature determines the concentration of evaporated flammable liquid in the air.

Each flammable liquid requires a different concentration of its vapour in air to sustain combustion.

The flash point of a flammable liquid is the lowest temperature at which there can be enough flammable vapour to ignite, when an ignition source is applied.

There are two basic types of flash point measurement: open cup and closed cup.

In open cup devices the sample is contained in an open cup (hence the name) which is heated, and at intervals a flame is brought over the surface.

The measured flash point will actually vary with the height of the flame above the liquid surface, and at sufficient height the measured flash point temperature will coincide with the fire point. The best known example is the Cleveland Open Cup (COC).

There are two types of Closed cup testers: one is non-equilibrium Flash point, such as Pensky-Martens where the vapours above the liquid are not in temperature equilibrium with the liquid, and another one is equilibrium temperature flash point, where the vapours and the liquid temperature are in equilibrium condition.

In both these types the cups are sealed with a lid through which the ignition source can be introduced. Closed cup testers normally give lower values for the flash point than Open cup (typically 5-10 °C) and are a better approximation to the temperature at which the vapour pressure reaches the lower flammable limit (LFL).

Fuel------------Flash point---------------Auto ignition Temp
Ethanol---------12.8°C (55°F)---------------365°C (689°F)
Gasoline--------<−40°C (−40°F)------------246°C (475°F)
Kerosene------->38°C (162°F)--------------220°C (428°F)
Jet fuel--------->38°C (100°F)--------------210°C (410°F)
Diesel----------->62°C (143°F)--------------210°C (410°F)
Biodiesel-------->130°C (266°F)

CCAI & CII

The Calculated Carbon Aromaticity Index (CCAI) is an index of the ignition quality of residual fuel oil. The running of all internal combustion engines is dependent on the ignition quality of the fuel.

For spark-ignition engines the fuel has an octane rating.

For diesel engines it depends on the type of fuel, for distillate fuels the cetane numbers are used. Cetane numbers are tested using a special test engine and the existing engine was not made for residual fuels.

For residual fuel oil two other empirical indexes are used: CCAI and Calculated Ignition Index (CII). Both CCAI and CII are calculated from the density and kinematic viscosity of the fuel.

CCAI= D - 81 -141x Log [Log [kV + 0.85]] - 483 x Log [(t+273) / 323]

Where: D= density at 15°C (kg/m3); V= viscosity (cSt); t = viscosity temperature (°C).

Calculated Carbon Aromaticity index will normally give a value between 800 and 880. The lower the value is the better the ignition quality.

Fuels with a CCAI more than 880 are often problematic or even unusable in a diesel engine. CCAI are often calculated under testing of marine fuel.

The Calculated Ignition Index (CII) is an index of the ignition quality of residual fuel oil.

CII = (270.795 + 0.1038T) – 0.254565 D + 23.708 log log(V+0.7)

Where:D = density at 15°C (kg/m³); V = viscosity (cSt); T = viscosity temperature (°C).
CII was designed to give out numbers in the same order as the cetane index for distillate fuels.

Types of Gasoline Additives

A typical gasoline may contain

Oil-soluble Dye, initially added to leaded gasoline at about 10 ppm to prevent its misuse as an industrial solvent, and now also used to identify grades of product.

Antioxidants, typically phenylene diamines or hindered phenols, are added to prevent oxidation of unsaturated hydrocarbons.

Metal Deactivators, typically about 10ppm of chelating agent such as N,N'-disalicylidene-1,2-propanediamine is added to inhibit copper, which can rapidly catalyze oxidation of unsaturated hydrocarbons.

Corrosion Inhibitors, about 5ppm of oil-soluble surfactants are added to prevent corrosion caused either by water condensing from cooling, water-saturated gasoline, or from condensation from air onto the walls of almost-empty gasoline tanks that drop below the dew point.

If your gasoline travels along a pipeline, it's possible the pipeline owner will add additional corrosion inhibitor to the fuel.

Anti-icing Additives, used mainly with carburetted cars, and usually either a surfactant, alcohol or glycol.

