Oceanography

4. Procedure

2 ml of NH₄CL solution is added to 100ml of filtered water sample. This mixture is allowed to pass through the cadmium column. Every drop coming out of the column should not take more than 1 second. When 40ml of the solution has been drained from the column, the remaining reduced solution is collected in a measuring cylinder. 25ml of the reduced solution is collected in a conical flask and 0.5ml of sulphanilamide is first added. After 3 minutes, 0.5ml of diamine solution is added to develop azo dye. The OD of the solution is then measured at 543nm. The washing of the column is done with dilute NH₄CL solutuion, which is prepared by diluting 50ml of ammonium chloride solution to 1 litre.

Preparation nitrate stock and working standard solutions

1.011 g of KNO₃ is dissolved in distilled water and is made up to 1000ml. 1ml of chloroform is added as a preservative. 1 ml of this solution contains 140 µg or 10 µg-at NO₃-N. Working solution is prepared from stock solution by diluting 1 ml of stock solution to 100ml. 1 ml of this solution contains 1.4µg or 1 µg-at NO₃-N. Different volumes of working solution are diluted to 1000ml to get the following concentrations.

1. 1 ml diluted to 1000ml = 0.1 µg- at N/l =1.4µgNO₃-N/l

2. 5 ml diluted to 1000ml = 0.5 µg- at N/l = 7µgNO₃-N/l

3. 10 ml diluted to 1000ml =1 µg- at N/l =14µgNO₃-N/l

4. 50 ml diluted to 1000ml = 5 µg- at N/l = 70µgNO₃-N/l

Formation of calibration curve

100 ml each of the above working solution is reduced in the cadmium column and the resultant nitrite is converted into azo dye with sulphanilamide and diamine solutions. The OD values of the different standards are tabulated. The values are plotted in a graph by treating OD values in ‘X’ axis and the concentrations in the ‘Y’ axis. The linear graph obtained is used to fix the concentration of the seawater sample of unknown nitrate concentration.