⚗️ Sodium and potassium in water — flame emission spectrometry (589.0 nm and 766.5 nm) with a caesium buffer
In short
Up to 40 ml of the sample (filtered through 0. 45 µm) is added to a 50 ml flask containing 5 ml of caesium chloride solution, made up with water and aspirated into the flame of a flame photometer or of an AAS spectrometer in the emission mode; the emission intensity is measured at 589. The test is performed according to PN-ISO 9964-3:1994; Range: up to 10 mg/l Na and K (portion of 40 ml → 50 ml); lower limit < 0.1 mg/l; higher concentrations — a smaller portion. The procedure comprises 6 steps; it is used for: Drinking water and raw waters — sodium and potassium up to 10 mg/l directly, higher after reducing the portion, Wastewater — according to the national annex PN-ISO 9964-3/Ak (reference method under Dz.U. 2019 poz. 1311), Mineral, medicinal and process waters — cation balance, control of softening and demineralisation.
At a glance
- Standard: PN-ISO 9964-3:1994
- Category: Physicochemistry
- Procedure steps: 6
- Range: up to 10 mg/l Na and K (portion of 40 ml → 50 ml); lower limit < 0.1 mg/l; higher concentrations — a smaller portion
- Wavelengths: Na 589.0 nm, K 766.5 nm (interference filters or a monochromator)
- Ionisation buffer: CsCl 25 g/l in HCl — 5 ml per 50 ml (about 2 g Cs/l); required with an air/acetylene flame
Overview
A national supplement to this standard, PN-ISO 9964-3/Ak:1997, covering wastewater, has been published.
ISO 9964-3:1993 specifies a method for determining dissolved sodium and potassium by flame emission spectrometry (FES), intended for raw and drinking waters. It applies to samples with sodium and potassium concentrations up to 10 mg/l — at higher concentrations a smaller portion is taken for the analysis; the lower limits of determination are below 0.1 mg/l for both elements. The ions normally present in raw and drinking waters do not interfere if an ionisation suppressant is present. Precision (interlaboratory trial of 1992): for drinking water sodium 4.38 mg/l, potassium 0.37 mg/l; for river water 63.1 and 5.28 mg/l; for municipal wastewater 87.0 mg/l Na — with repeatability coefficients of variation from about 2.4 % to 30 % depending on the level. In Poland it is in force as PN-ISO 9964-3:1994, and for wastewater — the national annex PN-ISO 9964-3/Ak:1997; the 2019 regulation (Dz.U. 2019 poz. 1311 [Journal of Laws 2019, item 1311]) names PN-ISO 9964-3 and /Ak as the reference methods (flame emission spectroscopy) for sodium and potassium in wastewater, alongside AAS according to PN-ISO 9964-1 and 9964-2.
In laboratory practice sodium and potassium are commonly determined in a flame with interference filters for these elements (sodium 589.0 nm, potassium 766.5 nm) after adding a caesium chloride solution that suppresses ionisation; an alternative is ion chromatography of cations according to PN-EN ISO 14911 (sodium and potassium 1.0–100 mg/dm³).