Anti-wear Additives, these are used to control wear in the upper cylinder and piston ring area that the gasoline contacts, and are usually very light hydrocarbon oils. Phosphorus additives can also be used on engines without exhaust catalyst systems.

Deposit-modifying Additives, usually surfactants.

1. Carburettor Deposits, additives to prevent these were required when crankcase blow-by (PCV) and exhaust gas recirculation (EGR) controlswere introduced. Some fuel components reacted with these gas streams to form deposits on the throat and throttle plate of carburettors.

2. Fuel Injector tips operate about 100C, and deposits form in the annulus during hot soak, mainly from the oxidation and polymerisation of the larger unsaturated hydrocarbons. The additives that prevent and unclog these tips are usually polybutene succinimides or polyether amines.

3. Intake Valve Deposits caused major problems in the mid-1980s when some engines had reduced driveability when fully warmed, even though the amount of deposit was below previously acceptable limits. It is believed that the new fuels and engine designs were producing a more absorbent deposit that grabbed some passing fuel vapour, causing lean hesitation.

Intake valves operate about 300C, and if the valve is kept wet, deposits tend not to form, thus intermittent injectors tend to promote deposits.

Oil leaking through the valve guides can be either harmful or beneficial, depending on the type and quantity. Gasoline factors implicated in these deposits include unsaturates and alcohols. Additives to prevent these deposits contain a detergent and/or dispersant in a higher molecular weight solvent or light oil whose low volatility keeps the valve surface wetted.

4. Combustion Chamber Deposits have been targeted in the 1990s, as they are responsible for significant increases in emissions. Recent detergent-dispersant additives have the ability to function in both the liquid and vapour phases to remove existing deposits that have resulted from the use of other additives, and prevent deposit formation.

Note that these additives can not remove all deposits, just those resulting from the use of additives.

Octane Enhancers, these are usually formulated blends of alkyl lead or MMT compounds in a solvent such as toluene, and added at the 100-1000 ppm levels. They have been replaced by hydrocarbons with higher octanes such as aromatics and olefins. These hydrocarbons are now being replaced by a mixture of saturated hydrocarbons and and oxygenates.

Effect of Humidity in Octane No

An increase of absolute humidity of 1.0 g water/kg of dry air lowers the octane requirement of an engine by 0.25 - 0.32 MON.

Carburetter icing :

Carburettor icing is caused by the combination of highly volatile fuel, high humidity and low ambient temperature. The extent of cooling, caused by the latent heat of the vaporised gasoline in the carburettor, can be as much as 20C, perhaps dropping below the dew point of the charge.

If this happens, water will condense on the cooler carburettor surfaces, and will freeze if the temperature is low enough. The fuel volatility can not always be reduced to eliminate icing, so anti-icing additives are used. In the US, anti-icingadditives are seldom required because of the widespread use heated intakeair and fuel injection.

Two types of additive are added to gasoline to inhibit icing:-

Surfactants that form a monomolecular layer over the metal parts that inhibits ice crystal formation. These are usually added at concentrations of 30-150 ppm.-

Cryoscopic additives that depress the freezing point of the condensed water so that it does not turn to ice.

Alcohols ( methanol, iso-propyl alcohol, etc. ) and glycols ( hexylene glycol, dipropylene glycol ) are used at concentrations of 0.03% - 1%.

If you have icing problems, the addition of 100-200mls of alcohol to a full tank of dry gasoline will prevent icing under moderately-cold conditions.

If you believe there is a small amount of water in the fuel tank, add 500mls of anhydrous isopropyl alcohol as the first treatment, and isopropyl alcohol is also preferred for more severe conditions. Oxygenated gasolines using alcohols can also be used. It's important to ensure the alcohol is anhydrous.

Higher octane fuels give more power

On modern engines with sophisticated engine management systems, the engine can operate efficiently on fuels of a wider range of octane rating, but there remains an optimum octane for the engine under specific driving conditions.

