How to detect the pH value in water quality testing?

- Aug 27, 2020-

There are colorimetric method and glass electrode method for measuring the PH value of water.


1. Colorimetry


The PH test paper method is a simple and rough measurement method. There are two commonly used PH test papers. One is a wide range of PH test papers, which can measure the pH range of 1-14; the other is a precision PH test paper, which can accurately measure the pH value of a certain range. Measurement steps:


(1) Take a test paper and cut it into 4-5 pieces and place them on a clean and dry glass plate.


(2) Use a clean glass rod to dip a little water sample to be tested on the PH test paper,

PH Meter 

(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.


1. Glass electrode method


1. Measuring principle


The glass electrode method is used to determine the pH value of a water sample using a saturated calomel electrode as a reference electrode, a glass electrode as an indicator electrode, and a working battery with the measured water sample, and then measuring the working electromotive force with a PH meter, which is directly read by the PH meter PH value.

PH Meter

The glass electrode method is accurate and fast in measuring PH, and is less interfered by factors such as water color, turbidity, colloidal substances, oxidants, reducing agents and salinity.


2. Instruments and reagents


(1) Instrument


a. Acidity meter or ion meter.


b. Glass electrode, saturated calomel electrode


(2) Reagent


a. Preparation of standard buffer solution


① PH standard buffer solution A (PH4.008, 25℃): Weigh 10.12g of potassium hydrogen phthalate (KHC8H4O4) dried at 110-130℃ for 2-3 hours, dissolve it in water and dilute it to a volumetric flask 1L.


② PH standard buffer solution B (PH6.865, 25℃): Weigh 3.388g of potassium dihydrogen phosphate (KH2PO4) and 3.533g of disodium hydrogen phosphate (Na2HPO4) that have been dried at 110-130℃ for 2-3 hours, respectively, and dissolve In water and dilute to 1L in a volumetric flask.

pH meter 

3. Operation steps


(1) Sampling According to sampling requirements, take a representative water sample.


(2) Instrument calibration The operating procedure is carried out according to the instrument manual.


(3) To determine the pH of the water sample, first rinse the electrode with distilled water, rinse with the water sample, and then immerse the electrode in the sample, carefully shake the test cup or stir to accelerate the electrode balance, stand still, and record the pH value when the reading is stable .


1. Matters needing attention


(1) The accuracy of the measurement result is first determined by the accuracy of the pH standard value of the standard buffer solution. Therefore, the buffer solution should be prepared and stored in accordance with GB11076-89 "Method for preparing buffer solution for pH measurement".


(2) The glass electrode and Gangong electrode should be selected, processed and installed according to the specifications.


(3) It is best to measure the pH value of the water sample on site, otherwise, the sample should be kept at 0-4℃ after sampling, and the measurement should be carried out within 6h after sampling.