Volhard's method uses a back titration with potassium thiocyanate to determine the concentration of chloride ions in a solution. Before the titration an excess volume of a silver nitrate solution is added to the solution containing chloride ions, forming a precipitate of silver chloride.
The term ‘excess‘ is used as the moles of silver nitrate added are known to exceed the moles of sodium chloride present in the sample so that all the chloride ions present will react.
Ag+(aq) + Cl-(aq) --> AgCl(s)
The indicator Fe3+ (ferric ion) is then added and the solution is titrated with the potassium thiocyanate solution. The titrate remains pale yellow as the excess (unreacted) silver ions react with the thiocyanate ions to form a silver thiocyanate precipitate.
Ag+(aq) + SCN -(aq) --> AgSCN(s)
Once all the silver ions have reacted, the slightest excess of thiocyanate reacts with Fe3+ to form a dark red complex.
Fe3+(aq) + SCN -(aq) --> [FeSCN]2+(aq)
The concentration of chloride ions is determined by subtracting the titration findings of the moles of silver ions that reacted with the thiocyanate from the total moles of silver nitrate added to the solution. "This method is used when the pH of the solution after the sample has been prepared is acidic. If the pH is neutral or basic, Mohr’s method or the gravimetric method should be used".
waarom moet het volgens volhardt methode in een zuurmilieu?
ReplyDeleteTitration of sewage water with NH4CNS 0.05N, after adding
ReplyDelete10 drops of HNO3
5 ml AGNO3 0.1N
1 ml of NH4Fe(SO4)2*12H2O
T(0)=9.3 ml
T(sample)=7.2 ml
K(NH4CNS)=0.37
How would I express Cl(-) conctration in mg/L?
amoyalyair@hotmail.com
ReplyDeleteTitration of sewage water with NH4CNS 0.05N, after adding
10 drops of HNO3
5 ml AGNO3 0.1N
1 ml of NH4Fe(SO4)2*12H2O
T(0)=9.3 ml
T(sample)=7.2 ml
K(NH4CNS)=0.37
How would I express Cl(-) conctration in mg/L?
1st of all find he moles of cl- then the mass in [g].this will be small, multiply by 1000 to change the mass to mg. do you remember the volume of the waste water that you pipetted? change this to L by dividing by 1000. finally divide the mass is mg by volume in L.this is same as ppm
ReplyDeleteif the addition of 1st drop of thiocyanate gives end point, what does it mean and what should i do?
ReplyDelete