Dissolve 0.50 g of the dihydrochloride in 500 ml of distilled water. Store the solution in a dark bottle. The solution should be renewed once a month or directly a strong brown colouration develops.
F. EXPERIMENTAL
PROCEDURE
1. Add 2.0 ml of concentrated ammonium chloride to the sample in the
74 A PRACTICAL HANDBOOK 'OF SEAWATER ANALYSIS
Erlenmeyer flask (Note a) . Mix the solution and pour about 5 ml onto the top of the column and allow it to pass through (Note b).
2. Add the remainder of the sample to the column and place the drained Erlenmeyer flask under the collection tube (Sect. C, above). When 40 ml has passed through the colunm, drain the collection tube into the flask, rinse the flask with this effluent, drain it and replace beneath the collection tube (Note c).
3. Collect a further 50 ml in the collection tube and rapidly empty it into the Erlenmeyer flask (Note d). The column will not be quite empty and should then be allowed to drain until flow ceases (Note e).
4. As soon as possible after reduction add 1.0 ml of sulphanilamide solution from an automatic pipette (Note f). Allow the reagent to react for a period greater than 2 min but not exceeding 8 min (Note g). Add 1.0 ml of naphthylethylenedi-amine solution and mix immediately. Between 10 min and 2 hr afterwards measure the extinction of the solution in a 1-cm cell against distilled water (Note h) using a wavelength of 5430 A. If the extinction is greater than about 1.25 (ca. 30 p,g-at N/liter) measure in a 0.5-cm cell, or pipette 25.0 ml of solution into a clean dry flask and add 25.0 ml of distilled water. Mix and remeasure the extinction and double this value for use in the formula below. If the extinction value is less than 0.1 in a 1-cm cell repeat the reading using a 10-cm cell and use the latter value for calculation purposes. For extinctions between 0.1 and 0.2 in a 1-cm cell, measure-ment in a 5-cm cell may be found convenient. Unless adjacent samples are known to have extinction values within about 25% of each other the absorptiometer cell should be rinsed with each new solution before filling.
5. Correct the observed extinction by that of a reagent blank (using 1-, 5-, or 10-cm cells as appropriate) obtained as described in Section G. Calculate the nitrate present from the expression:
tg-at N/liter = (corrected extinction X F) — 0.95 C
where C is the concentration of nitrite present in the sample in itg-at N/liter. (Note and Note j.)
NOTES
(a) Continual use of the column leads to deactivation, presumably because of the coating of metal particles by hydroxide or carbonate. The slight acidification of the sample by the addi-tion of ammonium chloride greatly slows the deactivation process and a well-made column should be capable of reducing at least 100 samples. The volume of the samples is not critical up to 5 ml.
(b) This small preliminary addition ensures that the liquid in the top part of the column has the composition of the sample. When the bulk of the sample is then added and some of the interstitial liquid in the top of the column gets mixed into the sample no error results. Other-wise if a sample were to follow, say, a blank, some dilutions could occur. This precaution is only necessary for work of the highest accuracy when the concentration of nitrate in consécutive samples varies greatly.
(c) The passage of a full 40 ml has been found necessary to flush the column completely from the preceding sample although (Note b) this precaution is only important when concentra-tions vary greatly from sample to sample. A maximum of 8-10 columns can be handled con-veniently at one time and, if prepared properly, their drainage times should be very similar.
11.6. REACTIVE NITRATE 75 The operator must learn to judge the necessary short delay between adding samples to each column in order that there should be time for rejecting the first aliquot of effluent.
(d) This 50-ml volume is not critical to a few milliliters and flasks need not be drained completely of the column-washings before collecting this fraction. Under the above conditions reduction is about 93% complete and temperature variations between 10 and 35 C have no effect.
(e) There is no need to wash columns between samples, but if the columns are not to be reused for 1-2 hr or longer 50 ml of dilute ammonium chloride should be poured into the top and allowed to pass through the system. The columns must be stored completely covered by liquid.
(f) Reduced nitrite solutions are probably stable for several hours but for safety, especially in hot weather, the analysis should not be delayed. The method is now identical with 11.7.
(g) Temperature is not critical at this stage provided that it falls in the range 15-30 C.
The diazotizing reaction requires 2 min for completion but undesirable side reactions and de-composition become significant after about 10 min.
(h) Complete colour development requires 10 min. The colour is then stable for at least 2 hr.
(i) With good columns nitrite is reduced to the extent of 5% so that a correction of 0.95 times the nitrite concentration of the sample (11.7) is made. With deactivated columns this fraction increases, becomes erratic, and can exceed 20%.
(j) Amounts of sulphide of up to 2 mg S2-/Iiter are said not to interfere with this method although the repeated analysis of such water will deactivate columns by the production of cadmium sulphide.
G. DETERMINATION OF BLANK 1. CELL-TO-CELL BLANKS
When both sample and distilled water cells are filled with distilled water the extinction of one against the other should be 0.000. Slight optical defects may produce a slightly positive or negative value. The water in the distilled water cell should be changed every day as marked turbidities can result even in distilled water if it remains in the cell for too long.
2. REAGENT BLANKS
The blank from reagents is barely significant when working with 1-cm cells but should be checked occasionally. It assumes considerable importance when a 10-cm cell is used. Ordinarily distilled water should be satisfactory but may contain an appreciable quantity of nitrate so, for use in the determination of reagent blanks when small amounts of nitrate are being determined, the distilled water should be redistilled from a little alkaline permanganate, rejecting the first few milliliters of distillate. Such water is assumed to contain no nitrate.
Carry out the method exactly as described in Section F paragraphs 1-4 using the 1-, 5-, or 10-cm cells that are appropriate. Add the concentrated ammonium chloride solution to 100 ml of redistilled water in a clean Erlenmeyer flask and use a column previously flushed with at least 50 ml of dilute ammonium chloride solution just before use. The blank extinction corrected by any cell-to-cell blank should not exceed about 0.1 using a 10-cm cell.
3. TURBIDITY BLANK
If the nitrate is sufficiently low to warrant the use of a 10-cm cell a check should.be kept on turbidity and samples must be filtered before analysis if the turbidity extinction is appreciable.
76 A PRAC'TICAL HANDBOOK OF SEAWATER ANALYSIS
H. CALIBRATION
As
there isa
slight salt effectin
this method, calibration should be carriedout
using synthetic sea wateror
natural sea water witha nitrate concentration
less than 1kg
-at N/liter. When analyzingfor
very smallconcentrations of nitrate
(say less than 0.5 ,ug-at N/liter)the factor
may be determinedon
"spiked" sea waterbut
evidence atthe moment points
to this being unnecessary.The
concentration—extinction relationship is strictly linear
and the factor
need therefore be obtained at onlyone
levelof nitrate concentration. As a
safeguard against column deactivationa standard
should beput
through each column atthe commencement of
each day's work. There should generally beno
significant difference betweenthe factors
obtainedfor
each columnbut the
accumulative meanfor
eachone
can be used,if
desired, to minimize any errors that wouldarise
should there be slight differences.1. SYNTHETIC SEA WATER