🧪 pH of water and wastewater by the potentiometric method to PN-EN ISO 10523

Physical chemistry PN-EN ISO 10523

Overview

SCALE: THIS IS THE MOST FREQUENTLY CITED STANDARD IN THE SCOPE DATABASE. Measurement of 22 Sep 2026: 556 entries at 364 laboratories — that is, every fourth laboratory in the database (1,281) holds this standard in its accreditation scope. Matching by the BEGINNING of the recorded string gives 543 entries at 358 laboratories; the "Laboratories" tab shows the second figure.

STATUS (state of the PKN catalogue cards on 22 Sep 2026). PN-EN ISO 10523:2012 — published 28 Mar 2012, 16 pages, ICS 13.060.50, technical body KT 119 (Water Quality — Fundamental Problems), sector SZŚ. The "Implements" field: EN ISO 10523:2012 [IDT] and ISO 10523:2008 [IDT]. The card carries no withdrawal note and no "Replaced by" line — the standard is CURRENT.

THE STANDARD EXISTS IN ENGLISH AND GERMAN — THERE IS NO POLISH VERSION. The PKN catalogue holds two cards for this edition: "PN-EN ISO 10523:2012 - English version" and "PN-EN ISO 10523:2012 - German version". There is no card for a Polish version (the Polish-version address returns an error page, checked 22 Sep 2026). The most widely used test method in Polish accredited laboratories therefore has no Polish text of the standard — the Polish title is given by the catalogue card itself, but the content is in English or German.

SCOPE OF APPLICATION — FOUR LIMITS THAT MUST BE READ TOGETHER (from the text of ISO 10523:2008, clause 1). The method is given for rain water, drinking and mineral waters, bathing waters, surface and ground waters, municipal and industrial wastewater and LIQUID sewage sludge, subject to: • a pH range from 2 to 12, • a solvent ionic strength BELOW I = 0.3 mol/kg (conductivity at 25 °C below 2,000 mS/m), • a temperature from 0 °C to 50 °C. Outside those limits the standard does not apply — and the ionic-strength limit falls exactly where typical waters end and brines, saline industrial effluents and concentrated eluates begin.

WHAT THE STANDARD MEASURES, AND WHY TEMPERATURE IS PART OF THE RESULT. The determination consists in measuring the potential difference of an electrochemical cell with a pH meter. The standard states plainly: the pH of a sample DEPENDS ON TEMPERATURE through the dissociation equilibrium, and therefore the temperature of the sample is ALWAYS reported together with the pH result. This is not a housekeeping recommendation — the pH of the same water at 5 °C and at 25 °C are two different numbers, both correct. An important distinction from the note to clause 7: temperature compensation in pH meters is based on the Nernst equation and accounts for the dependence of the ELECTRODE SLOPE on temperature — but it does NOT compensate the dependence of the solution's own pH on temperature. An instrument with "automatic temperature compensation" therefore does not relieve anyone of reporting the measurement temperature.

GLASS ELECTRODE ONLY. In its introduction the standard states that, because of its practical importance, universality and accuracy, it describes EXCLUSIVELY measurement using a glass pH electrode. Indicator methods (strips, coloured solutions) are not methods according to this standard.

INTERFERENCES — THE LONGEST CLAUSE OF THIS STANDARD AND THE COMMONEST SOURCE OF BAD RESULTS (clause 5): • ageing of, and deposits on, the electrode membrane (calcium carbonate, metal hydroxides, oil, grease) cause an APPARENT reduction of electrode slope, a long response time and cross-sensitivity to ions; • deposits or precipitates on or within the diaphragm (silver chloride, silver sulfide, proteins) break the electrical contact with the measured solution; a faulty diaphragm can be detected by measuring the dilution effect of solutions; • in waters of low conductivity high diffusion potentials arise, and stirring and electrode memory effects (back-diffusion of solution into the reference electrode) distort the result — special electrodes are then used (e.g. with a ground-joint diaphragm or with an internal bridge free of AgCl); • in waters of low buffering capacity the pH changes very easily — through uptake or loss of carbon dioxide from the air, or through leaching of alkaline substances from the glass of the vessel; measurement in a closed flow-through system is then recommended; • in ground and mineral waters RICH IN CARBON DIOXIDE degassing during measurement may alter the original pH; the pH of anoxic water containing iron(II) or sulfides changes on contact with air; • in suspensions the sample should be allowed to settle in a completely filled, closed vessel, and the measurement made in the clear supernatant layer; • evolution of gas near the electrode disturbs the measurement.

