⚗️ Sulfates in Water
In short
Gravimetric determination of sulfates (SO₄²⁻) in water by barium chloride precipitation method. The test is performed according to PN-ISO 9280:2002; Measurement range: ≥10 mg SO₄²⁻/L. The procedure comprises 10 steps (about 7 h 17 min–17 h 17 min in total); it is used for: Drinking water quality control (limit ≤250 mg SO₄²⁻/L), Groundwater testing (natural mineralization indicator), Mine and post-mining water monitoring.
At a glance
- Standard: PN-ISO 9280:2002
- Category: Physicochemistry
- Procedure steps: 10
- Total time of stages: 7 h 17 min–17 h 17 min
- Measurement range: ≥10 mg SO₄²⁻/L
- Accuracy: ±1–2% (at concentration >50 mg/L)
- Ignition temperature: 800 ± 25°C
Overview
Sulfates (SO₄²⁻) are one of the main anions occurring in natural waters. They originate from: mineral dissolution (gypsum CaSO₄, anhydrite), sulfide oxidation (pyrite FeS₂), atmospheric precipitation (acid rain — SO₂→H₂SO₄), industrial wastewater (pulp and paper, mining, galvanizing), and agriculture (sulfur fertilizers).
Drinking water standard: ≤250 mg SO₄²⁻/L (Polish Ministry of Health Regulation) — higher concentrations cause bitter taste and laxative effects. At high concentrations sulfates intensify concrete and metal installation corrosion (sulfate corrosion).
Gravimetric ISO 9280 method is reference (most accurate) method for sulfate determination. It involves BaSO₄ precipitation, filtration, ignition, and precipitate weighing. It is time-consuming but indispensable for arbitration and rapid method validation (turbidimetric).
Method principle
Sulfate ions in acidified sample (HCl) react with barium chloride (BaCl₂) forming white, crystalline, practically insoluble barium sulfate precipitate:
SO₄²⁻ + Ba²⁺ → BaSO₄↓ (white)
BaSO₄ precipitate is filtered through ashless filter paper or filter crucible, washed with hot water (removing Cl⁻ and Ba²⁺ ions), dried and ignited in muffle furnace at 800°C to constant mass.
SO₄²⁻ [mg/L] = (m_BaSO₄ × 0.4116 × 1000) / V_sample
where 0.4116 = conversion factor (M_SO₄/M_BaSO₄ = 96.06/233.39). Sample is acidified with HCl and boiled to ensure slow precipitation and obtain large, well-filtering crystals (minimizing coprecipitation).
Applications
- Drinking water quality control (limit ≤250 mg SO₄²⁻/L)
- Groundwater testing (natural mineralization indicator)
- Mine and post-mining water monitoring
- Water aggressiveness toward concrete assessment (sulfate corrosion)
- Industrial wastewater control
- Reference / arbitration method (rapid method validation)
Key parameters
| Parameter | Value |
|---|---|
| Measurement range | ≥10 mg SO₄²⁻/L |
| Accuracy | ±1–2% (at concentration >50 mg/L) |
| Ignition temperature | 800 ± 25°C |
| Weighing accuracy | 0.1 mg (analytical balance) |
| Analysis time | 8–12 hours (with ignition and cooling) |
| Method | Precipitation gravimetry (reference) |
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
-
Sample preparation
Measure 200–500 mL sample (select volume for expected concentration, so BaSO₄ mass is 50–300 mg). Filter through 0.45 μm filter if sample turbid.
-
Acidification
Add HCl (1+1) to pH ~1–2 (indicator paper or pH meter). Typically 5–10 mL HCl per 200 mL sample. Acidification prevents BaCO₃ coprecipitation.
-
Boiling and precipitation
Bring sample to boil on hot plate. To boiling sample add SLOWLY (dropwise) hot BaCl₂ solution (10%), mixing constantly. Add excess (~2 mL over calculated). Boil 15–30 minutes.
-
Precipitate aging
Leave solution with precipitate on hot plate (80–90°C) for 2–4 hours or in warm place overnight. Aging improves precipitate crystallinity (Ostwald ripening).
-
Filtration
Filter through ashless filter paper (Whatman 42) or filter crucible G4. Do not transfer entire precipitate — leave it in beaker as long as possible.
-
Precipitate washing
Wash precipitate with hot deionized water (min. 6 portions of 10 mL each). After each wash check filtrate for Cl⁻ presence (1 drop AgNO₃ — no turbidity = Cl⁻ washed out).
-
Drying and ignition
Ashless filter: place in porcelain crucible, dry at 105°C, ash carefully on burner, ignite in muffle furnace 800°C/1h. Filter crucible: dry 105°C, ignite 800°C/1h.
-
Cooling and weighing
Cool crucible in desiccator to room temperature (min. 30 min). Weigh on analytical balance (0.1 mg). Repeat ignition and weighing to constant mass (difference <0.5 mg).
