⚗️ Sodium in water — flame atomic absorption spectrometry (589.0 nm) with caesium chloride as an ionisation buffer
In short
Into a 100 ml flask containing 10 ml of caesium chloride solution (25 g of CsCl/l in HCl), 2 ml of the sample (filtered through 0. The test is performed according to PN-ISO 9964-1:1994 (ISO 9964-1:1993); Range: 5–50 mg/l Na (dilution 2 ml → 100 ml); optimum concentration of the measurement solution 0.1–1.0 mg/l. The procedure comprises 7 steps; it is used for: Drinking water and raw waters — sodium 5–50 mg/l (other ranges by changing the dilution), Wastewater — according to the national supplement PN-ISO 9964-1/Ak (the reference method under Dz.U. 2019 poz. 1311), Mineral and medicinal waters, control of softening (the exchange of Ca/Mg for Na) and of desalination.
At a glance
- Standard: PN-ISO 9964-1:1994 (ISO 9964-1:1993)
- Category: Physicochemistry
- Procedure steps: 7
- Range: 5–50 mg/l Na (dilution 2 ml → 100 ml); optimum concentration of the measurement solution 0.1–1.0 mg/l
- Wavelength: 589.0 nm; slit below 0.3 nm; Na lamp, photomultiplier sensitive in the red
- Flame: air/acetylene
Overview
ISO 9964-1:1993 specifies a method for determining dissolved sodium by flame atomic absorption spectrometry (AAS), intended for the analysis of raw and drinking waters. The method applies to samples with a sodium concentration of 5–50 mg/l; the range can be extended downwards or upwards by choosing a dilution factor other than the one given in clause 8. The ions normally present in raw and drinking waters do not interfere with the determination of sodium by AAS. In Poland the standard is in force as PN-ISO 9964-1:1994, and for wastewater the national supplement PN-ISO 9964-1/Ak:1997 was published; the 2019 regulation (Dz.U. 2019 poz. 1311 [Journal of Laws 2019, item 1311]) names PN-ISO 9964-1 and PN-ISO 9964-1/Ak (AAS) and PN-ISO 9964-3 (flame emission spectroscopy) as the reference methods for sodium in wastewater.
In the PWr textbook "Chemia wody" [Water chemistry] sodium and potassium are commonly determined by flame AAS with lamps for these elements (sodium 589.0 nm, potassium 766.5 nm) on the basis of PN-ISO 9964: caesium chloride solution is added to the sample in order to suppress the ionisation, the sample is aspirated directly into the air/acetylene flame and the absorbance is measured at 589.0 nm, the concentration being read from a calibration curve established on the particular instrument. An alternative is the ion chromatography of cations according to PN-EN ISO 14911 (sodium 1.0–100 mg/dm³).
Method principle
Caesium chloride solution is added to the sample as an ionisation suppressant, the sample is aspirated directly into the air/acetylene flame of an atomic absorption spectrometer and the absorbance is measured at a wavelength of 589.0 nm. Reagents: hydrochloric acid of about 11 mol/l (1.18 g/ml) or nitric acid of 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 1000 mg/l — 2.542 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 a polyethylene bottle (or a commercial solution); standard solution 10 mg/l (10 ml of the stock solution made up to 1000 ml), freshly prepared. Apparatus: an AAS spectrometer with a burner for an air/acetylene flame, a hollow-cathode lamp for sodium and a photomultiplier sensitive in the red, with a recommended spectral slit width below 0.3 nm; borosilicate glassware and polyethylene vessels cleaned with 10 % (V/V) nitric acid and rinsed with water, used solely for this method. Samples are taken into clean polyethylene bottles (ISO 5667-1, -2); preservation with acid is not usually necessary — where other metals are determined in the samples as well, they may be acidified with HCl or HNO₃ to about pH 1, provided that all the samples, standards and blanks contain the same acid at the same concentration. Samples containing particles are filtered through an acid-washed 0.45 µm filter (or centrifuged). Calibration solutions: into a series of 100 ml flasks, 10 ml of caesium chloride solution each and 0; 1.0; 2.0; 4.0; 6.0; 10.0 ml of the 10 mg/l standard → 0; 0.1; 0.2; 0.4; 0.6; 1.0 mg/l Na; the curve (absorbance against concentration) is usually linear in the range 0.1–1.0 mg/l. Calculation with a linear curve: ρ_Na = (A − A₀)/b · V_p/V_m (mg/l), where A and A₀ are the absorbances of the sample and of the blank, b the slope of the curve (l/mg), V_m the volume of the portion (usually 2 ml) and V_p the volume of the flask (100 ml); c_Na = ρ_Na/23.0 (mmol/l).
Applications
- Drinking water and raw waters — sodium 5–50 mg/l (other ranges by changing the dilution)
- Wastewater — according to the national supplement PN-ISO 9964-1/Ak (the reference method under Dz.U. 2019 poz. 1311)
- Mineral and medicinal waters, control of softening (the exchange of Ca/Mg for Na) and of desalination
- Laboratories with a classical flame AAS — determination alongside potassium (766.5 nm) according to part 2
- Boiler and process waters — control of the sodium after demineralisation
Key parameters
| Parameter | Value |
|---|---|
| Range | 5–50 mg/l Na (dilution 2 ml → 100 ml); optimum concentration of the measurement solution 0.1–1.0 mg/l |
| Wavelength | 589.0 nm; slit below 0.3 nm; Na lamp, photomultiplier sensitive in the red |
| Flame | air/acetylene |
| Ionisation buffer | CsCl 25 g/l in HCl — 10 ml per 100 ml of measurement solution (about 2 g Cs/l) |
| Calibration | 0; 0.1; 0.2; 0.4; 0.6; 1.0 mg/l Na; linear from 0.1 to 1.0 mg/l; check of the slope about every 10 samples |
| Sample preparation | filtration through 0.45 µm (with an acid-washed filter) or centrifugation; no preservation (or HCl/HNO₃ to about pH 1, in the same way for the samples and the standards) |
Standard
- Standard number
- PN-ISO 9964-1:1994 (ISO 9964-1:1993)
- Title (PL)
- Jakość wody — Oznaczanie sodu i potasu — Część 1: Oznaczanie sodu metodą absorpcyjnej spektrometrii atomowej
- Title (EN)
- Water quality — Determination of sodium and potassium — Part 1: Determination of sodium by atomic absorption spectrometry
Step-by-step procedure
-
Sampling
Take the sample into a clean polyethylene bottle (ISO 5667-1, -2); preservation with acid is not usually needed; if other metals are determined as well, acidify with HCl or HNO₃ to about pH 1 — the standards and the blanks must then contain the same acid.
