PH value is one of the most commonly used water quality testing indicators. The pH value of natural water is mostly in the range of 6-9; the pH value of drinking water is required to be between 6.5 and 8.5; the pH value of some industrial water should be guaranteed at 7.0-8.5. Between, otherwise it will have corrosive effects on metal equipment and pipelines. PH value and acidity and alkalinity are both different and related. The PH value indicates the acidity and alkalinity of the water, and the acidity or alkalinity is the content of the acid or alkali substance contained in the water. Changes in the pH of the water indicate the extent of water pollution.
The pH of water is determined by a colorimetric method and a glass electrode method.
The PH test method is a simple and rough measurement method. There are two kinds of commonly used PH test papers, one is a wide range of PH test papers, the range of PH can be measured is 1-14; the other is precision PH test paper, which can accurately determine a range of PH values. Determination steps:
(1) Take a test strip and cut it into 4-5 pieces and put it on a clean and dry glass plate.
(2) Dip a small amount of water sample to be tested on the pH test paper with a clean glass rod.
(3) After a while, observe the color of the test paper and compare it with the standard color card to determine the pH value of the water sample.
Second, the glass electrode method
1, the principle of measurement
The pH value of the water sample was determined by the glass electrode method. The saturated calomel electrode was used as the reference electrode, the glass electrode was used as the indicator electrode, and the working water sample was used to form the working battery. Then the working electromotive force was measured by the PH meter and directly read by the pH meter. PH value.
The glass electrode method is accurate and rapid in measuring PH, and is less interfered by factors such as water color, turbidity, colloidal substance, oxidant, reducing agent and salinity.
2, instruments, reagents
a, acidity meter or ion meter.
b, glass electrode, saturated calomel electrode
a, the preparation of standard buffer solution
1 PH standard buffer solution A (pH4.008, 25 ° C): Weighed 10.12 g of potassium hydrogen phthalate (KHC8H4O4) which was dried at 110-130 ° C for 2-3 h, dissolved in water and diluted in a volumetric flask. 1L.
2 PH standard buffer solution B (pH6.865, 25 ° C): Weighed 3.388 g of potassium dihydrogen phosphate (KH2PO4) and 3.533 g of disodium hydrogen phosphate (Na2HPO4), which were dried at 110-130 ° C for 2-3 h, respectively. Dilute to 1 L in water and in a volumetric flask.
3, the operation steps
(1) Sampling According to the sampling requirements, take a representative water sample.
(2) Instrument calibration The operation procedure is carried out according to the instrument's instruction manual.
(3) Determination of water sample PH first rinse the electrode with distilled water, rinse with water sample, then immerse the electrode in the sample, carefully shake the test cup or stir to accelerate the electrode balance, stand still, record the PH value when the reading is stable .
Third, matters needing attention
(1) The accuracy of the measurement results is first determined by the accuracy of the standard pH value of the standard buffer solution. Therefore, the buffer solution should be prepared and stored in accordance with GB11076-89 "Preparation Method for pH Measurement Buffer Solution".
(2) Glass electrodes and Gangong electrodes should be selected, handled and installed according to the specifications.
(3) It is best to measure the pH of the water sample on site. Otherwise, the sample should be kept at 0-4 °C after sampling and measured within 6 hours after sampling.