Older cars without such systems are more restricted in their choice of fuel, as the engine can not automatically adjust to accommodate lower octane fuel.

Because knock is so destructive, owners of older cars must use fuel that will not knock under the most demanding conditions they encounter, and must continue to use that fuel, even if they only occasionally require the octane.

If you are already using the proper octane fuel, you will not obtain morepower from higher octane fuels. The engine will be already operating at optimum settings, and a higher octane should have no effect on the management system.

Your driveability and fuel economy will remain the same. The higher octane fuel costs more, so you are just throwing money away. If you are already using a fuel with an octane rating slightly below the optimum, then using a higher octane fuel will cause the engine management system to move to the optimum settings, possibly resulting in both increased power and improved fuel economy.

You may be able to change octanes between seasons ( reduce octane in winter ) to obtain the most cost-effective fuel without loss of driveability. Once you have identified the fuel that keeps the engine at optimum settings, there is no advantage in moving to an even higher octane fuel.

The manufacturer's recommendation is conservative, so you may be able to carefully reduce the fuel octane. The penalty for getting it badly wrong, and not realising that you have, could be expensive engine damage.

RON and MON

AKI = Anti Knocking Index = (RON +MON) /2

RON - MON = Sensitivity.

Because the two test methods use different test conditions, especially the intake, mixture temperatures and engine speeds, then a fuel that is sensitive to changes in operating conditions will have a larger difference between the two rating methods.

Modern fuels typically have sensitivities around 10.

The US 87 (RON+MON)/2 unleaded gasoline is recommended to have a 82+ MON, thus preventing very high sensitivity fuels.

Recent changes in European gasolines has caused concern, as high sensitivity unleaded fuels have been found that fail to meet the 85 MON requirement of the EN228 European gasoline specification.

Engine Spec:

Automotive octane ratings are determined in a special single-cylinder, four stroke engine with a variable compression ratio ( CR 4:1 to 18:1 ) known as a Cooperative Fuels Research ( CFR ) engine.

Research Octane

The Research method settings represent typical mild driving, withoutconsistent heavy loads on the engine.Test Engine conditions Research Octane

Test Method: ASTM D 2699

Engine: Cooperative Fuels Research ( CFR )
Engine RPM: 600 RPM
Intake air temperature: Varies with barometric pressure ( eg 88kPa = 19.4C, 101.6kPa = 52.2C )
Intake air humidity: 3.56 - 7.12 g H2O / kg dry air
Intake mixture temperature : NA
Coolant temperature: 100 C
Oil Temperature : 57 C
Ignition Advance : fixed 13 degrees BTDC
Carburettor Venturi Set: according to engine altitude (eg 0-500m=14.3mm, 500-1000m=15.1mm )

Motor Octane rating

The conditions of the Motor method represent severe, sustained high speed, high load driving. For most hydrocarbon fuels, including those with either lead or oxygenates, the motor octane number (MON) will be lower than the research octane number (RON).

Test Engine conditions Motor Octane
Test Method : ASTM D2700
Engine : Cooperative Fuels Research ( CFR )
Engine RPM : 900 RPM
Intake air temperature : 38 C or 100F
Intake air humidity: 3.56 - 7.12 g H2O / kg dry air
Intake mixture temperature : 149 C
Coolant temperature: 100 C
Oil Temperature : 57 C
Ignition Advance - variable Varies with compression ratio ( eg 14 - 26 degrees BTDC )
Carburettor Venturi : 14.3 mm

Why are two ratings of Octane?

The correct name for the (RON+MON)/2 formula is the "antiknock index",and it remains the most important quality criteria for motorists.

The initial knock measurement methods developed in the 1920s resulted in a diverse range of engine test methods and conditions, many of which have beensummarised by Campbell and Boyd.

In 1928 the Co-operative Fuel ResearchCommittee formed a sub-committee to develop a uniform knock-testing apparatus and procedure.

They settled on a single-cylinder, valve-in-head, water-cooled, variable compression engine of 3.5" bore and 4.5" stroke.

