pH value

The pH value is the negative logarithm of the hydrogen ion activity in a solution, that is:

When the concentration of H+in water is low, the pH value can be calculated using the concentration of H+instead of activity.

PH value is one of the most commonly used water quality indicators. The pH value of general drinking water is required to be between 6.5-8.5; The pH value of surface water is mostly in the range of 6.5-8.5; The pH value of certain industrial water must be maintained between 7.0-8.5 to ensure the normal operation of pipelines and equipment; In wastewater biochemical treatment, pH value is also one of the important indicators for evaluating the toxicity of toxic substances. In addition, pH value has a significant impact on the migration and transformation of pollutants in water bodies, and therefore needs to be controlled.

PH value is both related and different from acidity and alkalinity. The pH value indicates the strength of the acidity or alkalinity of water, while acidity or alkalinity refers to the content of acidic or alkaline substances in water. A solution with the same acidity, such as 0.1mol hydrochloric acid and 0.1mol acetic acid, both have an acidity of 100mmol/L, but their pH values are significantly different. Hydrochloric acid is a strong acid, almost 100% ionized in water, with a pH of 1; Acetic acid is a weak acid with an ionization degree of only 1.3% in water and a pH of 2.9.

The main methods for determining the pH value of water are glass electrode method (GB6920-86) and colorimetric method.

1. Colorimetric method

Different pH aqueous solutions display different colors in various acid-base indicators, and each indicator has a certain range of color change. Add appropriate indicators to a standard buffer solution with a known pH value to prepare a standard color column, and compare it with the test solution to determine the pH value of the water sample.

This method is not applicable to water samples that are colored, turbid, or contain high levels of free chlorine, oxidants, and reducing agents. If the pH value of the water sample is roughly measured, pH test strips can be used.

2. Glass electrode method

Using a pH glass electrode as the indicator electrode and a saturated calomel electrode as the reference electrode, and combining both with the measured solution to form a primary cell, the electromotive force is:

In the formula: calomel - the electrode potential of a saturated calomel electrode, which does not vary with the hydrogen ion activity (aH+) in the measured solution, can be regarded as a constant value;

Glass - The electrode potential of pH glass electrode varies with the activity of hydrogen ions in the measured solution. Glass can be expressed using the Nernst equation, so the above equation is expressed as (at 25 ℃):

E battery=calomel - (0+0.0591gaH+)=K+0.059pH

It can be seen that as long as the E battery is measured, the pH of the tested solution can be determined. In practical measurements, it is difficult to accurately obtain the K value, so calculation methods are not used. Instead, a solution with a known pH value is used as a standard for calibration, and the pH of the measured solution is directly measured using a pH meter.

The internal resistance of pH glass electrodes is generally as high as tens to hundreds of megaohms, so pH meters that match it are high impedance input transistor millivolt meters or electronic potential difference meters. To correct the influence of temperature on pH measurement, pH meters are equipped with temperature compensation devices. To simplify operation, facilitate use, and be suitable for on-site use, composite pH electrodes have been widely used to make various pocket and pen pH meters. At present, advanced pH meters at home and abroad can measure pH values ranging from -2.00 to+19.00, and some can even measure pH values with a range of -9.00 to+23.00, with a measurement accuracy of 0.01.

The glass electrode measurement method is accurate, fast, and minimally affected by factors such as water color, turbidity, colloidal substances, oxidants, reducing agents, and salinity.