PH measurement method of water quality PH meter
1 Scope of application
1.1 This method is applicable to the determination of pH value of drinking water, surface water and industrial wastewater.
1.2 The color, turbidity, colloidal substance, oxidant, reducing agent and high salt content of water do not interfere with the determination; however, in a strongly acidic solution with pH<1, there will be a so-called "acid error", which can be determined according to acidity; In the alkaline solution with pH>10, due to the presence of a large amount of sodium ions, an error occurs and the reading is low, which is usually called "sodium difference". To eliminate the "sodium difference", in addition to using a special "low sodium difference" electrode, it is also possible to use a standard buffer solution that is similar to the pH value of the measured solution to calibrate the instrument.
The temperature affects the potential of the electrode and the ionization balance of the water. Attention must be paid to adjusting the compensation device of the instrument to be consistent with the temperature of the solution, and to make the temperature error between the measured sample and the standard buffer solution used for calibrating the instrument within ±1°C.
2 definitions
pH is operationally defined. For the solution x, measure the electromotive force EX of the concentrated solution H solution × |H2|pt of the galvanic cell reference electrode 1KC1. Replace the solution X of Zhuzhi pH(X) with the standard pH solution S, and measure the electromotive force ES of the battery, then
pH(X)=pH(S)+(Es—Ex)F/(RTIn10)
Thus, pH as defined is a dimensionless quantity. pH has no theoretical meaning, its definition is a practical one. However, in the limited range of dilute aqueous solutions where the concentration of the substance is less than 0.1mo1.dm-3, it is neither strongly acidic nor strongly alkaline
pH=-Log10[C(H+)y/(mo1.dm3)]±0.02
In the formula, C((H+) represents the concentration of hydrogen ion H+, and y represents the activity coefficient of a typical 1,-1 valent electrolyte in solution.
3 principles
The pH value is obtained by measuring the electromotive force of the battery. The battery usually consists of a saturated calomel electrode as the reference electrode and a glass electrode as the indicator electrode. At 25°C, for every pH unit change in the solution, the potential difference changes to 59.16 millivolts, which is directly expressed as a pH reading on the instrument. There is a compensation device on the instrument for temperature difference.
4 reagents
4.1 Preparation method of standard buffer solution (referred to as standard solution)
4.1.1 Quality of reagents and distilled water
4.1.1.1 In the analysis, unless otherwise specified, it is required to use analytically pure or superior pure reagents. When purchasing pH standard substances in bags certified by the China Institute of Metrology, you can refer to the instructions for use.
4.1.1.2 The distilled water used to prepare the standard solution should meet the following requirements: boil and cool the distilled water with conductivity less than 2×10-6S/cm, and the pH should be between 6.7 and 7.3.
4.1.2 When measuring pH, according to the three possibilities of water sample being acidic, neutral and alkaline, the following three standard solutions are often prepared:
4.1.2.1 For pH standard solution (pH4.008 25°C)
Weigh 10.12 grams of potassium phthalate hydrogen (KHC8H4O4) which was first dried at 110-130°C for 2-3 hours, dissolve in water and dilute to 1 liter in a volumetric flask.
4.1.2.2 pH standard solution B (pH6.865 25°C)
Weigh 3.388 grams of potassium dihydrogen phosphate (KH2PO4) and 3.533 grams of disodium hydrogen phosphate (Na2HPO4) which were first dried at 110-130°C for 2-3 hours, dissolve them in water and dilute to 1 liter in a volumetric flask.
4.1.2.3 pH standard solution C (pH9.180 25°C)
In order to make the crystal have a certain composition, 3.80g of borax (Na2B4O7.10H2O) should be weighed and placed in a desiccator to balance for two days and nights together with saturated sodium bromide (or sodium chloride plus sucrose solution (room temperature), dissolved in water And dilute 1L in a volumetric flask.
4.2 When the pH value of the sample to be tested is too high or too low, refer to Table 1 to prepare a standard solution that is similar to the pH value to calibrate the instrument.
4.3 Storage of standard solution concentration
4.3.1 The standard solution should be kept airtight in polyethylene bottles.
4.3.2 It is generally advisable to store the standard solution concentration for 1 to 2 months at room temperature. When turbidity, mold or precipitation are found, it cannot be used any longer.
4.3.3 Store in a refrigerator at 4°C, and the used standard solution is not allowed to be poured back, which can prolong the service life.
4.4 The pH value of the standard solution varies slightly with temperature. The pH (S) value of some commonly used standard solutions.
5 instruments
5.1 pH meter or ion concentration meter. Instruments used for routine testing should be accurate to at least 0.1 pH units, with a pH range from 0 to 14. If there are special needs, an instrument with higher precision should be used.
5.2 Glass electrode and twenty-mercury electrode.
6 Sample storage
Best measured on site. Otherwise, the sample should be kept at 0 ~ 4 ℃ after sampling, and the measurement should be carried out within 6 hours after sampling.
7 steps
7.1 Calibration of the instrument: the operation procedure shall be carried out according to the instruction manual of the instrument. First 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.
Calibrate the instrument with a standard solution that does not differ by more than 2 pH units from the pH of the water sample. Remove the electrode from the standard solution, rinse thoroughly and blot dry with filter paper. Then immerse the electrode in the first standard solution, the pH of which is about 3 pH units different from the second standard solution, if the difference between the indicated value of the instrument response and the pH (S) value of the first standard solution is greater than 0.1pH Unit, it is necessary to check whether there is a problem with the instrument, electrode or standard solution. When all three are normal, the sample can be used for determination.
7.2 Sample determination
When measuring the sample, first rinse the electrode carefully with distilled water, then rinse with water sample, then immerse the electrode in the sample, shake or stir carefully to make it even, let it stand, and record the pH value when the reading is stable.
