🎨 Water Color
In short
Determination of apparent and true color of water by visual and spectrophotometric methods on the platinum-cobalt scale (Pt-Co). The test is performed according to PN-EN ISO 7887:2012; Measurement range (method C): 1–70 mg Pt/L. The procedure comprises 10 steps (about 1 h 38 min in total); it is used for: Drinking water quality control (permissible value 15 mg Pt/L), Monitoring of coagulation and filtration processes in treatment plants, Assessment of activated carbon adsorption efficiency.
At a glance
- Standard: PN-EN ISO 7887:2012
- Category: Physicochemistry
- Procedure steps: 10
- Total time of stages: 1 h 38 min
- Measurement range (method C): 1–70 mg Pt/L
- Wavelength: 410 nm
- Cuvette optical path: 50 mm (5 cm)
Overview
Water color is one of the most important organoleptic parameters directly affecting consumer acceptance of water. It is expressed in mg Pt/L (platinum-cobalt scale, also called Hazen or APHA scale). The PN-EN ISO 7887 standard describes four methods for color determination — from simple visual observation (method A) to precise spectrophotometric measurements (methods B, C, D).
Water color results from the presence of dissolved organic substances (humic and fulvic acids), iron and manganese compounds, and other pollutants. A distinction is made between apparent color — observed in an unfiltered sample containing the influence of suspension and turbidity — and true color — measured after filtration through a 0.45 µm membrane filter, reflecting only dissolved substances.
The permissible color of drinking water in Poland is 15 mg Pt/L (Regulation of the Ministry of Health, Journal of Laws 2017, item 2294). Exceeding this value may indicate organic contamination, presence of metals, or problems in the treatment process. Color determination is mandatory in drinking water quality monitoring, control of treatment processes (coagulation, activated carbon adsorption), and assessment of surface waters.
Method C (spectrophotometric at 410 nm with reference to the Pt-Co scale) is most commonly used in accredited laboratories. It provides objective, repeatable results independent of the analyst's subjective assessment.
Method principle
The method is based on comparing the color of a water sample with platinum-cobalt scale standards. A standard solution of 500 mg Pt/L is prepared from potassium hexachloroplatinate(IV) (K₂PtCl₆) and cobalt(II) chloride (CoCl₂·6H₂O) in hydrochloric acid. Method C (spectrophotometric) involves measuring the absorbance of the sample at a wavelength of 410 nm in a cuvette with an optical path of 50 mm and converting to platinum concentration in mg Pt/L using a calibration curve. Method D (visual) involves comparing the sample with a series of standard dilutions in Nessler cylinders observed from above against a white background.
Applications
- Drinking water quality control (permissible value 15 mg Pt/L)
- Monitoring of coagulation and filtration processes in treatment plants
- Assessment of activated carbon adsorption efficiency
- Testing of surface and groundwater
- Swimming pool water quality control
- Assessment of the impact of humic substances on water quality
Key parameters
| Parameter | Value |
|---|---|
| Measurement range (method C) | 1–70 mg Pt/L |
| Wavelength | 410 nm |
| Cuvette optical path | 50 mm (5 cm) |
| Resolution | 1 mg Pt/L |
| Permissible value (drinking water) | ≤15 mg Pt/L |
| Repeatability | ±2 mg Pt/L (range 0–50 mg Pt/L) |
Standard
- Standard number
- PN-EN ISO 7887:2012
- Title (PL)
- Jakość wody — Badanie i oznaczanie barwy
- Title (EN)
- Water quality — Examination and determination of colour
Step-by-step procedure
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Preparation of standard solution
Dissolve 1.246 g K₂PtCl₆ and 1.000 g CoCl₂·6H₂O in 100 mL conc. HCl and make up to 1 L with water. The resulting solution has a color of 500 mg Pt/L.
-
Preparation of calibration series
From the 500 mg Pt/L solution, prepare dilutions: 5, 10, 15, 20, 30, 40, 50, 70 mg Pt/L in volumetric flasks.
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Sample filtration (true color)
Filter the sample through a 0.45 µm membrane filter. Discard the first 50 mL of filtrate. For apparent color, skip filtration.
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Bringing sample to room temperature
Equilibrate the temperature of sample and standards to 20–25°C. Temperature difference should not exceed 2°C.
