💎 Boron in Water
Determination of dissolved boron in water by spectrophotometric method with azomethine-H at 410 nm. Fast, sensitive and economical method for the range 0.01–1.0 mg B/L.
Overview
Boron (B) is an element whose presence in drinking and environmental water raises increasing toxicological interest. The permissible boron concentration in drinking water is 1.5 mg/L (according to EU Directive 2020/2184; previously 1.0 mg/L according to WHO). Excess boron shows toxic action on the reproductive and developmental system — it is teratogenic in animal studies.
Sources of boron in water are natural deposits of boron minerals (borax, kernite), geothermal and volcanic waters, detergents and cleaners (boron as a component of detergents), boron fertilizers in agriculture, industrial wastewater (glass, ceramics, insecticide production), and leachates from waste disposal sites. In Poland, elevated boron concentrations occur, among others, in geothermal water areas (Podhale) and in post-mining waters.
The azomethine-H method is a classical spectrophotometric technique recommended by ISO for routine boron determination in drinking and environmental waters. Azomethine-H (salicylic acid + H acid) reacts with boric acid in a buffered environment (pH 5.2–5.3) forming a yellow complex, whose absorbance is measured at 410 nm. The method is simple, cheap and does not require expensive equipment.
Alternative methods for boron determination are ICP-OES (λ = 249.77 nm) and ICP-MS, which offer higher sensitivity and wider range, but require much more expensive equipment. The azomethine-H method is preferred in laboratories not having ICP techniques.
Method principle
Boron dissolved in water (in the form of boric acid H₃BO₃ or borate ions) reacts with azomethine-H (derivative of salicylic acid and H-acid) in an environment buffered with ammonium acetate (pH 5.2–5.3) forming a yellow boron-azomethine complex. The absorbance of this complex is measured spectrophotometrically at a wavelength of 410 nm (or 415 nm). Color intensity is proportional to boron concentration according to the Beer-Lambert law in the range 0.1–3 mg B/L. Addition of EDTA eliminates interference from heavy metal ions (Fe, Cu, Al).
Applications
- Drinking water quality control (B ≤1.5 mg/L)
- Monitoring of geothermal and mineral waters
- Control of wastewater from detergent and glass production
- Testing of irrigation water in agriculture (boron toxicity to plants >1 mg/L)
- Monitoring of surface and groundwater
- Control of boron removal process from water (reverse osmosis, ion exchange)
- Testing of mine waters and landfill leachates
Key parameters
| Parameter | Value |
|---|---|
| Wavelength | 410 nm (or 415 nm) |
| Measurement range | 0.01–1.0 mg B/L (directly) |
| Detection limit (LOD) | ~0.01 mg B/L |
| Color development time | 2 hours (room temperature) |
| Reaction pH | 5.2–5.3 (acetate buffer) |
| Cuvette optical path | 10–50 mm |
Standard
- Standard number
- PN-EN ISO 9390:2001
- Title (PL)
- Jakość wody — Oznaczanie boranów — Metoda spektrometryczna z azometyną-H
- Title (EN)
- Water quality — Determination of borate — Spectrometric method using azomethine-H
Step-by-step procedure
1. Preparation of B standard solution
Dissolve 0.5716 g H₃BO₃ (dried at 40°C) in boron-free water and make up to 1 L. Concentration: 100 mg B/L. Store in PP vessel.
2. Preparation of calibration series
From 100 mg B/L solution prepare standards: 0, 0.1, 0.2, 0.5, 0.8, 1.0 mg B/L in 50 mL PP flasks.
3. Preparation of azomethine-H solution
Dissolve 3.8 g azomethine-H and 8 g ascorbic acid in 1 L deionized water. Store in refrigerator (4°C) max 7 days.
4. Preparation of buffer
Dissolve 500 g ammonium acetate in 1 L deionized water. Check pH = 5.2–5.3.
5. Color reaction
To 3.5 mL sample (or standard) add: 0.5 mL EDTA 0.025 mol/L, 2.5 mL azomethine-H solution, 1 mL buffer. Mix and leave for 2 h at room temperature.
6. Zeroing spectrophotometer
Fill cuvette with blank (water + reagents without boron). Zero instrument at λ = 410 nm.
7. Measurement of calibration curve
Measure absorbance of calibration standards at 410 nm. Plot relationship A vs B concentration. Linearity in range 0.1–3 mg B/L.
8. Sample measurement
Measure absorbance of samples (after 2 h incubation) at 410 nm. Read boron concentration from calibration curve.
9. Quality control
Blank, duplicate, control sample (standard of known concentration). Recovery: 90–110%. Duplicate RPD <10%.
10. Calculation and reporting
B concentration from calibration curve. For concentrations >1 mg B/L — dilute sample. Result in mg B/L with 2 decimal places.
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| UV-Vis Spectrophotometer | Hach DR6000, Shimadzu UV-1900i, Agilent Cary 60 | 15 000–60 000 PLN |
| Optical cuvettes | Hellma 100-QS quartz or glass 10 mm and 50 mm | 150–600 PLN/pc. |
| Automatic pipettes | Eppendorf Research Plus 0.5–5 mL and 100–1000 µL | 800–1 800 PLN |
| Volumetric flasks class A | DURAN / Brand 50, 100, 250 mL — polypropylene or PTFE (boron adsorbs on borosilicate glass!) | 40–120 PLN/pc. |
| Boron-free vessels (PP, PTFE) | All vessels must be free from boron — do not use borosilicate glass! | 10–50 PLN/pc. |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Azomethine-H | 5765-43-5 | Solution: 3.8 g azomethine-H + 8 g ascorbic acid in 1 L deionized water. Valid 1 week at 4°C. |
| Ammonium acetate (buffer) | 631-61-8 | 500 g CH₃COONH₄ in 1 L water — buffer pH 5.2–5.3. Ensures optimal reaction pH. |
| Ascorbic acid | 50-81-7 | Stabilizer in azomethine-H solution — prevents reagent oxidation |
| EDTA (disodium salt) | 6381-92-6 | 0.025 mol/L solution — eliminates Fe³⁺, Cu²⁺, Al³⁺ ion interference |
| Boric acid (standard) | 10043-35-3 | Pure H₃BO₃, dried at 40°C — for preparing 100 mg B/L standard solution |
| Deionized water free from boron | — | Prepared in boron-free vessels, checked for B content (<0.005 mg/L) |
| Hydrochloric acid 1 mol/L | 7647-01-0 | HCl — for acidifying samples and standards |
Health and safety (OHS)
- Boric acid — toxic to reproduction cat. 1B (H360FD) — use gloves
- Azomethine-H — irritating, avoid skin contact and dust inhalation
- EDTA — irritating, use gloves
- Hydrochloric acid — corrosive, work under fume hood
- Safety goggles and lab coat mandatory