💎 Boron in Water
In short
Spectrometric determination of borate in drinking water and in not heavily polluted ground, surface and saline waters by the azomethine-H method, which at a pH of about 6 forms a yellow complex with borate, measured at the absorption maximum of 410–420 nm. The test is performed according to ISO 9390:1990; Range: 0.01–1 mg B/l; higher concentrations after dilution (1.1). The procedure comprises 7 steps; it is used for: Drinking water — boron 1.5 mg/l (2.4 mg/l where desalinated water predominates or where groundwater has a geologically high boron content), Journal of Laws 2026, item 748, Annex 1 Part B item 10, Ground, surface and saline waters that are not heavily polluted (1.1), Surface waters affected by waste water discharges — borate from washing agents (introduction).
At a glance
- Standard: ISO 9390:1990
- Category: Physicochemistry
- Procedure steps: 7
- Range: 0.01–1 mg B/l; higher concentrations after dilution (1.1)
- Wavelength: absorption maximum in the range 410–420 nm, checked for each new batch of reagent (clause 3, 7.1)
- Cells: 10 mm; 50 mm for concentrations up to about 0.2 mg B/l (7.1)
Overview
STATUS (as of 29.10.2026). ISO 9390:1990 — first edition (13.09.1990), no newer edition. The standard has NOT been adopted into the collection of Polish Standards: the PKN search for “ISO 9390” returns no item (a check with the same query for “ISO 9297” returns PN-ISO 9297:1994; read on 29.10.2026). The Polish title of the standard is a working translation.
INTRODUCTION AND SCOPE (introduction, clause 1). The natural borate content of groundwater and surface water is small; in surface water it can increase significantly due to waste water discharges, because boron compounds are ingredients of domestic washing agents. The method covers concentrations from 0.01 mg to 1 mg of boron per litre — the range can be extended by dilution — and applies to drinking water and to ground, surface and saline waters that are not heavily polluted.
INTERFERENCES (1.2). Interferences are unlikely when analysing drinking water. Mg, Zn, Ca, Na, K, phosphate, sulfate and nitrate do not interfere; Mn, Zr, Cr, Ti, Cu, V, Al, Be and Fe may cause high results. Interferences caused by colour, humic acids and undissolved substances are removed by a suitable procedure (e.g. destruction of the colour or filtration through a column filled with activated carbon).
CONTAMINATION WITH BORON (clause 6, 7.3). Samples are not taken in borosilicate glass containers. Borate is widespread in the environment: polyethylene flasks and plastics pipettes are preferable; old borosilicate glass well rinsed with hydrochloric acid is suitable only for acidic solutions — never for neutral or alkaline solutions or for prolonged storage. Detergents and soaps for glassware and lab coats should be borate free, the use of towels and tissues for drying is avoided, and toiletries and talcum powder used by technicians often contain borate. Blanks are carried out at least in duplicate and should agree.
LEGAL REQUIREMENTS. Drinking water: boron 1.5 mg/l, and 2.4 mg/l when desalinated water is the predominant source of water in the supply system or in regions where geological conditions may lead to a high boron content in groundwater (Journal of Laws 2026, item 748, Annex 1 Part B item 10); measurement uncertainty 25 % (Annex 4 Part B item 11). Waste water: the reference methodologies for boron are PN-EN ISO 11885 (ICP-OES) and PN-EN ISO 17294-2 (ICP-MS) — ISO 9390 is not listed there (Journal of Laws 2019, item 1311, Annex 12 item 13).
REFERENCE DOCUMENT. The full text of the standard is available in the publisher’s preview (4 pages): scope, principle, reagents, apparatus, sampling, procedure with calibration, calculation and precision data from an interlaboratory trial carried out in Germany (Table 1).
Method principle
Azomethine-H — the condensation product of H-acid (8-amino-1-naphthol-3,6-disulfonic acid) and salicylaldehyde — reacts with dissolved forms of borate at a pH of about 6 to form a yellow complex, whose absorbance is measured spectrometrically at the absorption maximum in the range 410–420 nm (clause 3).
