Chemistry of water -Part 21-

Total organic carbon (TOC):

The amount of carbon in an organic compound is known as total organic carbon (TOC), and it's usually utilized to give an overall idea of the water's quality. The Total Organic Carbon (TOC) Analyzer relies on the oxidation of mineral and organic compounds to produce carbon dioxide and water, where the carbon dioxide gas resulting from the oxidation reaction is analysed using an infrared device. There are two different kinds of equipment used for this, in the first type, the oxygen oxidation process takes place at a high temperature (950 ° C) using a metal catalyst. While in the second type, a strong oxidizer is used in the presence of ultraviolet radiation in a glass reactor. The results obtained in both cases represent the total carbon (organic and mineral).

Calibration of ammoniacal nitrogen:

To calibrate ammonia nitrogen in water, there are several methods, and according to the concentration of nitrogen in the water and the presence of other electrolytes such as chlorine and iron (which interfere in the calibration process), the appropriate method is chosen. The titration can be performed directly on the aqueous sample or it can be preceded by a distillation process.

1- Direct titration (without distillation):

The concentration of ammoniacal nitrogen in water can be determined by measuring the ionic activity of ammonia under specific experimental conditions such as the concentration of the electrolytes and the pH of the medium.

It is possible to employ the second approach, which is based on the interaction of ammonium with phenol and hypochlorous acid in an alkaline medium (PH= 10.4 - 11.5). Coloured indophenol compounds formed through the previous transformation, at a wavelength of 360 nm, its absorbance is measured with a spectrophotometer. This method is used for aqueous samples containing a concentration higher than two micrograms per liter of ammoniacal nitrogen.
1 ml of the basic solution and 1 ml of the phenol solution are added to 20 ml of the water to be calibrated. The solution is kept for at least 6 hours in the dark, after which the absorbance is measured at a wavelength of 630 nm.

2 grams of Phenol

• The chemical reactions that take place:

NH3 + HOCl === NH2Cl + H2O

NH2Cl + C6H5OH === C6H4NClO + 2H2O+ 2HCl

C6H5OH + C5H4NClO === C12H9NO2 + HCl

2- Calibration method after sample distillation:

The ammonium is extracted with water vapor while being displaced in an alkaline media. In a distillation flask, a portion of the water sample that needs to be calibrated is put. 20 ml of a sodium carbonate solution with a 100 g/L concentration are added to the water, along with a few drops of an anti-foaming agent. The distillation process takes place for at least 20 minutes.

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.]

• "Chapter A2. Selection of Equipment for Water Sampling" (PDF). National Field Manual for the Collection of Water-Quality Data (Report). Techniques of Water-Resources Investigations, Book 9. Reston, VA: U.S. Geological Survey. March 2003.

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