Our aim was to determine percentage of the chloride ion in the sample by Volhard and Mohr methods.
Volhard method , Mohr method
Volumetric methods based upon the formation sparingly soluble silver salts are among the oldest known ; these procedures were and still are routinely employed for the analysis for silver as well as for the determination of such ions as chloride , bromide , visually detectable change in the solution.
For example , the formation of a second precipitate of distinctive color is the basis for end-point detection with the Mohr method. The formation of colored complex is used for the end-point detection with the Volhard method . Adsorption of some organic dyes can be used in end-point detection with fejans method.
Most application of precipitation titrations are based upon the use of a standart silver nitrate solution and are sometimes called argentometric methods.
Volhard Method :
10 mL of NaCI sample was added to 100 mL distilled water. Then 1 mL nitric acid , 5 mL ferric alum solution , 10 mL 0.1M and 15 mL nitrobenzene added to this solution respectively. After that the sample solution was titrated with ammonium thiocyanate until red color persist.
10 mL of NaCI sample was added to 50 mL distilled water. The 2 mL potassium chromate solution was added to it . After that the sample solution was titrated with 0.1M .
RESULTS AND CALCULATION :
The experiment is based upon the determination of the chloride ion with the precipitate titrations. In the precipitate titrations end –point is detected by chemical indicators which procedures a visual detectable change , usually of color or turbity in the solution. The three of types of visual indicator are Mohr , volhard and fejans methods. In the experiment Mohr method and Indirect Volhard method is applied.
In the indirect Volhard method an ions such as are precipitated with excess silver ion , then the is back titrated with a potassium thiocyanate solution. The precipitates of AgI , AgBr and AgSCN are need to be removed from the solution before back titration because AgCI is more soluble than AgSCN , AgCI should be removed by filtration before back titration. But in order to remove AgCI we used nitrobenzene to mask the silver chloride.
In Mohr method chromate ion is used as the indicator in the titration of chloride with silver . In Mohr method , adjusement of concentration of chromate ion and the pH of the solution are important factors to care about. The pH must be about 7-8 .If it is lower chromate ion will change into dichromate and use more then needed silver ion to reach equivalent point. The concentration of chromate should bu usually about 0.005 M . If the conc. of chromate is high the yellow color of chromate ion cancels the appearences of the red colored silver chromate precipitate.
In the experiment gauss errors are possible. While reading the menicus personal judgement is important