Chemistry of water -Part 26-

Titration of negative electrolytes in water:

1- Chloride anion titration (Cl-):

a) Mohr's method:
In the presence of potassium chromate, the chloride ions are titrated in a neutral medium with a standard solution of silver nitrate, and the completion of the reaction is indicated by the appearance of the distinctive red color of silver chromate.
For this process, the following reagents are used: concentrated nitric acid, calcium carbonate, potassium chromate solution (10 percent), silver nitrate solution (3.17 g in a liter of distilled water).
If the water sample is alkaline, its pH is adjusted using nitric acid, but if it is acidic, its pH is adjusted using calcium bicarbonate until a moderate water is reached (PH= 7).
1 ml of potassium chromate solution is added to 100 ml of the aqueous sample after adjusting its pH. A silver nitrate solution is used to calibrate the sample until a red coloration is seen. Let us assume that the volume of the silver nitrate solution used for calibration is V, so the concentration of chloride ions is equal to:

C (mg/l)= (V. t. (35.5). V') / 1000

where V' is the volume of the aqueous sample.

b) Titration of chloride ions in an acidic medium:
Mercuric nitrate in the presence of a diphenylcarbazone indicator is used to titrate chloride ions in an acidic medium. Where the following reagents are used: nitric acid, bromophenol solution, mercury nitrate solution, sodium chloride solution, diphenylcarbazone solution C13H12N4O (to be prepared by dissolving 0.5 g of diphenylcarbazone and 0.5 g of basic copper carbonate in 500 ml of alcohol.)

Basic copper(II) carbonate

Ten drops of a bromophenol solution are added to 50 ml of water to be calibrated, and nitric acid is added until the yellow color appears, and the same previous steps are carried out on a mixture consisting of 49 ml of distilled water, to which 1 ml of sodium chloride solution is added, and let it be volume V The nitric acid needed to acidify the sample water, and V' needed to acidify the distilled water.
The two previous solutions are calibrated using mercury nitrate until the color changes and becomes violet. If N ml of mercury nitrate solution is required for calibration of sample 1 and M ml for calibration of sample 2, then the concentration of chloride ions will be equal to:

C (mg/l)= ((0.71). M. 1000) / N. 50

References:

• [AQUAPROX- Livre: Traitement des eaux de refroidissement. Imprimé en France par EMD S.A.S- 53110 Lassay-les-Chateaux. N° d'imprimeur: 15566- Dépot légal: juin 2006. N° 842- Cyclus print 90°]

• [Introduction to Water Chemistry (Pollution- Treatment- Analysis). Dr. Nasser Al-Hayek. Publication of the Higher Institute for Applied Sciences and Technology (HIAST). Syrian Arab Republic, 2017.]

• Folco Laverdière, Anja Holstein, Laurent Thiebaut, Robert Mallee, Guillaume Gravejat, Benjamin Des clozeaux: Les principales methodes d’analyse,1999, p5.