Water Quality Color Determination – Platinum-Cobalt Colorimetric Method

1. Subject Matter and Scope

This standard specifies two methods for determining color. This standard measures the color of a sample after 15 minutes of clarification. pH value has a significant impact on color; therefore, pH value should be measured simultaneously with color determination.

1.1 The platinum-cobalt colorimetric method adopts the international standard ISO 7887—1985, "Water quality – Examination and determination of color". The platinum-cobalt colorimetric method is suitable for clean water, lightly polluted water with a slightly yellow tint, relatively clean surface water, groundwater, and drinking water.

1.2 The dilution ratio method is suitable for heavily polluted surface water and industrial wastewater.

The two methods should be used independently and are generally not comparable.

This standard is not applicable when the color hue of the sample and the standard solution are inconsistent.

2. Definitions

The definitions in this standard are taken from International Commission on Illumination Publication No. 17 (CIE publication No. 17) and are as follows:

2.1 Color of water: An optical property that alters the composition of the transmitted visible light spectrum.

2.2 Apparent Color of Water: The color produced by dissolved substances and insoluble suspended matter, determined using the original sample without filtration or centrifugation.

2.3 True Color of Water: The color produced solely by dissolved substances. Determined using a sample filtered through a 0.45 μm membrane filter.

2.4 Standard Unit of Colorimetry, Degree: One degree is defined as the color produced when 2 mg of cobalt(IV) chloride hexahydrate and 1 mg of platinum [in the form of hexachloroplatinic(IV) acid] are present in each liter of solution.

3. Platinum-Cobalt Colorimetric Method

3.1 Principle: A color standard solution is prepared using potassium chloroplatinate and cobalt chloride. This solution is compared with the sample under daylight to determine the color intensity, i.e., colorimetry.

The colorimetry of the sample is expressed as the degree value of the corresponding colorimetric standard solution (3.2.3).

Note: The standard degree derived from this standard unit is sometimes referred to as "Hazen international" or "Pt-Co standard" [GB 3143 "Determination of Color of Liquid Chemical Products (Hazen unit - Platinum-Cobalt color number)"], or milligrams of platinum per liter.

3.2 Reagents Unless otherwise specified, only optically pure water (3.2.1) and analytically pure reagents are used in the determination.

3.2.1 Optically pure water: Soak a 0.2 μm filter membrane (used in bacteriological studies) in 100 mL of distilled or deionized water for 1 hour. Filter 250 mL of distilled or deionized water using this solution. Discard the initial 250 mL. Use this water to prepare all standard solutions and as dilution water thereafter.

3.2.2 Colorimetric Standard Stock Solution, equivalent to 500°C: Dissolve 1.245 ± 0.001 g potassium hexachloroplatinate (K₂PtCl₆) and 1.000 ± 0.001 g cobalt chloride (IV) hexahydrate (CoCl₂·6H₂O) in approximately 500 mL of water (4.1), add 100 ± 1 mL of hydrochloric acid (p = 1.18 g/mL), and dilute to the lower mark with water in a 1000 mL volumetric flask.

Store the solution in a sealed glass bottle in the dark, at a temperature not exceeding 30°C. The solution should be stable for at least 6 months.

3.2.3 Colorimetric Standard Solutions: Add 2.50, 5.00, 7.50, 10.00, 12.50, 15.00, 17.50, 20.00, 30.00, and 35.00 mL of stock solution (3.2.2) to a set of 250 mL volumetric flasks using a pipette, and dilute to the mark with water (3.2.1). The colorimetric values of the solutions are 5, 10, 15, 20, 25, 30, 35, 40, 50, 60, and 70 degrees, respectively.

Store the solutions in tightly sealed glass bottles in the dark. The temperature should not exceed 30°C. These solutions are stable for at least one month.

3.3 Instruments

3.3.1 Common laboratory instruments and the following instruments.

3.3.2 Stoppered colorimetric tubes, 50 mL. All tubes should be of uniform size, with optically transparent glass and no shadows on the bottom.

3.3.3 pH meter, accuracy ±0.1 pH unit.

3.3.4 Volumetric flask, 250 mL.

3.4 Sampling and Sample

All glassware that comes into contact with the sample must be cleaned with hydrochloric acid or surfactant solution, and finally rinsed with distilled water or deionized water and drained.

Collect the sample in a glass bottle with a volume of at least 1 L. Perform the measurement as soon as possible after sampling. If storage is necessary, store the sample in the dark. In some cases, avoid contact with air. Also, avoid temperature changes.

3.5 Procedure

3.5.1 Sample

Pour the sample into a 250 mL (or larger) graduated cylinder, let it stand for 15 min, and pour off the supernatant as the sample for measurement.

3.5.2 Measurement

Fill a set of stoppered colorimetric tubes (3.3.2) to the mark with the colorimetric standard solution (3.2.3). Fill another set of stoppered colorimetric tubes to the mark with the sample (3.5.1).

Place the stoppered colorimetric tubes on a white surface at a suitable angle so that light is reflected upwards from the bottom of the tube through the liquid column.

Observe the liquid column vertically downwards and find the standard solution whose color is closest to that of the sample.

If the color is ≥70 degrees, dilute the sample appropriately with optically pure water (3.2.1) to bring the color into the range of the standard solution before measurement.

Measure the pH value of another sample.

3.6 Representation of Results

Report the value of the standard solution closest to the sample in the international standard unit of color (3), accurate to 5 degrees in the range of 0–40 degrees (excluding 40 degrees). Accurate to 10 degrees in the range of 40–70 degrees.

Report the pH value at the same time as reporting the sample color.

The color intensity (A0) of the diluted sample, expressed in degrees, is calculated using the following formula:

Where: V1—Volume of the diluted sample, mL;

V0—Volume of the sample before dilution, mL;

A1—Observed value of the color intensity of the diluted sample, degrees.