SOP FOR DETERMINATION OF COLOR IN WATER

METHOD : SINGLE WAVWLENGTH BY SPECTROPHOTOMETRIC 

Principle

Colour is determined spectrophotometrically at a wavelength between 450 and 465 nm, with platinum cobalt solutions as standards. True colour of samples and platinum cobalt standards follow Beer’s Law.

Interferences

The primary interference is from colloidal suspended particles that scatter light at the wavelength of the spectrophotometric method. Hence, it is mandatory to remove suspended particles for this method.

pH affects the light absorbance of organic matter in solution and also affects the solubility of substances. Preferably, do not adjust sample pH as long as it is between 4 and 10. If pH is adjusted, bring it to 7.0 and note the adjustment.

Apparatus

Spectrophotometer — The spectrophotometer shall be capable of operating between 450 and 465 nm with lowest count for adjustable wavelength of 1 nm. Matched glass cells with path lengths of 25 mm and above shall be used. Beer’s law allows flexibility in selecting cell path length.

pH Meter — For sample pH measurement.

Filter and Filter Assembly

Reagents

4.1 Reagent Grade Water, conforming to IS 1070.

4.2 Potassium Chloroplatinate, K₂PtCl₆, analytical grade.

4.3 Cobaltous chloride, CoCl₂.6H₂O, analytical grade.

4.4 Hydrochloric Acid, HCl, conforming to analytical reagent grade of IS 265.

4.5 Sodium Hydroxide, NaOH, conforming to IS 376.

Preparation of Standards

Standard chloroplatinate Stock Solution

Dissolve 1.246 g of potassium chloroplatinate and 1.00 g crystalized cobaltous chloride in water containing 100 ml concentrated HCl and dilute to 1000 ml with distilled water. This stock solution is equivalent to 500 colour units (500 CU).

Prepare standards having CU of 1, 2, 5, 10, 15, 20, 25, 30, 40, 50, 60 and 100 by diluting 0.2, 0.4, 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 8.0, 10.0, 12.0 and 20.0 ml stock standard chloroplatinate solution with water to a volume of 100 ml. Transfer to nessler tubes for use as standards.

Protect these standards against evaporation and contamination by use of clean inert stoppers. Keep them in the dark when not in use and do not keep for more than 1 month. The standards should also be protected against absorption of ammonia, which causes increase in colour.

Procedure (By Spectrophotometric Method)

Preparation of Standard Curve

• Choose a wavelength between 450 to 465 nm to develop the standard curve as the absorbance of Pt-Co has a broad maximum absorbance within this wavelength range.
• Fill one cell with distilled water to adjust absorbance to zero in the instrument. Then read absorbance of each colour standard and prepare a standard curve of CU versus absorbance.

Spectrophotometer Measurement

• Use the same wavelength and same length of glass cell used for developing the standard curve. Fill one spectrophotometer cell with distilled water to adjust absorbance to zero in the instrument.
• Rinse the other cell with sample and then fill up. Place cell in spectrophotometer and read absorbance.
• Determine sample colour using absorbance readings and standard curve.

Safety Guidelines

Proper safety & health precaution are taken while doing the analysis by using safety goggles,        apron, safety shoes & using fume chamber for fume generating chemicals.

Reference

IS 3025 (Part 4) : 2021