Anions in potable waters - Determination of Anions in Natural and Treated Waters - page 489

concentration of the analyte adjusted by appropriate dilutions or additions. The
concentration restriction applies also to rinsing of electrodes and their storage. The
electrodes are stored in a buffer containing fluoride at a concentration corresponding to
the lower limit of the fluoride concentrations measured.
Between immersions, the electrode stems are wiped and the electrode tips gently
blotted with tissue paper to remove adhering liquid drops. The electrodes are not rinsed
with water or a buffer solution. The electrodes are first immersed in 40ml of the solution
to be measured (analyte or reference solution) which is agitated for 1min and discarded.
A fresh 40ml portion of the solution is then used for electropotential measurement.
The precision of the electrode potential measurement by this procedure depends mainly
on temperature constancy. The standard deviation of

Table 7.6 Sequential programme of fluoride determination

External standard
method

Analyte addition method

Electrode
A

Electrode
B

Electrode
A

Electrode
B

F − concn (mg of F−L −1 ) of
standardising solution used
for electrode storage

0.02 0.10 0.02 0.10

Sequential programme for F − concentration
determination

(I) Conditioning concn, mg of
F−L −1 time, min

0.01 0.05 0.01 0.05

3 3 3 3

(II) Reference solution concn,
mg of F−L −1 time, min

0.02 0.10 0.025 0.10

1+10 1+10 1+10 1+10

1+10 a 1+10 a

(III) Analyte time, min record mV 1 0m

in after adding
analyte

record mV 10min
after adding
analyte

Concentration range of F −
determined

Lower limit, mg of F−L −1 0.02 b 0.10 b

0.025 0.10

Higher limit, mg of F−L −1 0.10 c 1.0 c

depends on analyte volume added

a 1min in 40ml of solution, discard solution, 10min in another 40ml of solution, record mV
reading
b If F − concentration of the analyte is below this limit, add NaFi solution to increase F −
concentration