SAMPLING AND STORAGE — THE STANDARD SAYS WHERE THE MAIN UNCERTAINTY LIES (clause 8). The pH may change rapidly through chemical, physical and biological processes, and therefore measurement IMMEDIATELY AT THE POINT OF SAMPLING is recommended. If that is not possible: the bottle is filled without gas exchange, COMPLETELY, and closed without a bubble; samples are transported and stored cool (2–8 °C) and in the dark (ISO 5667-3); in the laboratory the measurement is made as soon as possible and the effect of transport and storage on the result is checked. A sentence worth knowing before any dispute over a result: sampling and transport are usually the main contributors to the uncertainty of a laboratory pH measurement, which is why FIELD results often carry a SMALLER uncertainty than laboratory ones. That reverses the intuition by which a measurement "in the laboratory" is more accurate than one "in the field".

TWO-POINT CALIBRATION — A REQUIREMENT, NOT A CHOICE (9.2). The electrode is calibrated at TWO points, with buffers from the expected pH range of the sample; the buffers are chosen so that the expected sample result lies BETWEEN their values. The first buffer (usually pH 7) serves to set the zero point, the second the slope. After each measurement and between measurements the electrode and the temperature sensor are rinsed thoroughly with water. Requirements for the instrument and electrodes (clause 7): a pH meter with adjustable zero point, slope and temperature, an input resistance greater than 10^12 Ω and a readout resolution of 0.01 or better; a thermometer with an overall uncertainty not exceeding 0.5 °C (a Pt 100 / Pt 1000 sensor is recommended); the zero point of the glass electrode should not depart from its nominal value by more than ΔpH = 0.5, and the practical slope is to be at least 95 % of the theoretical slope.

WHAT THE ACCREDITATION SCOPES SHOW — AND WHERE THEY GO BEYOND THE STANDARD. The methodology is almost always simply "pH" (449 entries); some laboratories add the measuring range and the technique: "pH Range: 4.0 – 10.0 Potentiometric method" (28), "2.0 – 12.0" (14), "3.0 – 10.0" and "3.0 – 12.0" (4 each), "4.0 – 9.0" (3). The range declared by a laboratory may therefore be NARROWER than the range of the standard (2–12) — that is normal and reflects validation at that particular provider, but for an extremely acidic or alkaline sample one has to check whether it falls within the range stated in the accreditation scope, not merely within the range of the standard. Test objects: most often wastewater (72), water and wastewater (66), water intended for human consumption in various wordings (several dozen), swimming-pool water (30). Separately: 20 entries concern WASTE — there the standard usually stands together with the PN-EN 12457 series (leaching), so what is measured is the pH of the ELUATE, and an eluate is water; 8 entries concern sludge (the standard covers LIQUID sludge), and 6 entries concern SOIL, which the scope of this standard does NOT cover. Other documents serve for soil pH; those six entries are material to be checked in the specific scopes, not evidence of an error — we do not know their full context.

FLAWS OF RECORDING. The standard itself appears in the scopes in several forms differing in spacing and dash: "PN-EN ISO 10523:2012" (496 entries), "PN-EN ISO 10523" without the year (10), "PN-EN ISO 10523: 2012" with a space after the colon (3), "PN – EN ISO 10523:2012" and "PN–EN ISO 10523:2012" with a dash in place of the hyphen (2 each), "EN ISO 10523:2012" without the PN element (2). A search on an exact character string misses more than a dozen entries — which is why the figures in this article are counted with a pattern covering every form.

Method principle

The measurement consists in determining the potential difference of an electrochemical cell made up of a measuring (glass) electrode and a reference electrode; the potential of the measuring electrode is a function of the hydrogen-ion activity in the solution. pH is the negative decimal logarithm of hydrogen-ion activity. The instrument is calibrated at two points with buffers bracketing the expected result: the first buffer sets the zero point, the second the slope of the electrode characteristic. Because pH depends on temperature through the dissociation equilibrium, the temperature of the sample is always reported together with the result.

