Anions in non saline waters - Determination of Anions in Natural and Treated Waters - page 127

2.17.1 Titration methods
Three basic approaches have been made to the determination of chloride in water:

• Mohr titration;
• Mercuric nitrate and diphenyl carbazone titration;
• Technicon autoanalyser method (mercuric thiocyanate-ferric salt spectrophotometric
method).

The precision and bias of these three standard approaches to the determination of chloride
in river water has been evaluated [165,166] in trials carried out in a number of
laboratories in the UK. The following targets were set in this exercise:

• maximum tolerable bias −10% of the chloride concentration or 0.5% mg L −1 chloride,
whichever is the greater;
• maximum tolerable standard deviation −5% of the chloride concentration, or 0.25% mg
L −1 chloride, whichever is the greater.

The results were analysed statistically to provide estimates of the standard deviation of
individual analytical results in any batch of analyses. These estimates are summarised in
Table 2.6 which shows that all laboratories and methods achieved the target standard
deviation.
Portions of a standard chloride solution and two river water samples were analysed
once on each of five days. The results, summarised in Tables 2.6 and 2.7, show that the
confidence intervals of the mean results for a given solution do not all overlap, that is,
some inter-laboratory bias is present. To assess whether or not the bias of any laboratory
exceeded the target value, the following procedure was adopted. The mean result and its
90% confidence interval from laboratory i was denoted by X i ± L i and the true value of
the standard chloride solution or, for the two river waters, the overall mean of all
laboratories, was denoted by . The value of the maximum possible bias (95% confidence
level) was then calculated as:

Table 2.6 Results of precision tests

Lab. Standard solution

1

Standard
solution 2

River water' Spiked' river

water

SD a
(mg
CIL
−1 )

SD a
(mg
CIL
−1 )

SD a
(mg
CIL
−1 )

Analytical
method

Concn
(mg
CIL
−1 )

Rel
b
SD
%

Concn
(mg
CIL
−1 )

SD
a
(mg
Cl L

Relb
SD
%

Concn
(mg
CIL
−1 )

Relb
SD
%

Concn
(mg
CIL
−1 )

R

S