⚗️ Sulfates in Water

Physicochemistry PN-ISO 9280

Overview

STATUS (as of 28.10.2026). PN-ISO 9280:2002 — Polish version, published 17.04.2002, implements ISO 9280:1990 [IDT]; it replaced PN-C-04566-09:1974. In the PKN catalogue the standard has no withdrawal note. At the international level, ISO 9280:1990 was withdrawn on 13.02.2001 (stage 95.99), and the catalogues give no successor; the Polish standard remains in the PN collection and is referred to in the waste water regulation (below).

SCOPE (clause 1). Gravimetric determination of sulfate in all types of water, including seawater and most industrial effluents. In the specified test portion 10–5000 mg/l of sulfate (as SO₄²⁻) is determined, higher concentrations after dilution of the laboratory sample. The lower limit of detection is 10 mg/l.

PRINCIPLE (clause 2). The sample is acidified with hydrochloric acid and boiled with barium chloride solution for at least 20 min to promote coagulation of the barium sulfate precipitate; the precipitate is filtered through a tared sintered-glass crucible, washed free from chloride, dried at 105 °C and weighed when cool. The increase in the mass of the crucible is the barium sulfate formed from the sulfate in the sample.

INTERFERENCES (clause 8). Sulfide and sulfite can raise the result if they are oxidized to sulfate when the sample is unduly exposed to air; those present at the beginning of the analysis are removed by boiling during sample pretreatment (as hydrogen sulfide and sulfur dioxide). Organic substances in substantial amounts (e.g. at a permanganate index above 30 mg/l O₂) can interfere by absorption or coprecipitation — they are removed according to 9.1 (evaporation and ashing at about 700 °C). Table 3 gives the maximum amounts of chromate, phosphate, nitrate, silica, calcium and iron(III) in the test portion below which interference is unlikely.

INSOLUBLE SULFATE (6.2.2, 9.2). If the material retained on the filter paper during pretreatment may contain insoluble sulfate and it is to be included, the filter paper is burnt in a covered platinum dish over a low bunsen flame or in a muffle furnace brought from room temperature up to 500 °C, the residue is fused with 4 g of anhydrous sodium carbonate (15 min), leached with water and the sulfate in the solution is determined; the result is added to the dissolved sulfate.

LEGAL REQUIREMENTS. Regulation of the Minister of Health of 22 May 2026 (Journal of Laws 2026, item 748): sulfate 250 mg/l; the water should not be corrosive (Annex 1 Part C Table 2 item 9); measurement uncertainty — 15 % of the parametric value (Annex 4 Part B). The Regulation of the Minister of Maritime Economy and Inland Navigation of 12 July 2019 (Journal of Laws 2019, item 1311) lists PN-ISO 9280 as the reference method (gravimetric method) for sulfate in waste water (Annex 12, item 52).

SULFATE IN WATER (textbook “Chemia wody”, Wrocław University of Science and Technology, 2024). Sulfate comes from sedimentary rocks, from industrial waste water and mine waters, and also from the oxidation of sulfides and sulfites. In large amounts it is aggressive towards the materials of the water supply network — it accelerates the corrosion of metals and the dissolution of cement linings — and drinking water with a high concentration may have an unpleasant taste and a laxative effect.

WHAT WE DO NOT GIVE HERE. We do not reproduce the figures of the precision table (Table 2: standard solutions and samples from interlaboratory tests in Germany, 50–5000 mg/l) or of Table 3.

Method principle

Sulfate in the sample acidified with hydrochloric acid reacts with barium ions to form sparingly soluble barium sulfate. Slow addition of hot barium chloride solution to the boiling sample reduces coprecipitation, and subsequent heating aids coagulation of the precipitate and renders it more crystalline (note 3 to 6.3).

The precipitate is filtered through a tared sintered-glass crucible, washed with cold water until free from chloride, dried at 105 ± 2 °C to constant mass and weighed. Sulfate concentration ρ(SO₄²⁻) = m · 1000 · 0.4116 / V [mg/l], where m is the mass of barium sulfate after subtracting the blank (g), V is the volume of the test portion (ml), 0.4116 is the gravimetric factor.

Applications

• Drinking water — sulfate 250 mg/l (Journal of Laws 2026, item 748, Annex 1 Part C)
• Waste water — PN-ISO 9280 is the reference method for sulfate (Journal of Laws 2019, item 1311, Annex 12)
• All types of water, including seawater and most industrial effluents (clause 1)
• Waters with a high sulfate content — up to 5000 mg/l, higher after dilution

Key parameters

Standard

Standard number

PN-ISO 9280:2002

Title (PL)

Jakość wody — Oznaczanie siarczanów(VI) — Metoda grawimetryczna z chlorkiem baru

Title (EN)

Water quality — Determination of sulfate — Gravimetric method using barium chloride

Step-by-step procedure

1. Sampling

   Glass or polyethylene bottles, filled completely to exclude air and the oxidation of sulfide and sulfite; analysis on the day of collection or storage at 2–5 °C for not more than one week (5.1).

2. Test portion

   For dissolved sulfate, suspended solids are allowed to settle or the sample is filtered through a fine ashless filter paper. Pipette 10–200 ml of sample containing not more than 50 mg of sulfate (5.2, 6.1).

3. Pretreatment

   In a 500 ml beaker add 2 drops of methyl orange, neutralize with hydrochloric acid or sodium hydroxide, add 2 ± 0.2 ml of hydrochloric acid, make up with water to 200 ± 20 ml and boil for at least 5 min; if insoluble material is present — filter the hot mixture through a fine ashless filter paper and wash the paper with a small amount of hot water (6.2.1).

4. Precipitation

   Slowly add by pipette 10 ± 1 ml of hot (about 80 °C) barium chloride solution to the boiling solution; heat for at least 1 h, cover, allow to cool and stand overnight at 50 ± 10 °C (6.3).

5. Filtration

   Dry the crucible for 1 h at 105 °C, cool in a desiccator and weigh to the nearest 0.0002 g; filter the precipitate with gentle suction, dislodge the remaining precipitate with a rubber-tipped glass rod while rinsing with cold water, and wash the precipitate in the crucible with chloride-free cold water (6.4.1).

6. Chloride test

   Collect about 5 ml of the filtrate in a beaker containing about 5 ml of silver nitrate solution — no turbidity means that the precipitate is free from chloride; otherwise continue washing. Also check the rim of the underside of the crucible (6.4.2).

7. Drying and weighing

   Dry for about 1 h at 105 ± 2 °C, cool in a desiccator and weigh; dry for a further 10 min and weigh again until two successive masses differ by not more than 0.0002 g. Washing the precipitate with three 5 ml portions of ethanol shortens drying (6.5).

8. Blank test

   Crucible check: 200 ± 20 ml of water is taken through 6.3–6.5; the difference in crucible mass is the blank mass (6.6).

9. Calculation and result

   m = m₂ − m₁ − m_b; ρ(SO₄²⁻) = m · 1000 · 0.4116 / V in mg/l. A result below 100 mg/l is rounded to 1 mg/l, above 100 mg/l to 10 mg/l, to not more than three significant figures (clauses 7, 11).

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• Barium chloride is poisonous and harmful if swallowed (warning to 3.2)
• Sodium hydroxide solution is hazardous on contact with skin and eyes (warning to 3.3)
• The 6 mol/l hydrochloric acid is prepared with care (3.1); handle hydrochloric acid and silver nitrate according to the safety data sheets

Frequently asked questions