Determining the pH value of water without using a pH meter

Ivan Tikhonov

Annotation

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.

       (1)

 

    (2)

 

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.

It is necessary to determine the pH value of the heat network water. The heat network circuit is closed. Temperature chart is 95-70 0C. Make-up water undergoes a single-stage softening. There is no dosing of the reagent to increase the pH of the make-up water.

Since the circuit is closed, there is a gradual removal of free carbon dioxide from the circuit through the air vents due to heating of the circulating water (the solubility of CO2 decreases) and, accordingly, the pH value of the circulating water increases to values of about 8.2-8.3.

In reality, there were constant leaks in the circuit and the circuit was fed with softened water with a pH of about 7.0-7.1 (the CO2 concentration of the make – up water is about 10-15 mg/l). Accordingly, the pH value of the circulating water decreased.

Uneven feeding and constant fluctuation of the pH value of the circulating water caused doubts about the accuracy of pH measurements using the pH meter. As a result, I performed chemical analyses using the methods presented above. Data on the results of analyses are presented in table 1. The pH value was measured using a pH meter.

Table 1

Parameter unit of measure Make-up water Circulation water
Hardness mg-eq/l 0,11 0,18
Alkalinity (НСО3) mg-eq/l 2,0 2,7
рН un. рН 7,1 7,8
СО2 mg-eq/l 0,35 (15 мг/л) 0,07 (3,0 мг/л)
Conductivity  µS/cm 421 459

As can be seen from the table, circulating water contains significantly less CO2 than make-up water. As a result, the pH value of the circulating water increases.

Let’s calculate the pH value using equation 1.

 

 

 

 

The pH values measured and calculated are almost the same.

In fact, if there is no feeding, the pH of the circulating water rises to 8.3-8.4, and the carbon dioxide corrosion of the pipelines stops.

One more point should be noted. The alkalinity of the circulating water in this example is greater than the alkalinity of the make-up water. Although the circuit receives only make-up water. This is probably due to the fact that when the circuit is not fed for a long time, then there is a complete removal of CO2 and the pH rises to 8.3-8.4. As a result, hydrate appears in the circulating water due to the hydrolysis of carbonates formed at such pH values. In the case of feeding the circuit with make-up water that has free carbon dioxide, the process of binding free carbon dioxide into bicarbonate by hydrate of the circulating water occurs. Accordingly, the bicarbonate alkalinity of the circulation circuit increases. Also, the supply of CO2 to the circulation circuit is possible from the processes of oxidation of organic matter contained in the circulating water. Accordingly, the bicarbonate alkalinity of the circulation circuit increases. Also, the supply of CO2 to the circulation circuit is possible from the processes of oxidation of organic matter contained in the circulating water. This should be taken into account when determining the alkalinity for calculating the pH of circulating water. In other words, it is necessary to analyze the alkalinity of the circulating water, and not to assume that the alkalinity of the circulation circuit is equal to the alkalinity of the source water.

It should also be taken into account that if the water also contains significant amounts of hydrogen sulfide or ammonia, this will also affect the pH. Water with this peculiarity will require a comprehensive approach to identify the causes of pollution and develop treatment technology. However, with a high amount of bicarbonate in water relative to hydrogen sulfide and ammonia it is the carbon dioxide balance that will determine the pH of the water.

 

I hope this information can be useful.

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