The article presents a method for calculating the pH value of water based on the carbon dioxide balance of water. An example from the author’s practice is presented.
The pH value of natural water is usually determined by the ratio of various forms of carbon dioxide found in its composition. Various forms of carbon dioxide in water are formed as a result of adsorption and subsequent hydrolysis of carbon dioxide in water. As a result of hydrolysis, the following forms of carbon dioxide are formed:
CO2 – free carbonic acid,
НСО3 – semi-bound carbonic acid,
СО3 – bound carbonic acid.
The pH value of water can be determined by the Henderson-Hasselbalch equation.
Equation (1) describes the dissociation of carbon dioxide in water by the first stage, equation (2) by the second.
In this article, we will work with equation (1). The calculation of pH by equation (2) is described in the article by I. A. Tikhonov “The influence of various forms of carbon dioxide in water on its pH value” – https://tiwater.info/en/the-influence-of-various-forms-of-carbon-dioxide-in-water-on-its-ph-value/
It is obvious that to determine the pH value according to equation (1) it is necessary to know the values of free and semi-bound carbon dioxide in water.
You can determine the amount of semi-bound carbon dioxide (bicarbonate ion) by analyzing water for alkalinity according to ISO 9963-2:1994 Water quality – Determination of alkalinity. The determination is based on titration of the water sample with hydrochloric acid (0.1 N). As a result, only bicarbonate ions are present in the water in the pH range of the sample from 8.37 to 4.5. Accordingly, the amount of acid taken for titration of the sample from pH 8.37 to 4.5 determines the amount of bicarbonate ion.
To determine the free carbon dioxide in water, you can use the following method. An indicator-phenolphthalein-must be added to the water sample (100 ml). Then the sample should be titrated with a solution of caustic soda (0.1 N) until a stable weak pink color appears.
Caustic soda binds free carbon dioxide into bicarbonate by reaction (3). As a result, the pH value of the sample increases and when the pH value= 8.37, the sample is colored in a weak pink color.
NaOH+CO2 <-> NaHCO3 (3)
If you continue titrating such a sample with sodium hydroxide, the pH value will grow more than 8.37 and carbonates will appear in the water according to equation (4).
NaHCO3+NaOH <-> Na2CO3+H2O (4)
This shows how important it is to titrate the sample with sodium hydroxide to a pH value of 8.37. The use of phenolphthalein for indication instead of the pH meter is acceptable, but it may increase the error in determining the amount of free carbon dioxide.
The amount of caustic soda (0.1 N) used for titration of the sample is equal to the amount of free carbon dioxide (CO2) in mmol/l. That is, 1 ml of the caustic soda solution is equivalent to 1 mmol/l of CO2 (with a sample volume of 100 ml).
As a result, knowing the concentrations of HCO3 and CO2 using equation (1), you can determine the pH of the water sample. In equation (1), you can substitute concentrations, rather than activities, assuming that the activity coefficients of monovalent ions are equal.
One more point needs to be clarified. The presented method for determining the concentration of CO2 in water has been known for a long time and is recommended for determining CO2 in condensate-type waters because condensate is usually saturated with carbon dioxide and practically does not contain hardness salts (Ca, Mg). If there are hardness salts, calcium precipitation will occur, since the water saturation state changes towards the calcium carbonate in the direction of solid calcium carbonate release. The pH value will increase.
As a result, part of the caustic soda can be spent on the removal of calcium carbonate and, accordingly, the amount of carbon dioxide for such water can be determined more than actually exists.
For water with small hardness, there will be small error, but for water with hardness of more than 2 mmol/l, I recommend pre-softening of the sample.
Let’s take an example from my practice.
full version - https://tiwater.info/en/determining-the-ph-value-of-water-without-using-a-ph-meter/
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