The knock indicator was the bouncing-pin type. They selected operating conditions for evaluation that most closely match the current Research Method, how ever correlation trials with road octanes in the early 1930s exhibited such large discrepancies that conditions were changed ( higher engine speed, hot mixture temperature, and defined spark advance profiles ), and a new tentative ASTM Octane rating method was produced.

This method is similar to the operating conditions of the current Motor Octane procedure. Over several decades, a large number of alternative octane test methods appeared. These were variations to either the engine design, or the specified operating conditions.

During the 1950-1960s attempts were made to internationally standardise and reduce the number of Octane Rating test procedures. During the late 1940s - mid 1960s, the Research method became the important rating because it more closely represented the octane requirements of the motorist using the fuels/vehicles/roads then available.

In the late 1960s German automakers discovered their engines were destroying themselves on long Autobahn runs, even though the Research Octane was within specification. They discovered that either the MON or the Sensitivity ( the numerical difference between the RON and MON numbers ) also had to be specified.

Today it is accepted that no one octane rating covers all use. In fact, during 1994, there have been increasing concerns in Europe about the high Sensitivity of some commercially-available unleaded fuels. The design of the engine and vehicle significantly affect the fuel octane requirement for both RON and MON.

In the 1930s, most vehicles would have been sensitive to the Research Octane of the fuel, almost regardless of the Motor Octane, whereas most 1990s engines have a 'severity" of one, which means the engine is unlikely to knock if a changes of one RON is matched by an equal and opposite change of MON.

Note that the Research method was only formally approved in 1947, but used unofficially from 1942.

What fuel does the Octane Rating?

The fuel property the octane ratings measure is the ability of the unburntend gases to spontaneously ignite under the specified test conditions.

Within the chemical structure of the fuel is the ability to withstand pre-flame conditions without decomposing into species that will autoignite before the flame-front arrives.

Different reaction mechanisms, occurring atvarious stages of the pre-flame compression stroke, are responsible for the undesirable, easily-autoignitable, end gases.

During the oxidation of a hydrocarbon fuel, the hydrogen atoms are removed one at a time from the molecule by reactions with small radical species(such as OH and HO2), and O and H atoms. The strength of carbon-hydrogenbonds depends on what the carbon is connected to.

Straight chain HCs such as normal heptane have secondary C-H bonds that are significantly weaker thanthe primary C-H bonds present in branched chain HCs like iso-octane.

The octane rating of hydrocarbons is determined by the structure of the molecule, with long, straight hydrocarbon chains producing large amounts of easily-autoignitable pre-flame decomposition species, while branched and aromatic hydrocarbons are more resistant.

This also explains why the octane ratings of paraffins consistently decrease with carbon number. In real life, the unburnt "end gases" ahead of the flame front encounter temperatures up to about 700C due to compression and radiant and conductive heating, and commence a series of pre-flame reactions.

These reactions occur at different thermal stages, with the initial stage ( below 400C ) commencing with the addition of molecular oxygen to alkyl radicals, followed by the internal transfer of hydrogen atoms within the new radical to form an unsaturated, oxygen-containing species.

These new species are susceptible to chain branching involving the HO2 radical during the intermediate temperature stage (400-600C), mainly through the production of OH radicals. Above 600C, the most important reaction that produces chain branching is the reaction of one hydrogen atom radical with molecular oxygen to form O and OH radicals.

The addition of additives such as alkyl lead and oxygenates can significantly affect the pre-flame reaction pathways.

Antiknock additives work by interfering at different points in the pre-flame reactions, with the oxygenates retarding undesirable low temperature reactions, and thealkyl lead compounds react in the intermediate temperature region to deactivate the major undesirable chain branching sequence. The antiknock ability is related to the "autoignition temperature" of the hydrocarbons.

Purpose of Octane Measurement

To obtain the maximum energy from the gasoline, the compressed fuel-air mixture inside the combustion chamber needs to burn evenly, propagating out from the spark plug until all the fuel is consumed. This would deliver an optimum power stroke.

