🔥 Sodium and potassium in water
Determination of sodium and potassium in water by flame photometry (flame emission spectrometry). Measurement of Na emission at 589 nm and K at 766 nm.
Overview
Sodium (Na⁺) and potassium (K⁺) are among the main cations present in natural waters. Sodium in drinking water affects taste (detectable above 200 mg/L) and has health significance (hypertension). Potassium is an essential micronutrient, but at high concentrations may indicate wastewater contamination.
The PN-EN ISO 9964-3 standard describes a flame emission spectrometry method (FES / flame photometry) for simultaneous or separate determination of sodium and potassium in water. It is a simple, fast, and economical technique, ideal for routine determinations in waters at concentrations of 0.01–2 mg/L (higher ranges possible after dilution).
The standard also includes ISO 9964-1 (Na by AAS method) and ISO 9964-2 (K by AAS method). Permissible sodium concentration in drinking water: 200 mg/L (Journal of Laws 2017 item 2294). Potassium has no established limit in Polish regulations.
Method principle
In flame photometry, the water sample is aspirated by a pneumatic nebulizer and introduced as an aerosol into a flame (propane–butane/air or acetylene/air, ~1800–2300°C). The thermal energy of the flame excites alkali metal atoms to higher energy states. When returning to ground state, atoms emit radiation at characteristic wavelengths: Na—589 nm (yellow D doublet line), K—766 nm (red). Emission intensity is proportional to element concentration. An interference filter isolates the appropriate wavelength, and a photodetector measures intensity. To eliminate ionization interferences, a suppressor (CsCl or LiCl) is added.
Applications
- Drinking water quality control (Na—limit 200 mg/L)
- Mineral and therapeutic water monitoring
- Groundwater testing—mineralization
- Hemodialysis water control (critical Na and K concentrations)
- Municipal and industrial wastewater monitoring
- Surface water testing—salinity
Key parameters
| Parameter | Value |
|---|---|
| Range—sodium | 0.01–2 mg/L Na (without dilution) |
| Range—potassium | 0.01–2 mg/L K (without dilution) |
| Wavelength—Na | 589 nm (D line) |
| Wavelength—K | 766 nm |
| Flame type | Propane/air or acetylene/air |
| Analysis time | 10–15 seconds per reading |
Standard
- Standard number
- PN-EN ISO 9964-3:2001
- 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
1. Photometer startup
Turn on air compressor. Open fuel gas. Ignite photometer flame. Wait for stabilization (~10 min). Select Na filter (589 nm).
2. Standard preparation
From 1000 mg/L Na standard solution, prepare series: 0.1; 0.5; 1; 2; 5; 10 mg/L. To each flask add CsCl (final Cs concentration 1000 µg/mL). Similarly for K.
3. Sample preparation
Dilute water samples to linear range (if needed). Add CsCl as for standards. NOTE: use PE/PP vessels—glass releases Na!
4. Calibration—sodium
Zero photometer on blank (H₂O + CsCl). Set sensitivity on highest standard. Aspirate successive standards and record emission. Plot calibration curve.
5. Na measurement in samples
Aspirate successive samples. Read Na concentration from calibration curve or directly from display (if photometer has concentration mode). Rinse nebulizer with water between samples.
6. Filter change to K (766 nm)
Switch filter to 766 nm (potassium). Repeat calibration and sample measurement analogously.
7. Quality control
Blank, CRM every 10 samples, duplicates. Repeatability <3% CV. Check for glass contamination (Na blank).
8. Shutdown
Aspirate deionized water (rinse). Turn off flame (close gas, then air). Turn off photometer and compressor.
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| Flame photometer | Jenway PFP7, BWB XP, Sherwood M410, Krüss FP8000 | 25,000–70,000 PLN |
| Oil-free air compressor | Jun-Air, Werther, 3–6 bar with regulator | 5,000–15,000 PLN |
| Fuel gas cylinder | Propane–butane (LPG) or acetylene | 100–300 PLN (exchange) |
| Automatic pipettes | Eppendorf Research Plus (100 µL – 10 mL) | 800–2,500 PLN |
| Volumetric flasks (50, 100, 250 mL) | Class A, borosilicate | 30–100 PLN/piece |
| PE/PP plastic vessels/tubes | Falcon 50 mL, sample cups | 1–5 PLN/piece |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Sodium standard solution 1000 mg/L Na | 7647-14-5 | Certified CRM (NaCl in HNO₃ 0.5 mol/L). Calibration series: 0.1; 0.5; 1.0; 2.0; 5.0; 10.0 mg/L Na. |
| Potassium standard solution 1000 mg/L K | 7447-40-7 | Certified CRM (KCl in HNO₃ 0.5 mol/L). Calibration series: 0.1; 0.5; 1.0; 2.0; 5.0; 10.0 mg/L K. |
| Cesium chloride (CsCl)—ionization suppressor | 7647-17-8 | 10 g/L Cs solution. Add to standards and samples (final concentration ~1000 µg/mL Cs). Cs easily ionizes in flame, providing excess electrons and suppressing Na/K ionization. |
| Deionized water | — | Free from Na and K. Conductivity <0.1 mS/m. Use PE/PP vessels (not glass—releases Na!). |
| Nitric acid (HNO₃) 1% (v/v) | 7697-37-2 | For acidifying samples. Suprapur purity (low Na/K blank). |
Health and safety (OHS)
- Fuel gas (propane–butane / acetylene)—highly flammable, explosive (H220). Check gas connection tightness.
- Open flame—fire and burn risk. Do not leave unattended.
- CsCl—irritating (H315, H319). Use gloves.
- HNO₃—corrosive (H314). Work under fume hood when preparing dilutions from concentrated acid.
- Gas cylinder—secure against tipping. Do not heat. Store vertically.
- Safety glasses and lab coat required.