Anions in non saline waters - Determination of Anions in Natural and Treated Waters - page 201

400 0.202 6(10) 21(9) 22
600 0.209 11(10) 26(9) 28
800 0.395 18(10) 33(9) 38
1000 0.474 13(9) 27(8) 30
a The degree of freedom for within- (S w ) and between batch (S b ) standard deviations are given in
parentheses; S t is the total standard deviation
Source: Reproduced with permission from the Royal Society of Chemistry [400]
intercept was considered to be the presence of an excess hydrazine, reducing nitrate to
nitrite prior to diazotisation. The concentration of hydrazine had been set at 190mg L −1 in
the reduction step. However it was decided to reoptimise this parameter. A hydrazine
concentration of 75mg L −1 in the reduction stage was then used in the optimisation of the
other parameters. The calibration, using the reoptimised concentration for the reduction
step, was now found to pass through zero.
The resulting method is linear up to 600µg L −1 of nitrate as N with a sensitivity of
0.05L absorbance unit per 100µg L −1 . Interpretation of the performance statistics given in
Table 2.25 shows a limit of detection of 14µg L −1 (95% confidence levels). A plot of the
concentration of total oxidised nitrogen, determined in a variety of non saline waters, vs
the concentration, measured for the same samples using the manual cadmium reduction
method [401] gave a good straight line ( r =0.967, n =107) passing through the origin.
Waughman [402] has described a procedure for determining nitrate in agricultural and
environmental survey waters. Nitrate in the sample solution is reduced to ammonia by
titanous sulphate and the ammonia is then released from the solution and diffused and
absorbed onto a nylon square impregnated with dilute sulphuric acid. The nylon is then
put into a solution which colours quantitatively when ammonia is present and a
spectrophotometer is used to measure the colour.
Reduction of nitrate in nitrite by cadmium or copperised cadmium followed by
estimation of the nitrite produced by production of a diazo compound and
spectrophotometric estimation is the most commonly used method for determining
nitrate. It has the advantage of sensitivity, freedom from interference and automated
analysis.
Gauguch and Heath [403] have described a rapid manual cadmium reduction method
for nitrate determination in small volumes (5ml) of non saline waters. It involves the
preparation of test tubes as reaction tubes which can be used 5-10 times before
reactivation of the cadmium filings is necessary. Once the tubes are prepared it is
possible to batch-process 30 samples per hour without loss of sensitivity. However,
caution is required when applying this method to water with high salinities.
In this method cadmium filings were amalgamated with a 1% (w/v) solution of
mercuric chloride for 3min after which the mercuric chloric solution was decanted. The
filings were then washed several times with distilled water and stored in the dark under a
0.10mol L −1 ammonium chloride solution. Reaction tubes were prepared by adding one
small scoop (0.76±0.07g) of the amalgamated cadmium filings into 18×150mm
borosilicate glass test tubes. Rinse the reaction tubes three times with 0.10mol L −1