The concentration is determined from a linear calibration graph by the factor f (reciprocal of the slope) — separately for the range 0–0.20 mg/l (50 mm cell) and 0–1.00 mg/l (10 mm cell) (7.4).
Applications
- Drinking water — boron 1.5 mg/l (2.4 mg/l where desalinated water predominates or where groundwater has a geologically high boron content), Journal of Laws 2026, item 748, Annex 1 Part B item 10
- Ground, surface and saline waters that are not heavily polluted (1.1)
- Surface waters affected by waste water discharges — borate from washing agents (introduction)
Key parameters
| Parameter | Value |
|---|---|
| Range | 0.01–1 mg B/l; higher concentrations after dilution (1.1) |
| Wavelength | absorption maximum in the range 410–420 nm, checked for each new batch of reagent (clause 3, 7.1) |
| Cells | 10 mm; 50 mm for concentrations up to about 0.2 mg B/l (7.1) |
| Reaction | 25 ml of sample + 10 ml of reagent, 2 h in the dark at (20 ± 1) °C; shorter at 30 °C (7.1) |
| Blank | absorbance 0.10–0.17 per 10 mm optical path length (7.2) |
| Parametric value (drinking water) | 1.5 mg/l; 2.4 mg/l in the cases given in the explanatory note (Journal of Laws 2026, item 748, Annex 1 Part B item 10) |
Standard
- Standard number
- ISO 9390:1990
- Title (PL)
- Jakość wody — Oznaczanie boranów — Metoda spektrometryczna z azometyną-H (tłumaczenie robocze — normy nie wprowadzono do PN)
- Title (EN)
- Water quality — Determination of borate — Spectrometric method using azomethine-H
Step-by-step procedure
-
Sampling
Do not take samples in borosilicate glass containers; preserve samples according to ISO 5667-3 (clause 6).
-
Blank and purity of reagents
Treat 25 ml of water as in 7.1; the absorbance of the blank should be 0.10–0.17 per 10 mm — if higher, check the borate content of the reagents and the distilled water. Check: make 25, 100 and 250 ml of water slightly alkaline with the same small amount (e.g. 200 mg) of Ca(OH)₂, evaporate the larger aliquots to just under 25 ml, adjust to 25 ml and determine — boron increasing in proportion to the volume comes from the water, relatively high but constant results — from the reagents (7.2).
-
Calibration 0–0.20 mg/l
To six 25 ml one-mark plastics flasks add 0, 1, 2, 3, 4 and 5 ml of boron standard solution 2 (0; 0.04; 0.08; 0.12; 0.16; 0.20 mg B/l), dilute to the mark with distilled water and determine as in 7.1, measuring in a 50 mm cell against distilled water (7.4.1).
-
Calibration 0–1.00 mg/l
Repeat with 0, 5, 10, 15, 20 and 25 ml of boron standard solution 2 (0; 0.2; 0.4; 0.6; 0.8; 1.0 mg B/l), measuring in a 10 mm cell. Both graphs must be linear — otherwise check the solutions and repeat; the factor f is the reciprocal of the slope of each graph (7.4.2–7.4.3).
-
Colour reaction
Transfer 25.0 ml of sample (or a smaller volume diluted to 25 ml with distilled water) into a 100 ml polyethylene flask, add 10 ml of azomethine-H reagent (the text of the standard refers here to 4.1), mix and allow to stand for 2 h in the dark at (20 ± 1) °C. The reaction can be shortened at 30 °C — then samples, blank and calibration solutions are treated in the same way, because the intensity of colour depends on temperature (7.1, note 1).
-
Measurement
Measure the absorbance at the absorption maximum (410–420 nm) against distilled water, after setting the zero with distilled water: in a 10 mm cell, and for concentrations up to about 0.2 mg B/l — in a 50 mm cell. Check the position of the maximum for each new batch of reagent (7.1).
-
Calculation
Borate content, mg B/l = (A1 − A0) · f · V1max / V1, where A1 — absorbance of the sample, A0 — absorbance of the blank, f — factor from the appropriate calibration graph, V1 — volume of the sample, ml, V1max — maximum volume of the sample, ml (8.1).