Method principle
The sample is aspirated into a gas flame whose thermal energy is sufficient for the sodium and potassium present to emit their characteristic radiation; the intensity is measured at a wavelength of 589.0 nm for sodium and 766.5 nm for potassium. With an air/acetylene flame the addition of caesium chloride solution as an ionisation buffer is necessary (with other flames it may be omitted). Reagents: concentrated hydrochloric acid (1.18 g/ml) or nitric acid about 16 mol/l (1.41 g/ml); caesium chloride solution — 25 g of CsCl in 50 ml of HCl and 450 ml of water, made up to 1 l (about 20 g Cs/l); sodium stock solution 2000 mg/l (5.084 g ± 0.005 g of NaCl dried for at least 1 h at 140 °C ± 10 °C, made up to 1000 ml; stable for at least 6 months in polyethylene) and potassium stock solution 2000 mg/l (3.814 g ± 0.005 g of KCl dried in the same way) — or commercial solutions; Na/K standard solution (10 ml of each stock solution made up to 1000 ml; 1 ml contains 20 µg Na and 20 µg K; freshly prepared). Apparatus: a flame photometer with a suitable interference filter for sodium and potassium, or an atomic absorption spectrometer in the emission mode; borosilicate glassware and polyethylene vessels soaked in 10 % (V/V) HNO₃, rinsed with deionised water and used solely for this method. Samples are taken into clean polyethylene bottles (ISO 5667-1, -2); preservation with acid is not necessary (where other metals are determined — HCl or HNO₃ to about pH 1, in the same way for samples, standards and blanks). Samples containing particles are filtered through an acid-washed 0.45 µm filter or centrifuged. Test solutions: into 50 ml flasks — up to 40 ml of the sample and 5 ml of CsCl solution, made up with water. Calibration solutions: 0; 1.0; 2.5; 5; 10; 15; 20; 25 ml of the Na/K standard + 5 ml of CsCl in 50 ml flasks → 0; 0.4; 1.0; 2.0; 4.0; 6.0; 8.0; 10.0 mg/l Na and K. Flame photometer: water → 0 % of the scale, the 10 mg/l standard → 100 %, repeated until the readings are stable; AAS in emission: zeroing with water. The curve is usually linear up to 10 mg/l (with a photometer it may show a slight curvature; the slope should be checked e.g. every 10 samples). Calculations: ρ_Na = (R − R₀)·V_p/(V_m·b), and analogously ρ_K (R, R₀ — responses of the sample and of the blank; V_m — the portion, usually 40 ml; V_p — the flask, 50 ml; b — the slope, in l/mg); c_Na = ρ_Na/23.0, c_K = ρ_K/39.1 mmol/l.
Applications
- Drinking water and raw waters — sodium and potassium up to 10 mg/l directly, higher after reducing the portion
- Wastewater — according to the national annex PN-ISO 9964-3/Ak (reference method under Dz.U. 2019 poz. 1311)
- Mineral, medicinal and process waters — cation balance, control of softening and demineralisation
- Laboratories equipped with a simple flame photometer (without hollow-cathode lamps) or an AAS in the emission mode
- Routine series of samples — simultaneous measurement of Na and K from the same test solution
Key parameters
| Parameter | Value |
|---|---|
| Range | up to 10 mg/l Na and K (portion of 40 ml → 50 ml); lower limit < 0.1 mg/l; higher concentrations — a smaller portion |
| Wavelengths | Na 589.0 nm, K 766.5 nm (interference filters or a monochromator) |
| Ionisation buffer | CsCl 25 g/l in HCl — 5 ml per 50 ml (about 2 g Cs/l); required with an air/acetylene flame |
| Calibration | 0; 0.4; 1.0; 2.0; 4.0; 6.0; 8.0; 10.0 mg/l Na and K; photometer: water = 0 %, 10 mg/l = 100 %; linearity up to 10 mg/l |
| Sample preparation | filtration through 0.45 µm (with an acid-washed filter) or centrifugation; no preservation |
| Precision (1992) | drinking water Na 4.38 mg/l, K 0.37 mg/l; river Na 63.1, K 5.28 mg/l; wastewater Na 87.0 mg/l |
Standard
- Standard number
- PN-ISO 9964-3:1994
- Title (PL)
- Jakość wody — Oznaczanie sodu i potasu — Część 3: Oznaczanie sodu i potasu metodą emisyjnej spektrometrii płomieniowej
- Title (EN)
- Water quality — Determination of sodium and potassium — Part 3: Determination of sodium and potassium by flame emission spectrometry
Step-by-step procedure
-
Sampling
Take the sample into a clean polyethylene bottle (ISO 5667-1, -2); preservation with acid is not necessary; where other metals are determined — HCl or HNO₃ to about pH 1, in the same way for samples, standards and blanks.
-
Test solutions
Filter samples containing particles through an acid-washed 0.45 µm filter (or centrifuge them). Into 50 ml flasks add up to 40 ml of the sample and 5 ml of caesium chloride solution; make up with water (at concentrations above 10 mg/l take a smaller portion).
-
Calibration solutions
Into 50 ml flasks pipette 0; 1.0; 2.5; 5; 10; 15; 20; 25 ml of the Na/K standard, add 5 ml of CsCl and make up with water (0–10.0 mg/l Na and K).
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Setting up the instrument
Fit the filter for the metal being determined; while aspirating a calibration solution, optimise the aspiration and the flame. Photometer: water → 0 %, the 10 mg/l standard → 100 %, repeated until the readings are stable; AAS in emission: zero with water.
