Oceanography

iii. Colorimetric standard sample

 From the working solution the following standards are prepared by making up the volume of the standard solution to 100ml. In five 100ml volumetric flasks 20ml, 16ml, 12ml, 8ml and 4 ml of the working solutions are added separately and the volume is then made up to 100ml. A blank is also run along with the standards. The concentration of sulphide in these standards will be as given below.

 20 ml is made up to 100 ml  = 100 µgS^2- = 31.2 µg at/l

 16 ml is made up to 100 ml = 80 µgS^2- = 25.0 µg at/l

 12 ml is made up to 100 ml = 60 µgS^2- = 18.7 µg at/l

  8 ml is made up to 100 ml = 40 µgS^2- = 12.5 µg at/l

  4 ml is made up to 100 ml = 20 µgS^2- = 6.3 µg at/l

 Blank made up to 100 ml   = 0 µgS^2- = 0.0 µg at/l (with no addition of working solution)

 The above standards and blank are individually siphoned out into 50ml volumetric flasks. One ml each of the dimethyl p-phenylene diamine solution and ferric chloride solution are added to each flask, flasks are stoppered, shaken and kept aside for 60 minutes. After the completion of the development of the blue colour, optical density value for each concentration is recorded at 670mm using a spectrophotometer. The value of OD are taken in the X axis and the concentrations are plotted in the Y axis. Then a linear graph is drawn. Using this standard graph, the concentration of hydrogen sulphide can be calculated if the OD value of the sample is known.