343
Micronutrients in the Oceans
ln K NH 4 = –6285.33/T + 0.0001635 T – 0.25444 + (0.46532 – 123.7184/T) S
+ (–0.01992 + 3.17556/T) S
(8.23)
8.3.1 Determination of Nitrogen Compounds
Before looking at nitrate cycles in the oceans, it is of value to briefly mention the methods
used to determine NO 3
– , NO 2
– , and NH 4
+ . The inorganic forms of N are normally determined by colorimetric methods using an auto analyzer. NO 2
– is determined by treating
the water sample with a solution of sulfanilamide. The resultant diazonium ion is coupled
with N-(1 naphthyl)-ethylenediamine to give a pink azo dye. The absorbance is measured
at 543 nm with a spectrophotometer. The reactions are as follows:
NH 2 –C 6 H 4 –SO 2 NH 2 + NO 2
– + 2H + → + N≡N–C 6 H 4 –SO 2 NH 2 + H 2 O
(8.24)
Diazonium ion
Sulfanilamide
NH 2 CH 2 CH 2 NH–C 10 H 7 + + N≡N–C 6 H 4 –SO 2 NH 2 →
Naphthyl- ethylenediamine
Diazonium ion
­
→ NH 2 CH 2 CH 2 NH–C 10 H 6 N∙N–C 6 H 4 –SO 2 NH 2 + H +
(8.25)
Pink azo dye
PO 4 (µM)
0
1
2
3
4
Depth (m)
0
1000
2000
3000
4000
5000
6000
N. Atlantic
N. Pacific
Figure 8.4
Profile of phosphate in the Atlantic and Pacific Oceans.