ISO 19396-1:2017 塗料とワニス— pH値の決定—パート1：ガラス膜を備えたpH電極 | ページ 2

3.1

pH

measure for the acidic or basic reaction of an aqueous solution or polymer dispersion

Note 1 to entry: Notation of pH: the p and the H are vertically on one line. The same is valid for pOH.

Note 2 to entry: The acidic reaction is determined by the activity of the existing “hydrogen ions”. The basic reaction is determined by the activity of the existing hydroxide ions. The direct relationship between the activities of the “hydrogen ions” and the hydroxide ions is described by the ionic product of the water.

3.2

pH value

decadal logarithm of the hydrogen ion activity multiplied with (−1)

with

where

		is the activity of the hydrogen ion, in mol/kg;
		is the standard molality (1 mol/kg);
		is the activity coefficient of the hydrogen ion;
		is the molality of the hydrogen ion, in mol/kg.

Note 1 to entry: The pH value is not measurable as a measure of a single ion activity. Therefore, pH (PS) values of solutions of primary reference material (PS, en: Primary Standard) are determined, which are approximate to it and can be attributed to it. This is based on a worldwide agreement; see ISO 80000-9:2009, Annex C.

3.3

potentiometric measuring chain

combination of electrochemical half cells

3.4

pH (combination) electrode

pH (single-rod) measuring chain

potentiometric measuring chain (3.3) providing a voltage which depends on the pH value (3.2) of the measuring solution

Note 1 to entry: One of the two electrochemical half cells is the pH measuring electrode, the second is a reference electrode (3.5) (see Figure 1). Both electrodes can be combined as a single-rod measuring chain in one unit.

Note 3 to entry: This document refers to pH electrodes with glass membrane. The electrode shaft should be made of material resistant to chemicals and solvents.

3.5

reference electrode

electrode providing a constant potential which is independent from the pH value (3.2) of the measuring medium

Note 1 to entry: At present, the most commonly used type is the silver/silver chloride reference electrode, whose potential is stabilized by a constant concentration of potassium chloride (KCl) in the reference electrolyte (3.7) .

3.6

reference element

galvanic cell which dips into the reference electrolyte (3.7) and transmits the reference potential to the pH meter

Note 1 to entry: The reference elements of the pH measuring electrode and of the reference electrode should be aligned so that identical temperature characteristics are given.

3.7

reference electrolyte

aqueous salt solution (generally potassium chloride solution), whose chloride ion activity determines the potential of the reference electrode (3.5)

Note 1 to entry: At the diaphragm (3.8) , the reference electrolyte has contact with the measuring solution. Potassium chloride solution is used as reference electrolyte, because K⁺ ions and Cl⁻ ions have almost the same ion mobility and, therefore, only slight diffusion potentials result.

Note 2 to entry: The reference electrolyte should flow out of the diaphragm in order to ensure a constant reference potential. Therefore, it shall be refilled occasionally. For reference electrodes (3.5) or pH electrodes (3.4) with thickened/gel or solidified electrolyte, refilling of the electrolyte can be omitted. Such reference electrodes or pH electrodes are called low-maintenance.

3.8

diaphragm

permeable material in the sides of the casing of reference electrodes (3.5) , which enables the electrolytic contact between reference electrolytes (3.7) and measuring solution and simultaneously impedes the exchange of electrolyte

Note 1 to entry: For types of diaphragms, see 5.3.

3.9

measuring electrode with glass membrane

electrode providing a potential which is a function of the pH value (3.2)

3.10

pH glass membrane

membrane made of special glass, on whose interface to the solution an electrical potential (electrode function) results, which is proportional to the pH (3.1) of the solution

3.11

temperature compensation

compensation of the temperature-dependent measuring signal only of the buffer solutions (3.15) with known temperature dependency

Note 1 to entry: By this, the temperature dependency of the pH value (3.2) of the measuring medium cannot be compensated. Therefore, the temperature is always recorded together with the pH value.

3.12

theoretical slope

k

Note 1 to entry: At 23 °C, k = −58,57 mV.

3.13

practical slope

k′

Note 1 to entry: The slope obtained during calibration is a characteristic for the quality of the pH electrode.

3.14

zero point

pH value (3.2) , pH₀, of a pH electrode (3.4) , for which the pH proportional voltage of the pH electrode is U = 0 mV at a given temperature

Note 1 to entry: The zero point can also be indicated in terms of a voltage (offset voltage).

Note 2 to entry: The zero point obtained during calibration is a characteristic for the quality of the pH electrode.

3.15

buffer solution

solution with a pH value (3.2) of known measurement uncertainty

Note 1 to entry: The buffer solution is used for calibration and adjustment of pH meters. Buffer solutions have a pH value that is largely non-sensitive to dilution and acid or alkali addition.

3.16

stability of measured value

change of the measurement signal over time, dU/dt, under unchanged measurement conditions

Note 1 to entry: The stability of measured value is specified in accordance with the reproducibility requirement of the measurement.