Anions in non saline waters - Determination of Anions in Natural and Treated Waters - page 286

precipitation of sulphate ions with barium and the complexation of excess barium with
EDTA at pH 10.5. A calcium electrode was used to indicate the end-point to avoid ionic
exchange. The electrode was regularly reconditioned in slightly alkaline calcium
chloride. The principal interference came from phosphates, but they were seldom present
at high enough concentrations to cause serious errors. The relative error was 3% for
0.0001N sulphate and 0.66% for 0.01N.

2.93.1.2 Direct titration of sulphate with barium ions

Sulphate in non saline waters has been determined [705] by direct titration with 0.01N
barium cyanate in acetone-water medium to the carboxyarsenazo (if phosphates absent)
or nitchromazo (if phosphates present) end-point. Cations contained in non saline water
were found to interfere and must be removed by prior cation exchange of the sample. In
another method [706], sulphate is separated from interfering ions by passing the sample
through a column of KU-2 resin (H + form), then the column is washed with water, the
eluate is neutralised to the yellow colour of 4-nitrophenol with hydrochloric acid and two
drops of acid in excess are added. To this solution is added an equal volume of acetone
and 10 drops of 0.05% aqueous orthanilic K (3-(2-carboxyphenylazo)-6-(2-
sulphophenylazo) chromotropic acid), and sulphate is titrated with 0.02N barium chloride
to a colour change from blue-violet to pale blue.
To overcome interference due to calcium ions in non saline water in the determination
of sulphate, Akos-Szabo and Inczedy [707] first mixed the sample with a cation exchange
resin (H+ form). To an aliquot of the clear liquid was added an equal volume of 96%
ethanol and 30mg solid barium chloride. Sulphate was then titrated with 10m mol L −1
barium acetate solution using a radio frequency titration technique.
Savvin et al. [708] titrated sulphate in non saline water with standard barium chloride
using 3-(2-carboxyphenylazo)-6-(2-sulphophenylazo) chromotropic acid as
metallochromic indicator. Cations are first removed using a cation exchange column (H +
form) and the detection limit is 1mg L −1 .
A standard UK method [709] determines sulphate in waters by indirect titrimetry with
disodium ethylene diamine tetraacetic acid. In this method cations are replaced by
hydrogen ions. On passing the sample through a bed of a strong acid cation exchange
resin in the hydrogen form sulphate ions in the eluate are precipitated with excess barium
chloride. The pH is adjusted to 11-12 with ethanolamine and excess barium determined
by titration with standard 0.01M ethylene diamine tetracetic acid solution using o-cresol
phthalein screened with naphthol green as indicator. Down to 1.5mg L −1 sulphate can be
determined using a 100ml sample. Phosphate and chromate interfere but compensation
procedures are available. Standard deviations range from 0.36mg L −1 sulphate at the 19.6
mg L −1 level to 0.68 mg L −1 sulphate at the 98.4 mg L −1 sulphate level.

2.93.1.3 Direct titration with lead ions

Duvivier [710] titrated sulphate with standard lead nitrate solution in methanol medium
and monitored the reaction with a lead-selective electrode. There was a maximal error of
1% in measuring 19.2mg L −1 sulphate. Interference from carbonate ions could be