Online alkalinity and acidity analyzerThe traditional understanding of acids and bases in chemistry is a process from appearance to essence. The acid-base proton theory proposed in 1923 holds that any substance that can release a proton (H+) is an acid; Any substance that can accept protons is a base. The relationship between acid and base can be expressed as follows:

Acid proton+base

The above relationship is also known as the acid-base half reaction equation, and the acid-base substances on both sides of the acid-base half reaction are called conjugated acid-base pairs. An acid releases a proton to form its conjugate base, or in other words, a base combines with a proton to form its conjugate acid. From this, it can be seen that acids and bases are interdependent and can also be converted into each other. If an acid has a stronger tendency to give protons, its conjugate base has a weaker tendency to accept protons; The stronger the tendency of a base to accept protons, the weaker the tendency of its conjugate acid to give protons.

（1） Acidity

Acidity refers to the total amount of substances in water that can neutralize strong bases. This type of substance includes inorganic acids, organic acids, strong acid weak base salts, etc. Natural water bodies have a certain degree of acidity due to the dissolution of carbon dioxide into the atmosphere, while wastewater discharged from industries such as machinery, mineral processing, electroplating, pesticides, printing and dyeing, and chemical engineering is acidic. The expansion of acid rain also increases the acidity of natural water bodies, disrupting the normal living and growth conditions of aquatic organisms and crops, causing fish death and crop damage. So, acidity is an important indicator for measuring water quality.

The methods for determining acidity include acid-base indicator titration, potentiometric titration, and acidimeter method.

Online alkalinity and acidity analyzer1. Acid base indicator titration method

Titrate the water sample with standard sodium hydroxide solution to a certain pH value, and calculate the acidity based on the amount consumed. Depending on the indicator used, there are usually two types of acidity: one is the total acidity (phenolphthalein acidity) measured using phenolphthalein as an indicator (with a color changing pH of 8.3), which includes strong and weak acids; The acidity measured using methyl orange as an indicator (with a color changing pH of about 3.7) is called strong acidity or methyl orange acidity. The unit is CaCO

₃Expressed in mg/L.

2. Potentiometric titration method

Using a pH glass electrode as the indicator electrode and a calomel electrode as the reference electrode, a primary cell was formed with the measured water sample and connected to a pH meter. The pH meter indicated 3.7 and 8.3, respectively, by titrating with a standard solution of sodium hydroxide. The two acidity levels were calculated based on the corresponding amount of sodium hydroxide solution consumed.

This method is applicable to the determination of acidity in various water bodies and is not limited by the color or turbidity of the water sample. When measuring, attention should be paid to the influence of factors such as temperature, stirring state, and response time. Take 50ml of water sample, which can measure 10-1000mg/L (as CaCO)

₃Calculate the acidity within the specified range.

（2） Alkalinity

The alkalinity of water refers to the total amount of substances in water that can neutralize strong acids, including strong bases, weak bases, strong bases and weak acid salts.

The alkalinity in natural water is mainly caused by bicarbonate, carbonate, and hydroxide, with bicarbonate being the main form of alkalinity in water. The main sources of pollution that cause alkalinity are wastewater discharged from industries such as papermaking, printing and dyeing, chemical engineering, and electroplating, as well as the loss of detergents, fertilizers, and pesticides during use.

The methods for determining alkalinity in water are the same as measuring acidity, including acid-base indicator titration and potentiometric titration. The former uses acid-base indicators to indicate the endpoint of titration, while the latter uses a pH meter to indicate the endpoint of titration.

The alkalinity measured by titrating the water sample with a standard acid solution until the phenolphthalein indicator changes from red to colorless (pH 8.3) is called phenolphthalein alkalinity. At this point, OH

^-Has been neutralized, CO₃^2-Neutralized to HCO₃^-When the titration continues until the methyl orange indicator changes from orange yellow to orange red (pH about 4.4), the measured alkalinity is called methyl orange alkalinity. At this time, HCO in the water₃^-It has also been neutralized, that is, all alkaline substances have been neutralized by strong acids, so it is also called total alkalinity.

Assuming that phenolphthalein is used as an indicator to titrate the water sample and the consumption of strong acid is P, and methyl orange is used as an indicator to titrate the water sample, the consumption of strong acid is M, and the sum of the two is T (see Figure 3-26), the five possible situations that may occur when measuring the total alkalinity of water are shown in Table 3-19.

Table 3-19 Relationship between the forms of alkalinity and titration results

Fig 3-Schematic diagram of alkalinity composition in water 26
According to the amount of acid consumed by titration using two indicators, various alkalinity and total alkalinity in the water can be calculated separately, with the unit commonly used being mg/L. It can also be used as CaCO₃Or expressed in mg/L as CaO.