Applications

• Testing of water intended for human consumption and of surface, ground, mineral and rain waters
• Control of municipal and industrial wastewater — pH is a condition of acceptance of effluent into the sewer system
• Swimming-pool water — pH governs the effectiveness of chlorine disinfection
• Assessment of the corrosive properties of water and control of treatment processes (coagulation, disinfection)
• pH of the eluate from waste tested to the PN-EN 12457 series (leaching)
• Liquid sewage sludge — within the range covered by the standard

Key parameters

Standard

Standard number

PN-EN ISO 10523:2012 (wersja ANGIELSKA i NIEMIECKA; wprowadza EN ISO 10523:2012 [IDT] oraz ISO 10523:2008 [IDT])

Title (PL)

Jakość wody — Oznaczanie pH

Title (EN)

Water quality — Determination of pH

Step-by-step procedure

1. Deciding whether the sample falls within the scope of the standard

   Check three conditions at once: pH in the range 2–12, ionic strength below 0.3 mol/kg (conductivity at 25 °C below 2,000 mS/m) and temperature 0–50 °C. Brines, heavily saline effluents and concentrated eluates fall outside.

2. Measurement at the sampling point where possible

   A STEP FROM CLAUSE 8 OF THE STANDARD: pH changes rapidly through chemical, physical and biological processes, so measurement immediately on sampling is recommended. The standard states that sampling and transport are usually the main contributor to the uncertainty of a laboratory measurement.

3. Taking the sample without gas exchange

   Fill the bottle by overflow (preferably with a tube reaching the bottom), completely, and close it without an air bubble. Avoid release of carbon dioxide. Transport and storage: 2–8 °C, in the dark, to ISO 5667-3.

4. Preparing the instrument and the buffers

   Follow the electrode manufacturer's instructions. Prepare buffers bracketing the expected result. Measure the temperature of the buffers and of the sample — so far as possible they should be at the same temperature. With an electrode without a built-in sensor, immerse a separate temperature sensor.

5. Two-point calibration and setting

   Immerse the electrode in the first buffer (usually pH 7), stir, switch off the stirrer and set the zero point. Rinse the electrode with water. Immerse it in the second buffer, stir, switch off the stirrer and set the slope. Rinse the electrode before, between and after measurements.

6. Measuring the sample

   Measure under conditions close to those of calibration (temperature, stirring, ionic strength) — the standard points to this as the way to limit deviations. In suspensions, let the sample settle in a completely filled, closed vessel and measure in the clear supernatant layer.

7. Reporting the result TOGETHER WITH THE TEMPERATURE

   A pH result without the measurement temperature is incomplete: pH depends on temperature through the dissociation equilibrium, and automatic compensation in the pH meter does NOT remove that dependence — it corrects the electrode slope only.

8. Supervision of the electrode

   Check the electrode's fitness by periodic maintenance and testing. Symptoms of wear: an apparent fall in slope, a lengthened response time, cross-sensitivity to ions — usually caused by deposits on the membrane (calcium carbonate, hydroxides, oil, grease) or in the diaphragm (silver chloride and sulfide, proteins).

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• A pH result reported without the temperature is incomplete — automatic temperature compensation in the pH meter does not replace that information, because it corrects the electrode slope only
• Exceeding an ionic strength of 0.3 mol/kg (conductivity > 2,000 mS/m) takes the measurement outside the scope of the standard, although the instrument will still display a number
• In waters of low buffering capacity the pH changes through contact with air and through the glass of the vessel — measurement in an open vessel may give a result materially different from the original
• In ground and mineral waters rich in CO2, degassing during measurement alters the original pH; in anoxic waters containing iron(II) or sulfides the pH changes on contact with air
• Single-point calibration is not a method under this standard — two-point calibration with buffers bracketing the expected result is required
• The range declared in an accreditation scope may be narrower than the range of the standard (2–12) — for an extreme sample, check the provider's range, not merely the standard's
• The standard covers neither soil nor solid waste; the pH of an eluate from waste is a measurement in water and falls within the standard, the pH of soil does not

Training and video materials

• pH Measurement & Calibration — Mettler Toledo (Source: Mettler Toledo)
• Using a pH Meter — JoVE Science Education (Source: JoVE)

Frequently asked questions