Analytical Procedures - Acid Colour
February 11, 2002

Introduction

The measurement of the colour of a liquid sample does not provide any information concerning the nature of the substance producing that colour.  An arbitrary standard scale is used as a means for matching of colour intensity.  A colour of 5 units means that the intensity of the colour of this sample is equal to the intensity of the colour of a sample of sulphuric acid 50% in water, containing 5 milligrams of platinum (as potassium chloroplatinate) per liter.

The true colour of a sample is due only to soluble substances and its accurate determination where suspended matter is present is practically impossible.  Filtration should no tbe used because of the decolourizing action.  The best results can be obtained by centrifuging prior to colour observation.  Photometers are not recommended for the determination of colour, because of interference from turbidity.

Equipment

• Matching tubes

Reagents

Potassium chloro platinate solution.  (Dissolve 1.246 g. of grade reagent and 1.00 g. of cobalt chloride (CoCl2) in water, add 100 ml of HCl and dilute to 1 liter. This solution has the conventional value of 500 units, according to APHA standard - American Public Health Association).

Procedure

• Dilute 1+1 the acid sample to be tested with distilled water and cool it.

• Fill one of the matching tubes to the mark with the prepared sample and place in the middle compartment of a base.

• Fill other two matching tubes on either side to the mark with colourless acid diluted 1+1 with water and add the proper quantity of chloro platinate solution on each tube to match the sample colour.

• Place the tubes on the base so that the matching tubes point toward a source of a day-light.

• Compare the colour of the sample with the colours of each side tube.  The colour value is read directly from the comparison of standard chloro platinate solutions.