hydrolyze phosphorous to orthophosphate from whatever form in which it may occur in the sea and that ultraviolet light "oxidation" will destroy all organic phosphate bonds but not affect polyphosphate linkages. As mentioned in
NOTES ON APPARATUSthe definition of particulate material is somewhat arbitrary. A
0.5micron
pore-sizemembrane filter can be used but a glass filter paper is more convenient and probably nearly as good.
Symbol Description
1. I.S.R. (inorganic, Orthophosphate ion.
soluble, and
Symbol
46 A PRACTICAL HANDBOOK OF SEAWATER ANALYSIS
Description (polyphosphate) substances with a
condensed phosphate the former, especially in areas where there is
Determination solution in sea water but which becomes soluble and reacts with acid molybdate in 5 min. Such material includes calcium and ferric phosphates and any phosphate from plant either in living material or detritus, that is norreactive to acidified molybdate for a period of 5 min. A little of the phosphorus in living cells may be lost. Most
11.2. DETERMINATION OF PHOSPHORUS 11.2.1. DETERMINATION OF REACTIVE PHOSPHORUS
INTRODUCTION
All methods for phosphate in sea water rely on the formation of a phospho-molybdate complex and its subsequent reduction to highly coloured blue compounds.
Methods using stannous chloride as a reductant at room temperature have been favoured as they are most sensitive and give less interference from easily hydrolysable organic compounds than do other techniques. There are complexities in these methods due to interference from arsenic and to concealed blanks arising from the reduction of molybdate in sea water in the absence of phosphate. An excellent program of comparative tests has been described by Jones and Spencer (J. Marine Biol. Assoc. U.K., 43: 251, 1963).
The procedure given below is taken from the recent publication of Murphy and Riley (Anal. Chim. Acta, 27: 31, 1962) and is so superior to other methods in terms of the rapidity and ease of analysis that it probably represents the ultimate in sea-going techniques.
METHOD A. CAPABILITIES
Range: 0.03-5 ,ag-at/liter
1. PRECISION AT THE 3 p,G-AT/LITER LEVEL
The correct value lies in the range:
Mean of n determinations -±-0.03/n1 itg-at/liter.
2. PRECISION AT THE 0.3 MG-AT/LITER LEVEL
The correct value lies in the range:
Mean of n determinations -±0.02/n1 gg-at/liter.
3. LIMIT OF DETECTION
The smallest amount of phosphate that can be detected with certainty is about 0.03 itg-at Pater.
Reject duplicate determinations if extinction values differ by more than 0.02 in the extinction range 0.5-1.0 or more than 0.01 in the extinction range 0.1-0.5.
If the duplicate extinction values differ by less than the above limits, take a mean value.
B. OUTLINE OF METHOD
The seawater sample is allowed to react with a composite reagent containing molybdic acid, ascorbic acid, and trivalent antimony. The iesulting complex heter-opoly acid is reduced in situ to give a blue solution thé extinction of which is measured at 8850 A.
C. SPECIAL APPARATUS AND EQUIPMENT
130-ml capacity screw-capped polyethylene bottles marked on the side at 100 ml ( -±-2 ml) with a band of black tape.
49
50 A PRACTICAL HANDBOOK OF SEAWATER ANALYSIS
D. SAMPLING PROCEDURE AND SAMPLE STORAGE
130-ml polyethylene bottles should be filled completely with sample after rinsing them twice with the water to be analysed. The analysis should be, commenced as soon as possible, preferably within -fr hr, certainly before 2 hr. Samples should be kept in a cool dark place and not warmed to room temperature until the analysis is to be commenced. If the analysis has to be delayed for more than about 1 hr refrigerate the samples to 0 C or less. Quick freezing in a 40% glycol bath at —20 C (freeze to this temperature within 30 min of collection) stabilizes samples for many months and this technique should be used for the most precise work on samples drawn from the euphotic zones in tropical or sub-tropical waters. There is conflicting evidence concerning the loss of phosphate to polyethylene when samples are stored at room temperature but evidence indicates that this should be avoided (Hassen-teufel, Jagitsch, and Koczy, Limnol. Oceanog., 8: 152, 1963).
E. SPECIAL REAGENTS
1. AMMONIUM MOLYBDATE SOLUTION
Dissolve 15 g of analytical reagent quality ammonium paramolybdate (NH4 ), Mai O„ • 41120 (preferably finely crystalline), in 500 ml of distilled water.
Store in a plastic bottle out of direct sunlight. The solution is stable indefinitely.
2. SULPHURIC ACID SOLUTION
Add 140 ml of concentrated (sp gr 1.82) analytical reagent quality sulphuric acid to 900 ml of distilled water. Allow the solution to cool and store it in a glass bottle.
3. ASCORBIC ACID SOLUTION
Dissolve 27 g of good quality ascorbic acid in 500 ml of distilled water. Store the solution in a plastic bottle frozen solid in a freezer. Thaw for use and refreeze at once. The solution is then stable for many months but should not be kept for more than a week at laboratory temperatures.
4. POTASSIUM ANTIMONYL-TARTRATE SOLUTION
Dissolve 0.34 g of good quality potassium antimonyl-tartrate (tartar emetic) in 250 ml of water, warming if necessary. Store in a glass or plastic bottle. The solution is stable for many months.
5. MIXED REAGENT
Mix together 100 ml ammonium molybdate, 250 ml sulphuric acid, 100 ml ascorbic acid, and 50 ml potassium antimonyl-tartrate solutions. Prepare this reagent for use and discard any excess. Do not store for more than about 6 hr. The above quantity is suitable for about 50 samples.
F. EXPERIMENTAL
PROCEDURE
1. Warm the samples to a temperature between 15 and 30 C in a thermostated water bath (Note a) and measure the extinction of samples to obtain a turbidity correction (Note b; see also Sect. G).
11.2.1. REACTIVE PHOSPHORUS 51