-
Preparation of the measurement solutions
Filter samples containing particles through an acid-washed 0.45 µm filter (or centrifuge them). Into 100 ml flasks add 10 ml of caesium chloride solution each, pipette in 2 ml of the sample and make up with water; where the concentration falls outside 0.1–1.0 mg/l, change the volume of the sample.
-
Calibration solutions
Into a series of 100 ml flasks containing 10 ml of CsCl add 0; 1.0; 2.0; 4.0; 6.0; 10.0 ml of the 10 mg/l standard and make up with water (0–1.0 mg/l Na).
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Setting up the spectrometer
While aspirating a calibration solution, optimise the aspiration rate, the type of flame and the position of the beam; zero the absorbance with water.
-
Calibration
Aspirate the calibration solutions (with water in between), measure the absorbance at 589.0 nm, plot the absorbance-against-concentration curve and calculate the slope b (l/mg); check the slope about every 10 samples.
-
Measurement of the samples and the blank
Aspirate the measurement solutions (with water in between) and record the absorbance; with each series run a blank with water instead of the sample.
-
Calculation
ρ_Na = (A − A₀)/b · V_p/V_m [mg/l] (V_m usually 2 ml, V_p 100 ml); if the curve is not linear, read the concentration from the graph and convert for the dilution; c_Na = ρ_Na/23.0 mmol/l. Give the result in the test report.
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| Atomic absorption spectrometer with an air/acetylene burner | set up according to the manufacturer's instructions, optimisation of the aspiration and of the flame | — |
| Hollow-cathode lamp for sodium and a photomultiplier sensitive in the red | spectral slit below 0.3 nm | — |
| Volumetric flasks of 100 ml and 1000 ml, pipettes of 2 ml and 10 ml (borosilicate glass) | cleaned with 10 % HNO₃, used solely for this method | — |
| Polyethylene bottles | sampling and storage of the samples, stock solution | — |
| Filters 0.45 µm washed with acid, or a centrifuge | removal of particles | — |
| Drying oven 140 °C ± 10 °C | drying of the NaCl | — |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Caesium chloride — solution of 25 g/l in HCl (about 20 g Cs/l) | — | suppression of the ionisation of sodium in the flame |
| Hydrochloric acid of about 11 mol/l or nitric acid of about 16 mol/l | — | CsCl solution, cleaning of the glassware (10 % HNO₃), preservation where applicable |
| Sodium stock solution 1000 mg/l (2.542 g of NaCl/l) and standard solution 10 mg/l | — | calibration |
| Deionised water | — | dilutions, zeroing, blank |
| Acetylene and compressed air | — | gases for the flame |
Health and safety (OHS)
- Acetylene — a gas installation with safety devices, ventilation, checks for tightness; never leave the burner unattended
- Concentrated acids (HCl, HNO₃) — goggles, gloves, fume hood
- The hot burner and the fumes from the flame — extraction above the spectrometer
Frequently asked questions
Which standard describes this test?
The test is performed according to PN-ISO 9964-1:1994 (ISO 9964-1:1993) — “Water quality — Determination of sodium and potassium — Part 1: Determination of sodium by atomic absorption spectrometry”.
How does this method work?
Caesium chloride solution is added to the sample as an ionisation suppressant, the sample is aspirated directly into the air/acetylene flame of an atomic absorption spectrometer and the absorbance is measured at a wavelength of 589.0 nm.
What is the measuring range and accuracy?
Range: 5–50 mg/l Na (dilution 2 ml → 100 ml); optimum concentration of the measurement solution 0.1–1.0 mg/l; Wavelength: 589.0 nm; slit below 0.3 nm; Na lamp, photomultiplier sensitive in the red; Flame: air/acetylene; Ionisation buffer: CsCl 25 g/l in HCl — 10 ml per 100 ml of measurement solution (about 2 g Cs/l).
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?
Atomic absorption spectrometer with an air/acetylene burner, Hollow-cathode lamp for sodium and a photomultiplier sensitive in the red, Volumetric flasks of 100 ml and 1000 ml, pipettes of 2 ml and 10 ml (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 5–50 mg/l (other ranges by changing the dilution); Wastewater — according to the national supplement PN-ISO 9964-1/Ak (the reference method under Dz.U. 2019 poz. 1311); Mineral and medicinal waters, control of softening (the exchange of Ca/Mg for Na) and of desalination; Laboratories with a classical flame AAS — determination alongside potassium (766.5 nm) according to part 2; Boiler and process waters — control of the sodium after demineralisation.
What safety precautions apply?
Acetylene — a gas installation with safety devices, ventilation, checks for tightness; never leave the burner unattended; Concentrated acids (HCl, HNO₃) — goggles, gloves, fume hood; The hot burner and the fumes from the flame — extraction above the spectrometer.
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 9964-1.