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Zero measurement
Fill the 50 mm cuvette with distilled water. Zero the spectrophotometer at λ = 410 nm.
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Measurement of calibration curve
Measure the absorbance of each calibration standard at 410 nm. Plot the relationship absorbance vs Pt concentration (mg/L).
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Sample measurement
Fill the cuvette with the sample. Measure absorbance at 410 nm. Read concentration from the calibration curve.
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Duplicate measurement
Repeat measurement for a second portion of the sample. Difference between duplicates should not exceed 2 mg Pt/L.
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Quality control
Measure a control sample (standard of known color, e.g., 15 mg Pt/L). Result must be within ±10% of certified value.
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Recording and reporting of result
Report result in mg Pt/L with accuracy to 1 mg Pt/L. Note whether measurement concerns apparent or true color.
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| UV-Vis Spectrophotometer | Hach DR6000, Shimadzu UV-1900i, Agilent Cary 60 | 15 000–60 000 PLN |
| Optical cuvettes 50 mm | Hellma 100-QS quartz, optical glass | 150–800 PLN/pc. |
| Nessler cylinders (method D) | Glass cylinders 200 mL with flat bottom, DURAN | 80–150 PLN/pc. |
| Membrane filtration system | Sartorius, Millipore — filters 0.45 µm | 2 000–5 000 PLN |
| Volumetric flasks class A | DURAN / Brand 100, 250, 500, 1000 mL | 40–120 PLN/pc. |
| Analytical balance | Mettler Toledo ME204, Radwag AS 220.R2 | 8 000–18 000 PLN |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Potassium hexachloroplatinate(IV) | 16921-30-5 | K₂PtCl₆ — for preparing 500 mg Pt/L standard solution; 1.246 g per 1 L |
| Cobalt(II) chloride hexahydrate | 7791-13-1 | CoCl₂·6H₂O — 1.000 g per 1 L standard solution |
| Concentrated hydrochloric acid | 7647-01-0 | HCl analytical grade — 100 mL per 1 L standard solution, for acidification |
| Distilled / deionized water | — | Free from color, quality at least grade 2 according to PN-EN ISO 3696 |
Health and safety (OHS)
- Potassium hexachloroplatinate — sensitizing, use gloves and mask when weighing
- Cobalt chloride — carcinogenic cat. 1B (H350), absolutely use personal protective equipment
- Concentrated hydrochloric acid — corrosive, work under fume hood, goggles and gloves
- Safety goggles and laboratory coat mandatory
Frequently asked questions
Which standard describes this test?
The test is performed according to PN-EN ISO 7887:2012 — “Water quality — Examination and determination of colour”.
How does this method work?
The method is based on comparing the color of a water sample with platinum-cobalt scale standards. A standard solution of 500 mg Pt/L is prepared from potassium hexachloroplatinate(IV) (K₂PtCl₆) and cobalt(II) chloride (CoCl₂·6H₂O) in hydrochloric acid.
What is the measuring range and accuracy?
Measurement range (method C): 1–70 mg Pt/L; Wavelength: 410 nm; Cuvette optical path: 50 mm (5 cm); Resolution: 1 mg Pt/L.
How long does the test take?
The times given for the individual stages add up to approximately 1 h 38 min. The procedure comprises 10 steps.
What equipment is required?
UV-Vis Spectrophotometer, Optical cuvettes 50 mm, Nessler cylinders (method D), Membrane filtration system, Volumetric flasks class A, Analytical balance. Indicative cost of the core instrument (UV-Vis Spectrophotometer): 15 000–60 000 PLN.
Where is this test used?
Drinking water quality control (permissible value 15 mg Pt/L); Monitoring of coagulation and filtration processes in treatment plants; Assessment of activated carbon adsorption efficiency; Testing of surface and groundwater; Swimming pool water quality control; Assessment of the impact of humic substances on water quality.
What safety precautions apply?
Potassium hexachloroplatinate — sensitizing, use gloves and mask when weighing; Cobalt chloride — carcinogenic cat. 1B (H350), absolutely use personal protective equipment; Concentrated hydrochloric acid — corrosive, work under fume hood, goggles and gloves; Safety goggles and laboratory coat mandatory.
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-EN ISO 7887.