Anions in sewage effluent - