Chemistry of water -Part 17-

Analysis of natural waters:

3- Alkalinity: "Alkalimetric title"

The alkalinity of the water is measured using a standard solution of a mineral acid with a phenolphthalein indicator that makes us obtain the phenolphthalein alkalinity (PA) or (TA). The transition of phenolphthalein from red to colourless marks the initial point of neutralization at a pH of 8.3, the neutralization of free bases, soda, and lime corresponds to it:

Ca(OH)2 + 2HCl ==== CaCl2 + H2O

as well as the transition to bicarbonate, which is the semi-neutralization of carbonates:

Na2CO3 + HCl ==== NaHCO3 + NaCl

To obtains the total alkalinity TAC (Or Methyl Orange alkalinity MA), a methyl orange indicator must be used, the value of pH 4,5 is the value at which the transition point occurs. Methyl orange changes colour from yellow to pink during this step, signalling the neutralization of the bicarbonates, or the neutralization of half of the initial amounts of both carbonates and bicarbonates.

Methyl orange

• Calculating the concentrations in hydroxides, bicarbonates, and carbonates is possible thanks to knowledge of the TA and the TAC, hydroxides are the cause of the alkalinity if TA and TAC are both equal, and the presence of carbonates and hydroxides indicates that the TA is less than the TAC.

Accordingly, the TA is equal to all of the hydroxides plus half of the carbonates, and the TAC is equal to both of them.

• All of the alkalinity is caused by carbonates if the TA is less than half the TAC.
• Carbonates and bicarbonates are present if the TA is less than half the TAC.
• The TAC results only from the bicarbonates if the TA is zero.

References:

• [General and inorganic chemistry book- M. Shkhashirou- H. Birqdad- Y. Qodsi- University publications. Algeria]

• [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°]

• [GÉOCHIMIE DES EAUX CONTINENTALES- Nathalie Gassama. ISBN 9782340-047402 ©Ellipses Édition Marketing S.A., 2021. 8/10 rue la Quintinie 75015 Paris]

• [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.