VPP 322: Aquatic Animal Diseases, Health Care and Management (1+1)

Chemical parameters - Salinity

SALINITY

• Salinity is the total amount of dissolved material in grams in one kilogram of sea water. It is usually estimated by adopting Harvey’s titration method (argentimetric method) or using a salinometer.

Principle

• The halogen compounds present in seawater (chloride, bromide and iodide) readily react with silver to give insoluble silver halides. Fluoride (a halogen ion) does not precipitate with silver. Hence, the total quantity of silver required to react with chloride, bromide and iodide is a true measure of the chlorinity of seawater. In this method, potassium chromate is used as the indicator. The main principle of this titrimetric method is that silver will not react with chromate till all the halogens (other than fluoride) present in seawater are converted into silver halides. Since silver chromate has a characteristic brownish red colour, the end point can be precisely noted.

Harvey ’s direct titration method

Reagents

• Silver nitrate solution
  • 27.25 of analar/GR/reagent grade silver nitrate is dissolved in a little distilled water and the volume is made up of 1 lit. In the absence of light and evaporation, this reagent is stable at least for one month.
• Potassium chromate solution
  • 8.0g of the salt is dissolved in 100 ml of distilled water. This reagent is stable for several months if it is kept in a refrigerator.

Procedure

• 10 ml of the water sample is taken in a conical flask and 1 ml of chromate solution is added to it. The entire sample now turns yellow. This solution is titrated against silver nitrate solution taken in a burette. The titration is stopped when the yellow colour vanishes with the formation of a brownish red colour. Values are then tabulated. The titration is repeated with fresh sample till concordant values are obtained.

Calculation

• Volume (ml) of silver nitrate used (at the time of end point) for the titration of 10 ml of water sample = Salinity (ppt) of the sample
• Salinity of the seawater sample may be expressed by giving the unit% or ppt (parts per thousand). For example , if 35 ml of silver nitrate is required for the titration, the salinity of the water sample will be 35 ppt.

Salinity corrections

• The following salinity corrections should be made for determining the exact salinity of the given seawater sample

  Salinity calculated (%)	Correction to be applied
  40	-0.15
  38	-0.08
  36	-0.03
  34	+0.03
  32	+0.07
  30	+0.11
  28	+0.15
  26	+0.17
  24	+0.20
  22	+0.22
  20	+0.23
  18	+0.23
  16	+0.23
  14	+0.20
  12	+0.19
  10	+0.16
  8	+0.15

• The salinity correction values are used mainly to rectify the errors caused by burettes and pipettes.