☠️ Arsenic in water
Determination of arsenic in water by hydride generation technique (HG-AAS) — reduction of As to arsine (AsH₃) and atomic absorption measurement. Range 1–10 µg/L.
Overview
Arsenic is a highly toxic metalloid classified by IARC as Group 1 carcinogen. Long-term exposure even to low concentrations in drinking water causes arsenicosis — skin lesions, neuropathy, skin, lung and bladder cancers. The permissible arsenic concentration in drinking water is 10 µg/L (Directive 2020/2184/EU, Journal of Laws 2017, item 2294).
PN-EN ISO 11969 specifies the method for determining arsenic (including organically bound arsenic) in drinking, ground, and surface waters by hydride generation technique with atomic absorption detection (HG-AAS). The measurement range is 1–10 µg/L, and higher concentrations can be determined after dilution.
The HG-AAS (Hydride Generation AAS) technique is specific to elements forming volatile hydrides — arsenic, selenium, antimony, bismuth, tin, germanium, and tellurium. It provides better sensitivity than conventional FAAS by separating the analyte from the sample matrix.
Method principle
Arsenic in the sample is reduced to volatile arsine (arsenic hydride, AsH₃) by reaction with sodium borohydride (NaBH₄) in acidic environment (HCl). The reaction occurs in a hydride generator:
As³⁺ + NaBH₄ + HCl → AsH₃↑ + NaCl + H₂↑ + H₃BO₃
Gaseous AsH₃ is separated in a gas-liquid separator and transferred by carrier gas stream (argon) to a heated quartz absorption cell (T ~ 900°C), where it undergoes atomization:
AsH₃ → As⁰ + 3/2 H₂
Free arsenic atoms absorb radiation at wavelength 193.7 nm from HCL lamp. Absorbance is proportional to As concentration in the sample.
To ensure complete conversion of As(V) to As(III) — which reacts with NaBH₄ — the sample is pre-reduced with potassium iodide (KI) in acidic environment.
Applications
- Drinking water quality control (standard: ≤ 10 µg/L)
- Study of groundwater in areas with elevated arsenic background
- Monitoring of surface waters (priority substances)
- Analysis of industrial wastewater (metallurgy, ore processing)
- Hydrogeological studies — arsenic of natural origin
- Health risk assessment at water intakes
Key parameters
| Parameter | Value |
|---|---|
| Measurement range | 1–10 µg/L (without dilution) |
| Detection limit | 0.15–0.5 µg/L |
| Recovery | 96.3–99.8% |
| Wavelength | 193.7 nm (As resonance line) |
| Repeatability | RSD < 5% |
| Atomization temperature | ~900°C (quartz cell) |
Standard
- Standard number
- PN-EN ISO 11969:2002
- Title (PL)
- Jakość wody — Oznaczanie arsenu — Metoda atomowej spektrometrii absorpcyjnej (technika wodorkowa)
- Title (EN)
- Water quality — Determination of arsenic — Atomic absorption spectrometric method (hydride technique)
Step-by-step procedure
1. Sample preparation
Collect sample in PE container. Acidify with HCl or HNO₃ to pH < 2. Filtration 0.45 µm for dissolved forms. Store in dark, analysis within 7 days.
2. Pre-reduction As(V) to As(III)
To 50 mL sample add 5 mL KI/ascorbic acid solution. Wait 45 min at room temperature (or 15 min at 60°C). As(V) → As(III).
3. Solution preparation
Prepare fresh NaBH₄ solution (0.6% in 0.5% NaOH) and 5 mol/L HCl. Standard solutions: 0; 1; 2; 5; 10 µg/L As in HCl.
4. Instrument setup
Install As lamp (193.7 nm). Connect hydride generator. Heat quartz cell to ~900°C. Set argon flow 100–200 mL/min.
5. Calibration
Measure series of standards. Verify linearity (R² > 0.998). Check sensitivity — absorbance of 10 µg/L standard should give clear signal.
6. Sample measurement
Pump sample + HCl + NaBH₄ to generator. AsH₃ separated in separator, transferred to cell. Absorbance reading. Rinse between samples.
7. Quality control
CRM every 10 samples. Control and blank sample. Duplicate every 20 samples. Recovery 90–110%.
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| AAS spectrometer with hydride generator | Agilent 280FS + VGA 77, Shimadzu AA-7800 + HVG-1, Thermo iCE 3500 + VP100 | 120 000–300 000 PLN |
| Hydride generator (HG) | Agilent VGA 77, Shimadzu HVG-1 — continuous or injection (FIA) systems | 40 000–80 000 PLN |
| Heated quartz cell | Quartz absorption cell with electric heater to ~900°C | 5 000–15 000 PLN |
| HCL lamp for arsenic | Hollow cathode lamp As, Agilent/Heraeus/Photron | 800–1 500 PLN |
| Peristaltic pump | For feeding sample, HCl and NaBH₄ to chemifold | 5 000–12 000 PLN |
| Gas-liquid separator | Glass or PTFE, for separating AsH₃ from solution | 500–2 000 PLN |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Sodium borohydride (NaBH₄) | 16940-66-2 | 0.6% (w/v) in 0.5% NaOH. Reductant — generates AsH₃. Prepare freshly |
| Hydrochloric acid (HCl) | 7647-01-0 | Suprapur purity, 5–6 mol/L, acidic environment for hydride generation |
| Potassium iodide (KI) | 7681-11-0 | 5% (w/v) in 5% ascorbic acid. Pre-reduction As(V) → As(III) |
| Ascorbic acid | 50-81-7 | 5% (w/v), KI stabilizer, prevents oxidation of iodides |
| As standard solution (1000 mg/L) | 7440-38-2 | Certified CRM. Dilute in HCl, store in dark |
| Sodium hydroxide (NaOH) | 1310-73-2 | For stabilizing NaBH₄ solution (prevents decomposition) |
Health and safety (OHS)
- Arsine (AsH₃) — extremely toxic gas! Absolutely work under fume hood
- Arsenic compounds — carcinogenic (IARC Group 1). Nitrile gloves, avoid contact and inhalation
- NaBH₄ — reacts violently with acids (H₂ evolution). Add carefully, away from ignition sources
- Concentrated HCl — corrosive, suffocating. Work only in fume hood
- As-containing waste — hazardous waste, collect separately
- Complete personal protective equipment: lab coat, goggles, gloves, fume hood