Anions in non saline waters - Determination of Anions in Natural and Treated Waters - page 195

2 Na 4 P 2 O 7

Pyrophosphate (as P) −0.000 −0.110 −1.696

2 (NaPO 3 ) 6 Hexametaphosphate

(as P)

−0.003 −0.110 −1.649

EDTA 5 Disodium

salt

0.000 −0.005 +0.011

EDTA 5 Disodium salt

2 KH 2 PO 4 Orthophosphate

(as P)

0.000 −0.099 −1.422

2 CrO 3

Cr(VI) +0.003 −0.112 −1.850

S 2-

1 Na + −0.00/5 −0.088 −0.482

S 2-

0.5 Na + −0.005 −0.041 −0.167

Alanine 1 Amino acid +0.003 +0.0008 +0.019

Cysteine 1 Cysteine

hydro-
choloride

−0.003 −0.055 −0.830

Acetate 2 Na + 0.000 - +0.011
a The braces denote the simultaneous presence of the indicated substances in the test solutions.
b If the other substances had no effect, the results would be expected (95% confidence) to lie
within the ranges 0.000±0.0005, 0140±0.007 and 2.100±0.068mg L −1 respectively.
Source: Reproduced with permission from the Royal Society of Chemistry [391]

of cadmium will produce an increase in the effective reduction capacity to which the
solution is exposed. That is to say interferences appear to decrease the surface activity an
effect that can be overcome by either prolonging the exposure time or increasing the
overall surface area of metal. Davison and Woof [391] studied only the effect of time.
For interference studies it was appropriate to use a reduction time, and an amount of
cadmium, that had been shown to be prone to interference from lake water. The results of
the interference studies is shown in Table 2.20. The common cations and anions do not
interfere, except hydrogen carbonate, and its effect is negligible below 50mg L −1 Of the
metal ions, only lead and copper interfered and then only at such high levels as would not
be found in unpolluted waters. When 2mg L −1 of copper were added the results tended to
be erratic. This is probably because of an inconsistent induced change in the redox
potential of reaction (1) above. Both silicate and phosphate ions, in their various forms,
interfered. In order to test whether the interference was due to the complexation of
cadmium ions, ethylenediamine-tetraacetic acid (EDTA) was added to complex the
cadmium ions preferentially. This addition had no effect and the EDTA itself did not
interfere. As expected, the strong oxidising agent chromium (VI) oxide did interfere in
the redox process. Sulphite ions have reducing properties, but the interference observed
was probably a result of the formation of cadmium sulphide which then blocked the
active sites. Of the organic compounds tested, only cysteine interfered.
In order to check whether the interfering compounds were affecting the reduction step