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It is not always possible to use Mohr method to determine concentration of chlorides. For example, Mohr method requires neutral solution, but in many cases solution has to be acidic, to prevent precipitation of metal hydroxides (like in the presence of Fe3+). In such cases we can use Volhard method, which is not sensitive to low pH.
In the Volhard method chlorides are first precipitated with excess silver nitrate, then excess silver is titrated with potassium (or sodium) thiocyanate. To detect end point we use Fe3+ cations, which easily react with the thiocyanate, creating distinct wine red complex.
There is a problem though. Silver thiocyanate solutility is slightly lower than solubility of silver chloride, and during titration thiocyanate can replace chlorides in the existing precipitate:
AgCl(s) + SCN- → AgSCN(s) + Cl-
To avoid problems we can filtrate precipitated AgCl before titration. However, there exist much simpler and easier procedure that gives the same result. Before titration we add some small volume of a heavy organic liquid that is not miscible with water (like nitrobenzene, chloroform or carbon tetrachloride). These liquids are better at wetting precipitate than water. Once the precipitate is covered with non polar liquid, it is separated from the water and unable to dissolve.
Precipitate solubility is not a problem during determination of I- and Br-, as both AgBr and AgI have much lower solubilities than AgSCN.
There are two reactions, as this is a back titration. First, we precipitate chlorides from the solution:
Ag+ + Cl- → AgCl(s)
Then, during titration, reaction taking place is:
Ag+ + SCN- → AgSCN(s)
In back titrations sample size is more difficult to calculate than during normal, direct titrations. For best accuracy excess of silver should be titrated with about 40-45 mL of titrant (assuming - as we usually do - that we are using 50 mL burette). However, that usually means we should use relatively large initial volume of silver solution. Assuming we will start with 50 mL of pipetted silver nitrate and we will titrate excess with about 25 mL of thiocyanate solution, and finally assuming both solutions used are 0.1M, aliquot taken for titration should contain about 0.09 g chloride anion (2.5 millimoles).
Note, that silver nitrate can be added not using single volume pipette, but from burette. If the amount of chlorides is approximately known, this way it is possible to control excess of silver nitrate and volume of the thiocyanate titrant.
end point detection
End point is detected with the use of iron (III) thiocyanate complex, which have very distinct and strong wine color.
To perform titration we will need 0.1M silver nitrate solution to precipitate chlorides, titrant - 0.1M potassium thiocyanate solution, nitric acid (1+1) to acidify solution, ammonium ferric sulfate solution that will be used for end point detection, nitrobenzene, and some amount of distilled water to dilute sample.
- Pipette aliquot of chlorides solution into 250mL Erlenmeyer flask.
- Add 5 mL of 1+1 nitric acid.
- Dilute with distilled water to about 100 mL.
- Add 50 mL of 0.1M silver nitrate solution.
- Add 3 mL of nitrobenzene.
- Add 1 mL of iron alum solution.
- Shake the content for about 1 minute to flocullate the precipitate.
- Titrate with thiocyanate solution till the first color change.
As in every back titration, to calculate amount of substance we have to subtract amount of titrated excess from the initial amount of reactant used. In the case of argentometry calculations are easy, as all substances used react on the 1:1 basis.
First we have to calculate number of moles of silver nitrate initially added to the chlorides sample. Assuming it was 50 mL of 0.1 M solution, it contained 5 millimole of silver. Then, excess was titrated according to the reaction equation:
Ag+ + SCN- → AgSCN(s)
Thus amount of excess silver is C×V, and amount of Cl is 0.005-C×V moles.
sources of errors
Apart from problems listed on the general sources of titration errors page, results of titration can be skewed by the already mentioned replacement of precipitated chlorides by silver thiocyanate. It shouldn't matter if the procedure was followed carefully.
Page was last modified on September 09 2009, 19:27:53.