-
Calculations
SO₄²⁻ [mg/L] = (m_BaSO₄ × 0.4116 × 1000) / V_sample [mL]. Account for blank (titration of water alone with reagents).
-
Quality control
Blank: precipitate mass <0.5 mg. Duplicate: RPD <3%. Na₂SO₄ control standard — deviation max ±2%. Check precipitation completeness (add drop of BaCl₂ to filtrate — no turbidity).
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| Analytical balance (0.1 mg) | Mettler Toledo ME204, Sartorius Entris II, Radwag AS 220.R2 | 5 000–15 000 PLN |
| Muffle furnace (800°C) | Nabertherm L 3/11, Snol 8.2/1100, POL-EKO SLF | 8 000–25 000 PLN |
| Laboratory dryer (105°C) | Binder FD 115, Memmert UF110 | 5 000–15 000 PLN |
| Desiccator | Glass with silica gel, ø 200–250 mm | 300–800 PLN |
| Filtration set | Porous filter crucible (G4) or Whatman 42 ashless filters | 200–500 PLN |
| Hot plate | For boiling sample with BaCl₂, temperature control | 1 000–3 000 PLN |
| Glass beakers 400–600 mL | DURAN borosilicate, wide, for precipitation | 25–60 PLN/pc |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Barium chloride BaCl₂·2H₂O | 10326-27-9 | Precipitating solution: 100 g/L (10%). Add hot to hot sample. WARNING: barium salts are toxic! |
| Hydrochloric acid HCl | 7647-01-0 | Diluted HCl (1+1) for sample acidification (pH ~1–2). Prevents carbonate coprecipitation. |
| Silver nitrate AgNO₃ (for test) | 7761-88-8 | Solution 0.1 mol/L — for checking wash completeness (absence of Cl⁻ in filtrate). |
| Ashless filter papers | — | Whatman 42 or equivalent — ash <0.01%. For BaSO₄ precipitate filtration. |
| Deionized water | — | Hot — for washing precipitate on filter (min. 6× 10 mL each). |
Health and safety (OHS)
- Barium chloride BaCl₂ — TOXIC! Lethal oral dose ~1 g. Do not eat or drink in laboratory. Gloves mandatory.
- Hydrochloric acid HCl — corrosive, irritating fumes. Work under fume hood.
- Muffle furnace 800°C — risk of severe burns. Use tongs and thermal gloves.
- Silver nitrate — corrosive, causes permanent stains. Nitrile gloves.
- Safety goggles, nitrile gloves, and coat mandatory.
- Barium-containing waste — collect separately as toxic waste.
Frequently asked questions
Which standard describes this test?
The test is performed according to PN-ISO 9280:2002 — “Water quality — Determination of sulfate — Gravimetric method using barium chloride”.
How does this method work?
Sulfate ions in acidified sample (HCl) react with barium chloride (BaCl₂) forming white, crystalline, practically insoluble barium sulfate precipitate: SO₄²⁻ + Ba²⁺ → BaSO₄↓ (white) BaSO₄ precipitate is filtered through ashless filter paper or filter crucible, washed with hot water (removing Cl⁻ and Ba²⁺ ions), dried and ignited in muffle furnace at 800°C to constant mass. SO₄²⁻ [mg/L] = (m_BaSO₄ × 0.
What is the measuring range and accuracy?
Measurement range: ≥10 mg SO₄²⁻/L; Accuracy: ±1–2% (at concentration >50 mg/L); Ignition temperature: 800 ± 25°C; Weighing accuracy: 0.1 mg (analytical balance).
How long does the test take?
The times given for the individual stages add up to approximately 7 h 17 min–17 h 17 min. The procedure comprises 10 steps.
What equipment is required?
Analytical balance (0.1 mg), Muffle furnace (800°C), Laboratory dryer (105°C), Desiccator, Filtration set, Hot plate, Glass beakers 400–600 mL. Indicative cost of the core instrument (Analytical balance (0.1 mg)): 5 000–15 000 PLN.
Where is this test used?
Drinking water quality control (limit ≤250 mg SO₄²⁻/L); Groundwater testing (natural mineralization indicator); Mine and post-mining water monitoring; Water aggressiveness toward concrete assessment (sulfate corrosion); Industrial wastewater control; Reference / arbitration method (rapid method validation).
What safety precautions apply?
Barium chloride BaCl₂ — TOXIC! Lethal oral dose ~1 g. Do not eat or drink in laboratory. Gloves mandatory.; Hydrochloric acid HCl — corrosive, irritating fumes. Work under fume hood.; Muffle furnace 800°C — risk of severe burns. Use tongs and thermal gloves.; Silver nitrate — corrosive, causes permanent stains. Nitrile gloves.; Safety goggles, nitrile gloves, and coat mandatory.; Barium-containing waste — collect separately as toxic waste.
Which laboratory can perform this test?
The test is performed by laboratories accredited to ISO/IEC 17025. On LabCoda you can find them by the standard number PN-ISO 9280.