The relationship between total alkalinity, carbonate hardness, and pH

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With the continuous development of the socio-economy, large amounts of wastewater are discharged without treatment, severely affecting river waters as a result of human activities; this alters their original hydrochemical characteristics and leads to increasing pollution. This changed situation has presented new challenges to water quality monitoring efforts. The purpose of water quality monitoring is to promptly, accurately, and comprehensively reflect the status and development trends of water environmental quality, thereby providing a scientific basis for water environment management, planning, and pollution control. It is an important fundamental task for the rational development, utilization, and protection of water resources; it serves as a sentinel and eyes-and-ears for controlling water pollution, protecting the water environment, and improving its quality. Research on water environment science is of great significance for developing the national economy and ensuring public health. 1 Overview 1) Alkalinity: The total amount of strong alkaline species in water that must be neutralized in order to lower the pH of treated natural water to the pH value corresponding to a pure CO2 aqueous solution. 2) Temporary hardness: Also known as carbonate hardness, it refers to the content of calcium and magnesium carbonates in water. Since the carbonate content in natural water is very low, carbonates are only present in alkaline water. Therefore, temporary hardness generally refers to the content of bicarbonates in water; when water is boiled, the bicarbonates therein decompose to form carbonate precipitates. 3) pH: A value that indicates the acidity or alkalinity of a solution; it represents the ratio of the total number of hydrogen ions to their total amount in the solution. 2 Detection Methods 2.1 Principle of the total alkalinity method: The water sample is titrated with a standard acid solution until it reaches a specified pH value; the endpoint is determined by the color change of the acid-base indicator when that pH value is reached. When titration is carried out until the phenolphthalein indicator changes from red to colorless, the pH of the solution is 8.3, indicating that the hydroxide ions in the solution have been neutralized and all carbonates have converted to bicarbonates ； When titration is carried out until the methyl orange indicator changes from light orange-yellow to orange-red, the pH value of the solution is 4.4–4.5, indicating that the bicarbonates in the water have been neutralized, including those present in the crude oil as well as those formed from carbonates. Based on the amount of standard hydrochloric acid titrant consumed upon reaching the above two endpoints, the carbonate and bicarbonate contents in water, as well as the total alkalinity, are calculated. When phenolphthalein is used as the indicator, the volume of standard hydrochloric acid solution consumed until a color change occurs is P (ml); when methyl orange is used as the indicator, the volume of standard hydrochloric acid solution consumed until a color change occurs is M (ml). 2.2 Principle of the temporary hardness method: The bicarbonates (and carbonates) present in water react as follows when methyl orange is used as an indicator: Ca(HCO3)2 + 2HCl → CaCl2 + 2CO2↑ + 2H2O. Carbonates also react in this process: CaCO3 + 2HCl → CaCl2 + CO2↑ + H2O. If organic acids, bicarbonates, or sodium carbonate (or potassium carbonate) are present in the water, they will consume hydrochloric acid. Therefore, before measuring carbonate hardness, it is necessary to determine the amounts of these substances and adjust the resulting values; otherwise, the calculated carbonate hardness will be higher than the actual value. 2.3 Principle of the pH value measurement method The pH value is determined by measuring the electromotive force of a cell. This battery typically consists of a saturated calomel electrode as the reference electrode and a glass electrode as the indicator electrode. At 25 °C, for every change of 1 pH unit in the solution, the potential difference changes by 59.16 mV; this value is then directly displayed on the instrument as a pH reading. Temperature differences are compensated for in the instrument. 3 Chemical reagents used in the work 3.1 Total alkalinity reagent: Phenolphthalein indicator: Weigh 0.5 g of phenolphthalein and dissolve it in 100 ml of 95% ethanol; then titrate with 0.1 mol/L sodium hydroxide solution until a pale red color appears. Methyl orange indicator: Weigh 0.1 g of methyl orange and dissolve it in 100 ml of distilled water. Standard hydrochloric acid titration solution: C(HCl) = 0.025 mol/L. 3.2 Carbonate hardness reagents: Hydrochloric acid: 0.1 mol/L. Sodium hydroxide: 0.1 mol/L. Methyl orange solution: 0.05%. 4 Testing in actual work 4.1 When pH < 4.4–4.5: Water samples taken on November 1, 2011, by the Benxi Water Environment Monitoring Sub-center for a large enterprise in Benxi City. This water sample was measured with a pH meter, and its pH value was 2.2. Total alkalinity determination: Take 100 ml of the water sample and place it in a 250 ml conical flask as the sample; add 4 drops of phenolphthalein and stir well. If the water sample is colorless ； Add another 3 drops of methyl orange and shake well; the water sample turns orange-red immediately. At this point, the amount of hydrochloric acid consumed is M+P=0, and the total alkalinity of the water sample is 0. Determination of carbonate hardness: Take 100 ml of water sample, place it in a 250 ml Erlenmeyer flask, and add 3 drops of methyl orange solution. The color of this water sample turned orange-red, and its carbonate hardness is 0. 