D. SAMPLING PROCEDURE
2. SYNTHETIC SEA WATER
Dissolve 25 g of analytical reagent quality sodium chloride and 8 g of mag-nesium sulphate heptahydrate, MgSO 4 • 7H20, in each liter of distilled water.
3. PROCEDURE
The evaporation and fuming procedure described in this method result in a slight, but reproducible, loss of phosphorus which may amount to several per cent.
If fuming is taken to dryness the loss approaches 15%. The explanation for this is obscure but it appears as if some phosphoric acid may co-distill with perchloric acid vapour and a certain fraction of the phosphate is condensed to polyphosphates which do not completely hydrolyse in ammonia solution and which do not react with molybdate. For safety, therefore, it is best to calibrate each batch of samples by taking standards throughout the entire method.
Add 50 ml of synthetic sea water to each of five flasks. Reserve two flasks for blank determinations and to each of the three others add 5.0 ml of the dilute phosphate solution (equivalent to 3 pg-at P/liter in 50 ml). Add 8.0 ml of perchloric acid solution to each flask and carry these standards and blanks through the full method as described in Section F, paragraphs 2-7, inclusive.
Calculate the factor F from the expression:
F 3.00
=
where Es is the mean extinction of the three standards and E, is the mean extinction of the two blanks (not corrected for cell-to-cell blanks).
11.4. DETERMINATION OF INORGANIC POLYPHOSPHATE INTRODUCTION
This procedure is based on the method of Strickland and Solorzano (Limnol.
Oceanogr. 13: 515, 1968). It relies on the fact that organic phosphate can be converted to orthophosphate without the hydrolysis of polyphosphates by the action of ultra-violet radiation (Armstrong, Williams, and Strickland, Nature, 211: 481, 1966) and then any polyphosphate can be converted to orthophosphate by acid hydrolysis.
The method measures inorganic and organic polyphosphates.
METHOD
A. CAPABILITIES
The range and precision of this method have not been determined in detail but appear to be little different from those given for 11.2.1.
B. OUTLINE OF METHOD
The filtered sample is exposed to short wavelength ultraviolet radiation until all organically combined phosphate is destroyed and a determination of ortho-phosphate is then made on an aliquot. The remainder of the sample is heated with acid until all polyphosphates are hydrolysed and the orthophosphate is again deter-mined. The difference between these two results measures the polyphosphate in the sample.
C. SPECIAL APPARATUS AND EQUIPMENT
Use 50-ml capacity stoppered graduated glass measuring cylinders.
Quartz tubes and Uv lamp equipment as described in the NOTES ON APPARATUS section. Temperatures should be kept below 70 C in the tubes.
D. SAMPLING PROCEDURE
See II.2.I,D.
E. SPECIAL REAGENTS
1. AMMONIUM MOLYBDATE SOLUTION Prepare as described in II.2.I,E.1.
2. SULPHURIC ACID SOLUTION Prepare as described in 11.2.1,E.2.
3. ASCORBIC ACID SOLUTION Prepare as described in II.2.I,E.3.
4. POTASSIUM ANTIMONYL-TARTRATE SOLUTION Prepare as described in II.2.I,E.4.
5. MIXED REAGENT
Prepare as described in 11.2.1,E.5.
63
64 A PRACTICAL HANDBOOK OF SEAWATER ANALYSI S 6 . HYDROCHLORIC ACID SOLUTIO N
Add 300 ml of distilled water to 200 ml of concentrated hydrochloric acid (sp gr 1 .18) . Store in a glass bottle .
7 . HYDROGEN PEROXID E
Use the best "30 per cent" analytical reagent quality solution .
F . EXPERIMENTAL PROCEDUR E
1 . Add about 105 ml of sample to a quartz irradiation tube after first rinsing the tube with a little sample . Add 1-2 drops of hydrogen peroxide, mix, and irradiate for 1-1 .5 hr (Note a) . Cool the tube and contents to room temperature .
2 . Rinse a clean 50-ml stoppered glass graduated cylinder with 1 or 2 ml of the irradiated water and then fill the cylinder to the 50-ml mark .
3 . To the remaining water in the tube add, with a bulb pipette, 1 .0 ml of hydrochloric acid solution . Mix and place the quartz tube in a boiling water bath for 2 hr (Note b) .
4. Cool the tube and contents to room temperature, use 1 or 2 ml to rinse a clean 50-ml stoppered glass graduated cylinder and then fill the cylinder to the 50 ml mark .
5 . Add 1 .0 ml of hydrochloric acid to the first cylinder, containing the irradi-ated but unhydrolysed sample (Note c), and then to both cylinders add 5 ml of mixed reagent from a 5 ml measuring cylinder, invert the cylinders two or three times to mix and set aside for at least 5 min .
6 . Continue determinations exactly as described in 11 .2 .1,17 .3 . If E, is the extinction of the irradiated and hydrolysed sample and E2 is the extinction of the irradiated sample ,
µg-at polyphosphate-P/liter = (El - EZ)F X 1 .02 where F is a factor obtained as described in II .2 .I,H (Note d) .
NOTES
(a) This treatment destroys all phosphorus-containing organic molecules and liberate s orthophosphate . Organic or inorganic polyphosphate linkages are not broken at seawater pH, provided that temperatures do not exceed 60-70 C .
(b) This treatment hydrolyses tripolyphosphate and linear polyphosphate glasses with up to 70 atoms of phosphorus, to give orthophosphate . The time is not critical and could possibly be shortened but 2 hr gives a safe minimum. If hydrolysis is undertaken in the quartz tubes fewer contamination and transfer errors are likely .
(c) This allows fc.° any blank that may be introduced by the hydrochloric acid . (d) The value for F is the same for this mct_ od as for the reactive phosphate method and need not be redetermined . A factor of 1 .02 allows for the addition of hydrochloric acid . G . DETERMINATION OF BLANK S
This is a "difference" method and has no blank correction (see Note c) .
H . CALIBRATION
See II .2 .I,H (Note d) .
11.5. DETERMINATION OF REACTIVE SILICATE