Oceanography

4. Procedure

The water sample is first filtered using a glass fibre (GF/c) filter paper. Normally 250ml of sample from coastal waters and 750ml of samples from offshore waters are filtered for this estimation. After the filtration, the filter paper is placed in a Kjedahl flask and 1 ml of the digestion mixture is added. Glass beads should also be added in this flask to avoid splashing of the digesting particulate matter during boiling. The digestion of this mixture is carried out for about 60 minutes to convert the particulate nitrogen to ammonium sulphate. After the digestion is over, 25 ml of distilled water is added to the digested organic matter. The filter paper remains are removed by centrifuging the diluted mixture. To this clear solution, 2% sodium hydroxide solution is added slowly till the pH of the solution is adjusted to 7. The final volume of the solution is made up to 100ml with ammonia—fre distilled water. Ammonia content of this solution is estimated following the procedure already described for the assessment of ammonia content of seawater. The particulate nitrogen content of the sample is calculated as follows:

Particulate nitrogen content of the seawater sample (µg- N/l) =

Ammonia-nitrogen content (µg-N) x 14000

Vol. of the sample filtered (ml)