GB/T 6920-1986Water quality--Determination of pH value--Glass electrode method (English PDF)

1 Scope of application

1.1 This method is applicable to the determination of the pH value of drinking water, surface water and industrial wastewater.

1.2 The color, turbidity, colloidal substance, oxidizing agent, reducing agent and high salinity of water do not interfere with the determination; however, in the strong acidic solution with pH < 1, there is so-called "acid error", which can be determined according to the acidity; in the alkaline solution with pH >10, due to the existence of a large amount of sodium ions, the error is generated; therefore, the reading is relatively low, which is commonly referred to as "sodium error". For the methods of eliminating the "sodium error", in addition to use the specially-made electrode of "low sodium error", it can also adopt the standard buffer solution which is similar to the pH value of the solution under test, in order to calibrate the instrument.

The temperature can affect the electric potential of the electrodes and the ionization balance of the water. It shall be noted that the compensation device of the adjusting instrument should be consistent with the temperature of the solution, and the temperature error of the standard buffer solution between the sample to be measured and the calibrating instrument should be within ± 1°C.

The pH is defined from the operation. For solution X, it should measure the galvanic cell's. reference electrode|KCl concentrated solution|| solution X |H2| Pt electromotive force EX. Replace the solution X, whose pH(X) is unknown, into the standard 1 This definition is cited from p.151 of GB 3100-3102-82 Quantities and units. pH solution S, and measure the battery's electromotive force ES, then, pH(X)=pH (S) + (Es - Ex) F / (RTln10)

Therefore, the defined pH is the dimensionless quantity. pH has no theoretical significance. the definition is the practical definition. However, within the limited range of the dilute aqueous solution whose amount-of-substance concentration is less than 0.1mol·dm3, when it is neither strong acidity nor the strong alkalinity (2 < pH < 12), then it has the followings according to the definition. pH=-log10[c(H+)y / (mol·dm3)] ± 0.02 In the formula, c(H+) represents the amount-of-substance concentration of the hydrogen ion (H+). y represents the activity coefficient of the typical 1-1-valence electrolyte in the solution.

2 Definitions 1

3 Principle

The pH value is obtained by measuring the electromotive force of the battery. This battery is usually composed of that the saturated calomel electrode is acted as the reference electrode AND the glass electrode is acted as the indicator electrode. At the temperature of 25°C, if the solution changes 1 pH unit, the change of the potential difference will be […]

4 Reagents

4.1 Preparation method of the standard buffer solution (abbreviated as standard solution)

4.1.2 When measuring the pH, according to the three possible conditions -- the water sample can present to acidity, neutrality and alkalinity -- the following three standard 2 The unit of electrical conductance is Siemens which is denoted by the symbol of "S". 1S = 1 ohm-1. solutions are often prepared.

4.4 The pH value of the standard solution can vary slightly with the changes of the temperature. The pH (S) values of certain common standard solutions are shown in Table 2.

5 Instruments

5.1 Acidity meter or ion concentration meter. The instrument for the routine test should accurate to 0.1pH unit. The pH range is from 0 to 14.If there is special requirement, the instrument with higher precision shall be used.

6 Sample preservation

It is best to conduct the site measurement. Otherwise, the samples should be kept at the 0~4°C after the sampling, and they should be measured within 6 hours after the sampling.

7 Steps

7.1 Instrument calibration. The operating procedure shall be carried out according to the operation instruction of the instrument. Firstly, adjust the water sample and the standard solution to the same temperature; record the measurement temperature; and adjust the temperature-compensation knob of the instrument to this temperature.

8 Degree of precision (see Table 3)

Repeatability 5 Reproducibility 6

9 Annotations

9.1 The glass electrode should be immersed in the distilled water for more than 24 hours before use.

9.2 When measuring the pH, the bulb of the glass electrode shall be completely immersed in the solution, and it should allow it to be slightly higher than the ceramic core end of the calomel electrode, in order not to bump the cup when stirring.

9.4 The liquid level of the saturated potassium chloride solution in the calomel electrode must be higher than the mercury body. It should have little potassium chloride crystal at the room temperature, in order to ensure the saturation of the potassium chloride solution, however, it is important to note that the potassium chloride crystal should not be too much, in order to prevent from blocking the access of the solution to be tested.

9.5 When measuring the pH, in order to reduce the dissolution or volatilization of the carbon dioxide from the air and the water sample, it should not open the bottle of water sample in advance before measuring the water sample.

10 Test Report

The test report shall include the followings. […]