-
Calibration and measurement
Aspirate the calibration solutions (with water in between), measure the response at 589.0 nm (Na) and 766.5 nm (K), plot the curves and calculate the slope b; aspirate the test solutions (with water in between) and a blank with each series; check the slope about every 10 samples.
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Calculation
ρ_Na = (R − R₀)·V_p/(V_m·b), ρ_K analogously [mg/l] (V_m usually 40 ml, V_p 50 ml); if the curve is not linear, read the concentrations from the graph and convert for the dilution; c_Na = ρ_Na/23.0, c_K = ρ_K/39.1 mmol/l. State the result and any deviations in the test report.
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| Flame photometer with interference filters for Na and K, or an AAS spectrometer in the emission mode | set up according to the manufacturer's instructions; optimisation of the aspiration and of the flame | — |
| Burner and gas supply (air/acetylene or another flame) | with other flames the caesium buffer may be omitted | — |
| Volumetric flasks of 50 ml and 1000 ml, pipettes (borosilicate glass), polyethylene bottles | washed with 10 % HNO₃, used solely for this method | — |
| Filters 0.45 µm washed with acid, or a centrifuge | removal of particles | — |
| Drying oven 140 °C ± 10 °C | drying of NaCl and KCl | — |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Caesium chloride — solution of 25 g/l in HCl (about 20 g Cs/l) | — | ionisation buffer |
| Concentrated hydrochloric acid (1.18 g/ml) or nitric acid about 16 mol/l (1.41 g/ml) | — | CsCl solution, washing of the vessels, preservation where applicable |
| Stock solutions Na 2000 mg/l (5.084 g NaCl/l) and K 2000 mg/l (3.814 g KCl/l) | — | standards (or commercial solutions) |
| Na/K standard solution, 20 µg/ml of each | — | calibration solutions (freshly prepared) |
| Deionised water | — | dilutions, zeroing, blank |
Health and safety (OHS)
- Acetylene and other flammable gases — tightness of the installation, ventilation, supervision of the burner
- Concentrated acids (HCl, HNO₃) — goggles, gloves, fume hood
- Hot burner and fumes from the flame — extraction above the instrument
Frequently asked questions
Which standard describes this test?
The test is performed according to PN-ISO 9964-3:1994 — “Water quality — Determination of sodium and potassium — Part 3: Determination of sodium and potassium by flame emission spectrometry”.
How does this method work?
The sample is aspirated into a gas flame whose thermal energy is sufficient for the sodium and potassium present to emit their characteristic radiation; the intensity is measured at a wavelength of 589.0 nm for sodium and 766.
What is the measuring range and accuracy?
Range: up to 10 mg/l Na and K (portion of 40 ml → 50 ml); lower limit < 0.1 mg/l; higher concentrations — a smaller portion; Wavelengths: Na 589.0 nm, K 766.5 nm (interference filters or a monochromator); Ionisation buffer: CsCl 25 g/l in HCl — 5 ml per 50 ml (about 2 g Cs/l); required with an air/acetylene flame; Calibration: 0; 0.4; 1.0; 2.0; 4.0; 6.0; 8.0; 10.0 mg/l Na and K; photometer: water = 0 %, 10 mg/l = 100 %; linearity up to 10 mg/l.
How long does the test take?
The procedure comprises 6 steps. The standard does not give a duration for every stage — the laboratory's own procedure decides.
What equipment is required?
Flame photometer with interference filters for Na and K, or an AAS spectrometer in the emission mode, Burner and gas supply (air/acetylene or another flame), Volumetric flasks of 50 ml and 1000 ml, pipettes (borosilicate glass), polyethylene bottles, Filters 0.45 µm washed with acid, or a centrifuge, Drying oven 140 °C ± 10 °C.
Where is this test used?
Drinking water and raw waters — sodium and potassium up to 10 mg/l directly, higher after reducing the portion; Wastewater — according to the national annex PN-ISO 9964-3/Ak (reference method under Dz.U. 2019 poz. 1311); Mineral, medicinal and process waters — cation balance, control of softening and demineralisation; Laboratories equipped with a simple flame photometer (without hollow-cathode lamps) or an AAS in the emission mode; Routine series of samples — simultaneous measurement of Na and K from the same test solution.
What safety precautions apply?
Acetylene and other flammable gases — tightness of the installation, ventilation, supervision of the burner; Concentrated acids (HCl, HNO₃) — goggles, gloves, fume hood; Hot burner and fumes from the flame — extraction above the instrument.
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 9964-3.