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| Spectrometer | for measurements in the range 410–420 nm, with cells of optical path length from 10 mm to 50 mm (clause 5) | — |
| Laboratory ware | of polypropylene, polyethylene or PTFE; 100 ml polyethylene flasks and 25 ml one-mark plastics flasks (clause 5, 7.1, 7.4) | — |
| Sample containers | not of borosilicate glass; preservation according to ISO 5667-3 (clause 6) | — |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Azomethine-H, solution | 5941-07-1 | dissolve 1.0 g of azomethine-H sodium salt (C₁₇H₁₂NNaO₈S₂) and 3.0 g of L(+)-ascorbic acid in water and dilute to 100 ml; in a polyethylene bottle at 4–6 °C stable for up to a week (4.1) |
| Buffer solution, pH 5.9 | — | mix 250 g of ammonium acetate, 250 ml of water, 80 ml of sulfuric acid (ρ = 1.21 g/ml), 5 ml of phosphoric acid (ρ = 1.71 g/ml), 1.0 g of citric acid (monohydrate) and 1.0 g of disodium EDTA (dihydrate) with gentle heating (4.2) |
| Reagent solution | — | equal volumes of the azomethine-H solution and the buffer; prepare on the day of use, store in a polyethylene bottle (4.3) |
| Borate stock solution, 1.0 g B/l | 10043-35-3 | dissolve 5.719 g of boric acid (H₃BO₃) in 1000 ml of water; store in a polyethylene bottle (4.4) |
| Boron standard solutions | — | solution 1 — 10.0 mg B/l: 10 ml of stock solution to 1000 ml; solution 2 — 1.0 mg B/l: 10 ml of solution 1 to 100 ml (4.5–4.6) |
| Calcium hydroxide, Ca(OH)₂ | 1305-62-0 | for checking the boron in distilled water and reagents (4.7, note 2 in 7.2) |
| Water | — | distilled or of equivalent purity, stored in polyethylene bottles; reagents of recognized analytical grade (clause 4) |
Health and safety (OHS)
- Sulfuric acid, phosphoric acid, boric acid and the other reagents are handled according to the safety data sheets
Frequently asked questions
Which standard describes this test?
The test is performed according to ISO 9390:1990 — “Water quality — Determination of borate — Spectrometric method using azomethine-H”.
How does this method work?
Azomethine-H — the condensation product of H-acid (8-amino-1-naphthol-3,6-disulfonic acid) and salicylaldehyde — reacts with dissolved forms of borate at a pH of about 6 to form a yellow complex, whose absorbance is measured spectrometrically at the absorption maximum in the range 410–420 nm (clause 3). The concentration is determined from a linear calibration graph by the factor f (reciprocal of the slope) — separately for the range 0–0.
What is the measuring range and accuracy?
Range: 0.01–1 mg B/l; higher concentrations after dilution (1.1); Wavelength: absorption maximum in the range 410–420 nm, checked for each new batch of reagent (clause 3, 7.1); Cells: 10 mm; 50 mm for concentrations up to about 0.2 mg B/l (7.1); Reaction: 25 ml of sample + 10 ml of reagent, 2 h in the dark at (20 ± 1) °C; shorter at 30 °C (7.1).
How long does the test take?
The procedure comprises 7 steps. The standard does not give a duration for every stage — the laboratory's own procedure decides.
What equipment is required?
Spectrometer, Laboratory ware, Sample containers.
Where is this test used?
Drinking water — boron 1.5 mg/l (2.4 mg/l where desalinated water predominates or where groundwater has a geologically high boron content), Journal of Laws 2026, item 748, Annex 1 Part B item 10; Ground, surface and saline waters that are not heavily polluted (1.1); Surface waters affected by waste water discharges — borate from washing agents (introduction).
What safety precautions apply?
Sulfuric acid, phosphoric acid, boric acid and the other reagents are handled according to the safety data sheets.
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 ISO 9390.