In real life, a series of pre-flame reactions will occur in the unburnt "end gases" in the combustion chamber before the flame front arrives. If these reactions form molecules or species that can autoignite before the flame front arrives, knock will occur.

Simply put, the octane rating of the fuel reflects the ability of the unburnt end gases to resist spontaneous autoignition under the engine testconditions used.

If autoignition occurs, it results in an extremely rapid pressure rise, as both the desired spark-initiated flame front, and the undesired autoignited end gas flames are expanding.

The combined pressure peak arrives slightly ahead of the normal operating pressure peak, leading to a loss of power and eventual overheating. The end gas pressure waves are superimposed on the main pressure wave, leading to a sawtooth pattern of pressure oscillations that create the "knocking" sound.

The combination of intense pressure waves and overheating can induce piston failure in a few minutes. Knock and preignition are both favoured by high temperatures, so one may lead to the other. Under high-speed conditions knock can lead to preignition, which then accelerates engine destruction

Octane Invention

Since 1912 the spark ignition internal combustion engine's compression ratio had been constrained by the unwanted "knock" that could rapidly destroy engines.

"Knocking" is a very good description of the sound heard from an engine using fuel of too low octane. The engineers had blamed the "knock" on the battery ignition system that was added to cars along with the electric self-starter.

The engine developers knew that they could improve power and efficiency if knock could be overcome. Kettering assigned Thomas Midgley, Jr. to the task of finding the exact cause of knock. They used a Dobbie-McInnes manograph to demonstrate that the knock did not arise from preignition, as was commonly supposed, butarose from a violent pressure rise *after* ignition. The manograph was notsuitable for further research, so Midgley and Boyd developed a high-speed camera to see what was happening. They also developed a "bouncing pin" indicator that measured the amount of knock.

Ricardo had developed analternative concept of HUCF ( Highest Useful Compression Ratio ) using a variable-compression engine. His numbers were not absolute, as there were many variables, such as ignition timing, cleanliness, spark plug position, engine temperature. etc.

In 1927 Graham Edgar suggested using two hydrocarbons that could be produced in sufficient purity and quantity.

These were "normal heptane", that was already obtainable in sufficient purity from the distillation of Jeffrey pine oil, and " an octane, named 2,4,4-trimethyl pentane " that he first synthesized. Today we call it " iso-octane " or 2,2,4-trimethyl pentane.

The octane had a high antiknock value, and he suggested using the ratio of the two as a reference fuel number. He demonstrated that all the commercially- available gasolines could be bracketed between 60:40 and 40:60 parts by volume heptane:iso-octane.

The reason for using normal heptane and iso-octane was because they both have similar volatility properties, specifically boiling point, thus the varying ratios 0:100 to 100:0 should not exhibit large differences in volatility that could affect the rating test.

Name ------------Melting Point(C)---- Boiling Point (C) --Density (g/ml) --Heat of Vaporisation
normal heptane --: -90.7 -------------- +98.4------------ 0.684 -----------0.365 @ 25C
iso octane --------:-107.45 ------------+ 99.3 ------------0.6919 ----------0.308 @ 25C
Having decided on standard reference fuels, a whole range of engines andtest conditions appeared, but today the most common are the Research OctaneNumber ( RON ), and the Motor Octane Number ( MON ).

Octane Rating

To rate a fuel, the engine is set to an appropriate compression ratio that will produce a knock of about 50 on the knockmeter for the sample when the air-fuel ratio is adjusted on the carburettor bowl to obtain maximum knock.

Normal heptane and iso-octane are known as primary reference fuels.

Two blends of these are made, one that is one octane number above the expected rating, and another that is one octane number below the expected rating.

These are placed in different bowls, and are also rated with each air-fuelratio being adjusted for maximum knock.

The higher octane reference fuel should produce a reading around 30-40, and the lower reference fuel shouldproduce a reading of 60-70.

The sample is again tested, and if it does not fit between the reference fuels, further reference fuels are prepared, and the engine readjusted to obtain the required knock.

The actual fuel rating is interpolated from the knockmeter readings.