Experiment 6 Mohr Method

Analytical Chemistry 2 2012 Experiment 6 Determination of Salinity by the Mohr Method M ethod

Introduction Mohr method is one of the classical methods for the analysis of soluble chloride. It belongs to a class of methods known as argentometric titrations  – which make use of Ag+ as titrant.

Procedure [1] Preparation of Reagents - Prepare 250 mL of 0.1 M AgNO 3 solution [MW 169.87 g/mol]. Good for one student. Use top loading balance. Protect solution from light. Store in amber reagent bottle. - Prepare 30 mL of 5 %(w/v) K 2CrO4 solution using distilled water as diluent. [Warning: All Cr-containing compounds are classified as suspected carcinogens. Dispose properly as HEAVY METAL WASTE] [2] Blank Correction - Prepare a blank sample by mulling 0.1 g CaCO 3 with 2-3 mL of water in a mortar. Transfer 3-5 drops of the suspension to a 250-mL Erlemeyer flask containing 100 mL of water and add 5 mL of the indicator. Add 0.1 M AgNO 3 dropwise (using half drops) until a color change from greeninsh yellow to brownish yellow is observed. Note the volume of silver nitrate added. SUBTRACT this volume from each trial made on the sample. Keep this solution as end point guide. [3] Standardization of 0.1 M AgNO 3 solution - Dry NaCl in the oven for one hour. Weigh triplicates of 0.14 to 0.15 g (to the nearest 0.1 mg) NaCl. Dissolve and dilute to mark in 100.00-mL volumetric flasks with distilled water. Pipet 10.00 mL of the standard solution in 250-mL Erlenmeyer flasks for analysis. Add 50 mL of distilled water and 5 mL of 5% K 2Cr2O4 indicator to the solution. Titrate with the prepared silver nitrate solution until the solution acquires a brownish yellow tint.

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Analytical Chemistry 2 2012 [4] Analysis of Sample - Submit clean and dry 50-mL reagent bottles to your instructor. Pipet 10.00 mL into a 100-mL volumetric flask and dilute to mark with distilled water. Titrate as in [3]. Do three replicates. [5] Computation of Results   

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   



  

Report the mean, sd, and %rsd of your analyses.

Precaution: Sample solutions must be essentially neutral. Check with pH paper.

Guide Questions 1. Briefly describe the underlying principles of the Mohr method. Write the pertinent equilibria involved. 2. Why should the Mohr titration be performed at essentially neutral pH values? 3. What is blank correction? 4. Why is CaCO3 suspension used for blank correction? 5. Why should the AgNO 3 solution be protected from light?

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