Anions in non saline waters - Determination of Anions in Natural and Treated Waters - page 288

Pollution Laboratory [713]

2.93.1.4 Spectrophotometric titration

Askne [713] has described a photometric titration procedure for determining low
concentrations (2×10 −5 -20×10 −5 mol L −1 ) of sulphate in non saline waters by measuring
the intensity of the orange-coloured complex formed between thorin and excess barium
ions from a solution of barium perchlorate.
The thorin method is based on the fact that sulphate ions in the sample solution are
precipitated as barium sulphate through the addition of barium perchlorate via a burette.
After the whole amount of sulphate is precipitated, further additions of barium ions form
an orange-coloured complex with added thorin, the absorbance of which is measured at
520nm.
The titration is carried out in a medium comprising aqueous sample (20% by volume)-
acetone (80% by volume). By addition of perchloric acid, the pH is adjusted to between
2.5 and 4. The sample should be cation exchanged (Dowex 50W×850-100 mesh) before
titration to prevent cations from interfering by forming coloured complexes with thorin.
The titration can be carried out either manually using a microburette or automatically
using an autotitrator unit linked to the spectrophotometer (Fig. 2.48).

Table 2.47 Chromogenic reagent, for the spectrophotometric determination of sulphate

Chromophore λ

max
nm

Interferences Limit of

detection

Ref.

305 Slight with 500mg L −1 acetate
Cl, Br, I and 100mg L −1 of
PO 4 , NO 3 , F, Interference by
IO 3 , SO 3 , SeO 4

Excess 2-amino perimadine HCl
added, unused reagent estimated

4-120mg
L −1

[715]

Molybdosulphate complex micro 6(-
p -acetyl-phenylazol)

[716]

−2-amino perimadine FeSO 4 +
Complex

325-
360

0.1nm [717]

Thorin-thymolblue - Interference by humic acid - [718]

Thorin and thorium (measure
decrease in extinction)

410 - 0.2-0.5mg

L −1

[719]

Thorn 520 All cations interfere, HBO 3 ,

As, ClO 4 ,NO 3 , Cl, F, CN,
PO 4 , S do not interfere

0-8mg L −1 [720,
721]

Barium dimethyl 654 No interference - [722]

Sulphonzo(III) - Cl<2×10 −2 mol L −1

NO 3 <3×10 −2 mol L −1 PO 4 <