Water Quality: pH Determination - Glass Electrode Method

Water Quality—Oetermination of pH Value—Glass Electrode Method

1. Scope of Application

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

1.2 The color, turbidity, colloidal substances, oxidizing agents, reducing agents, and high salt content of water do not interfere with the determination; however, in strongly acidic solutions with pH < 1, there will be a so-called "acid error," which can be determined according to acidity. In alkaline solutions with pH > 10, the presence of a large number of sodium ions will cause an error, resulting in a lower reading, commonly referred to as "sodium error." To eliminate "sodium error," in addition to using a specially designed "low sodium error" electrode, a standard buffer solution with a pH value similar to that of the solution being measured can be used to calibrate the instrument.

Temperature affects the electrode potential and the ionization equilibrium of water. Care must be taken to adjust the instrument's compensation device to be consistent with the solution temperature, and the temperature error between the sample being measured and the standard buffer solution used for instrument calibration should be within ±1℃.

2. Definition

pH is defined operationally. For solution x, measure the electromotive force EX of the 1KCl concentrated solution H solution × |H2|pt at the galvanic cell reference electrode. Replace solution X with a standard pH solution S, and similarly measure the cell's electromotive force ES. Then:

pH(X) = pH(S) + (Es - Ex)F/(RTIn10)

Therefore, the defined pH is a dimensionless quantity. pH has no theoretical meaning; its definition is a practical one. However, in a limited range of dilute aqueous solutions with a concentration less than 0.1 mol·dm⁻³, where the solution is neither strongly acidic nor strongly alkaline (Z < pH < 12), then by definition:

pH = -Log¹⁰［C（H⁺）y/（mol·dm³）］ ± 0.02

Where C（H⁺） represents the molar concentration of hydrogen ions (H⁺), and y represents the activity coefficient of a typical monovalent electrolyte in the solution.

3. Principle

The pH value is obtained by measuring the electromotive force of a battery. This battery typically consists of a saturated calomel electrode as the reference electrode and a glass electrode as the indicator electrode. At 25°C, for every 1 unit change in pH in the solution, the potential difference changes by 59.16 millivolts, which is directly expressed as a pH reading on the instrument. Temperature differences are compensated for by the instrument.

4. Reagents

4.1 Preparation Method of Standard Buffer Solutions (referred to as standard solutions)

4.1.1 Mass of Reagents and Distilled Water

4.1.1.1 In the analysis, unless otherwise specified, analytical grade or superior grade reagents are required. When purchasing packaged pH standard materials that have passed the verification of the National Institute of Metrology, China, the instructions for use can be followed.

4.1.1.2 The distilled water used to prepare the standard solution should meet the following requirements: boiled and cooled distilled water with a conductivity of less than 2 × 10⁻⁶ S/cm, and a pH between 6.7 and 7.3 is preferred.

4.1.2 When measuring pH, depending on whether the water sample is acidic, neutral, or alkaline, the following three standard solutions are commonly prepared:

4.1.2.1 pH standard solution (pH 4.008, 25℃)

Weigh 10.12 g of potassium phthalate (KHC₈H₄O₄) dried at 110–130℃ for 2–3 hours, dissolve it in water, and dilute to 1 liter in a volumetric flask.

4.1.2.2 pH standard concentration B (pH 6.865) 4.1.2.3 pH Standard Solution C (pH 9.180, 25℃)

Weigh 3.388 g of potassium dihydrogen phosphate (KH₂PO₄) and 3.533 g of disodium hydrogen phosphate (Na₂HPO₄), respectively, after drying at 110–130℃ for 2–3 hours. Dissolve them in water and dilute to 1 L in a volumetric flask.

4.1.2.3 pH Standard Solution C (pH 9.180, 25℃)

To ensure the crystals have a specific composition, weigh 3.80 g of borax (Na₂B₄O₇·10H₂O), which has been equilibrated for two days and nights together with saturated sodium bromide (or sodium chloride dissolved in sucrose (room temperature)) in a desiccator. Dissolve it in water and dilute to 1 L in a volumetric flask.

4.2 When the pH value of the sample being tested is too high or too low, a standard solution with a pH value approximately similar to the sample should be prepared according to Table 1 to calibrate the instrument.

4.3 Storage of Standard Solutions

4.3.1 The standard solution should be stored in a sealed polyethylene bottle.

4.3.2 Standard solutions at room temperature are generally best stored for 1-2 months. If turbidity, mold, or precipitation is observed, they should not be used.

4.3.3 Store at 4℃. Used standard solutions should not be poured back into the refrigerator; this will extend their shelf life.

4.4 The pH value of standard solutions varies slightly with temperature. The pH (S) values of some commonly used standard solutions are shown in Table 2.