4.2 When 4.4~4.5＜ pH＜8.3: 4.2.1 In cases where the water sample contains no bicarbonate (and potassium) or sodium carbonate (and potassium). These are water samples analyzed by the Benxi Water Environment Monitoring Sub-center on November 1, 2011, for a large enterprise in Benxi City. This water sample was measured using a pH meter; its pH value is 6.3. Total alkalinity determination: Follow the steps outlined in 4.1 for total alkalinity determination. When titration is carried out until the methyl orange indicator changes from light orange-yellow to orange-red, the amount of hydrochloric acid consumed is 4.2 ml. The total alkalinity of the water sample is calculated to be 52.6 mg/L. Determination of carbonate hardness: Follow the steps outlined in 4.1 Determination of carbonate hardness. The volume of hydrochloric acid consumed by this water sample was 1.00 ml. The carbonate hardness of this water sample is, and through calculation, the carbonate hardness of this water sample was found to be 50.0 mg/L. 4.2.2 Examples of water samples containing sodium bicarbonate (and potassium) or sodium carbonate (and potassium): water samples analyzed on November 11, 2011, by the Benxi Water Environment Monitoring Sub-center for a large enterprise in Benxi City. This water sample was measured with a pH meter, and its pH value was 6.2. Total alkalinity determination: Follow the steps outlined in 4.1 for total alkalinity determination. When titration is completed and the methyl orange indicator changes from light orange-yellow to orange-red, the amount of hydrochloric acid consumed is 3.70 ml. The total alkalinity of the water sample can be calculated, and it is found to be 46.3 mg/L. Determination of carbonate hardness: Take 100 ml of the water sample and place it in a 250 mL Erlenmeyer flask. Add 4 drops of phenolphthalein solution; if no red color appears, boil the mixture for 5–7 minutes. The appearance of a red color indicates the presence of bicarbonates, i.e., NaHCO3 or KHCO3. In this case, the determination should be carried out as follows: 1) Take 100 ml of the water sample, add 3 drops of methyl orange as an indicator, and titrate with 0.1 mol/L hydrochloric acid until a pale orange color is reached. Record the volume used (V1), which was 0.80 ml. 2) 100 ml of the water sample was taken, to which 0.80 mL of 0.1 mol/L sodium hydroxide solution (V1), an amount equal to that required for 0.1 mol/L hydrochloric acid, was added. The mixture was heated until it began to boil, then quickly cooled in a cold water bath. The precipitates such as CaCO3 formed were filtered out; the filter paper was rinsed with a small amount of distilled water. Afterwards, 3 drops of methyl orange were added to the filtrate, and it was titrated with 0.1 mol/L hydrochloric acid until a light orange color appeared. The volume of acid used (V2) was 1.10 ml. At this point, the carbonate content in the water sample was calculated, resulting in a carbonate hardness of 25.0 mg/L for this water sample. 4.3 In cases where pH > 8.3, these are water sample tests conducted on November 4, 2011, by the Benxi Water Environment Monitoring Sub-center for a large enterprise in Benxi City. This water sample was measured with a pH meter, and its pH value was 8.7. Total alkalinity determination: Take 100 ml of the water sample and place it in a 250 ml conical flask as the sample. Add 4 drops of phenolphthalein and stir well; then titrate with standard hydrochloric acid solution until the color just disappears. Record the volume of the standard hydrochloric acid solution used, which is 6.3 ml ； Add another 3 drops of methyl orange and shake well. When the titration reaches the point at which the methyl orange indicator changes from light orange-yellow to orange-red, the amount of hydrochloric acid consumed is M, which equals 3.8 ml. The total alkalinity of the water sample can be calculated, and it turns out to be 126 mg/L. Determination of carbonate hardness: Take 100 ml of water sample and place it in a 250 ml Erlenmeyer flask. Add 4 drops of phenolphthalein solution; if a red color appears, it indicates the presence of carbonates, namely Na2CO3 or K2CO3. Add hydrochloric acid for titration until the solution becomes colorless; record the volume used at this point as P, which is 0.9 ml. Then add 3 more drops of methyl orange solution and continue titrating until a light orange-red color is achieved; record the volume used at this point as M, which is 1.5 ml. At this point, the carbonate content in the water sample was calculated, resulting in a carbonate hardness of 30.0 mg/L for this water sample. 5 Conclusions 1) When pH carbonate hardness ； 3) Total hardness > Total alkalinity > Carbonate hardness ； When pH > 8.3, total alkalinity > carbonate hardness. 6 Conclusion At present, as we transition from engineering-oriented water management to resource-based water management, and from traditional to modern water management practices, we are faced with challenges such as water scarcity, severe water pollution, and deterioration of the aquatic ecosystem. The economic and social development in this new era imposes higher demands on water quality monitoring efforts. Understanding the relationship between total alkalinity, carbonate hardness, and pH allows one to determine the general trend and accuracy of the monitoring results. This enables accurate and timely provision of reliable monitoring data to relevant departments, offering a scientific basis for pollution prevention and control. The above content is from Rotor Douding Network: The relationship between total alkalinity, carbonate hardness, and pH (Authors: Sun Jianjun, Mao Yufeng). Online monitoring instruments for hardness, alkalinity, and pH: Hardness: PACON 5000; Alkalinity: PACON 5500; pH: innoCon 6800P