Water Quality Color Determination – Dilution Ratio Method

1. Subject Matter and Scope

This standard specifies two methods for color determination. 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 Platinum-Cobalt Colorimetric Method: This 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 Dilution Method: This 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 hues 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

The 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 Colorimetric Intensity, 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. Dilution Method

3.1 Principle

The intensity of the color is expressed as a multiple (dosage factor) when the sample is diluted with optically pure water (platinum-cobalt colorimetric method) to a point where the color is just barely visible when compared to optically pure water.

Simultaneously, the sample is visually observed to examine the color properties: the depth of color (colorless, light, or dark), hue (red, orange, yellow, green, blue, and purple, etc.), and, if possible, the transparency of the sample (transparent, cloudy, or opaque). These are described in words.

Results are expressed using a combination of dilution factors and textual descriptions.

3.2 Reagents

3.2.1 Optically pure water (platinum-cobalt colorimetric method).

3.3 Instruments

3.3.1 Commonly used laboratory instruments, including stoppered colorimetric tubes (platinum-cobalt colorimetric method) and a pH meter (platinum-cobalt colorimetric method).

3.4 Sampling and Samples

Same as platinum-cobalt colorimetric method.

3.5 Procedure

3.5.1 Sample

Same as platinum-cobalt colorimetric method.

3.5.2 Determination

Take the sample (platinum-cobalt colorimetric method) and optically pure water (platinum-cobalt colorimetric method) into stoppered colorimetric tubes, fill them to the mark, and 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, compare the sample and optically pure water, and describe the chromaticity and color tone of the sample, including transparency if possible.

Dilute the sample stepwise with optically pure water to different concentrations, placing each concentration in a stoppered colorimetric tube and filling it to the mark. Place the stoppered colorimetric tube on a white surface and compare it with optically pure water using the same method described above. Dilute the sample until it is just indistinguishable from optically pure water, and record the dilution factor at this point.

Dilution method: For samples with a color intensity greater than 50 times, pipette the sample into a volumetric flask and dilute to the mark with optically pure water, using the largest possible dilution ratio each time to ensure the diluted color intensity is within 50 times.

For samples with a color intensity less than 50 times, take 25 mL of sample in a stoppered colorimetric tube and dilute to the mark with optically pure water, using a dilution factor of 2 each time.

For samples or samples diluted to a very low color intensity, pour an appropriate amount of sample from a stoppered colorimetric tube into a graduated cylinder and measure it, then dilute to the mark with optically pure water, using a dilution factor less than 2 each time. Record the dilution factor values for each dilution.

Separately measure the pH value of the sample.

4. Representation of Results

Multiply the successive dilution factors and round the product to the nearest integer to express the sample's colorimetry.

Also, describe the sample's color intensity and hue in words, including transparency if possible.

Report the pH value along with the sample's colorimetry.