🔬 Manganese in water
Spectrophotometric determination of manganese in water using formaldoxime method. Measurement of orange Mn-formaldoxime complex absorbance at 450 nm.
Overview
Manganese is one of the key drinking water quality parameters. In groundwater, it occurs mainly as Mn²⁺ and can reach concentrations of several mg/L. Elevated manganese concentration (>0.050 mg/L) causes dark water discoloration, brownish-black deposits in installations, unpleasant metallic taste, and promotes growth of manganese bacteria.
The PN-EN ISO 6333 standard describes a spectrometric method for determining manganese with formaldoxime. The method is used for surface and drinking waters. Application range: 0.01–5 mg/L Mn. Samples with higher concentration should be diluted.
The permissible concentration of manganese in drinking water in Poland is 0.050 mg/L (Ministry of Health Regulation). It is one of the most frequently exceeded parameters in groundwater, especially in deep wells.
Method principle
Manganese(II) reacts with formaldoxime (CH₂=NOH, formaldehyde oxime) in alkaline medium (pH ~10), forming an orange complex whose absorbance is measured at 450 nm. Iron interferences are eliminated by adding EDTA and hydroxylamine hydrochloride, which complex Fe³⁺ and Fe²⁺, preventing their reaction with formaldoxime. Hydroxylamine simultaneously acts as a reducing agent, maintaining manganese in Mn²⁺ form.
Applications
- Drinking water quality control (Journal of Laws 2017 item 2294—limit 50 µg/L)
- Groundwater monitoring (wells, intakes)
- Manganese removal process control
- Surface water testing
- Water control in food and beverage industry
- Industrial wastewater monitoring
Key parameters
| Parameter | Value |
|---|---|
| Measurement range | 0.01–5.0 mg/L Mn |
| Wavelength | 450 nm |
| Detection limit | 0.01 mg/L |
| Accuracy | ±5% in 0.05–5.0 mg/L range |
| Reaction pH | ~10 (alkaline medium) |
| Color development time | 10–15 minutes |
Standard
- Standard number
- ISO 6333:1986
- Title (PL)
- Jakość wody — Oznaczanie manganu — Metoda spektrometryczna z formaldoksymem (brak odpowiednika PN-EN)
- Title (EN)
- Water quality — Determination of manganese — Formaldoxime spectrometric method
Step-by-step procedure
1. Sample preparation
Acidify sample with HNO₃ to pH < 2 immediately after collection. Filter through 0.45 µm if determining dissolved Mn.
2. Sample measurement
Transfer appropriate volume of sample (e.g., 25–50 mL) to 50 mL volumetric flask.
3. EDTA addition
Add 1 mL of 5% EDTA-Na₂ solution to mask iron interferences. Mix.
4. Formaldoxime addition
Add 2 mL of freshly prepared formaldoxime solution. Mix.
5. Alkalinization
Add 1 mL NaOH 10 mol/L to achieve pH ~10. Fill to mark with deionized water. Mix vigorously.
6. Color development
Wait 10–15 minutes for full development of orange complex color.
7. Absorbance measurement
Measure absorbance at 450 nm in 10 mm cuvette (or longer for low concentrations) against blank. Read concentration from calibration curve.
8. Calibration and quality control
Prepare calibration curve with min. 5 standards. Perform blank, CRM control samples, and duplicates every 10 samples.
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| UV-VIS spectrophotometer | Hach DR6000, Shimadzu UV-1900i, HACH DR3900 | 15,000–60,000 PLN |
| Glass / quartz cuvettes 10–50 mm | Hellma, borosilicate cuvettes | 50–400 PLN/piece |
| Automatic pipettes | Eppendorf Research Plus (1–10 mL) | 800–2,500 PLN |
| Volumetric flasks (50, 100 mL) | Class A, borosilicate | 30–80 PLN/piece |
| Analytical balance (0.1 mg) | Radwag AS 220.R2, Mettler Toledo ME204 | 5,000–15,000 PLN |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Formaldoxime (solution) | — | Prepare fresh: mix equal volumes of 2% formaldehyde and 4% hydroxylamine hydrochloride. Solution unstable—prepare daily. |
| Hydroxylamine hydrochloride (NH₂OH·HCl) | 5470-11-1 | 4% solution: 4 g in 100 mL water. Reducing agent and formaldoxime component. |
| Formaldehyde 37% (formalin) | 50-00-0 | 2% solution: dilute formaldehyde with deionized water. For formaldoxime preparation. |
| EDTA-Na₂ solution 5% | 6381-92-6 | Disodium EDTA salt—for masking iron interferences. 5 g EDTA-Na₂ in 100 mL water. |
| Sodium hydroxide (NaOH) 10 mol/L | 1310-73-2 | For alkalinizing reaction medium to pH ~10. |
| Manganese standard solution 1000 mg/L Mn | 7785-87-7 | Certified CRM (e.g., Merck Certipur) from MnSO₄. For calibration: 0.05; 0.1; 0.5; 1.0; 2.0; 5.0 mg/L. |
| Deionized water | — | Free from manganese, conductivity <0.1 mS/m. |
Health and safety (OHS)
- Formaldehyde (formalin)—carcinogenic cat. 1B (H350), toxic by inhalation (H331), irritating. Work under fume hood!
- NaOH 10 mol/L—strongly corrosive (H314). Causes severe skin burns and eye damage.
- EDTA-Na₂—irritating to eyes (H319). Use safety glasses.
- Safety glasses, nitrile gloves, and lab coat required.
- Formaldehyde-containing waste—collect separately as hazardous waste.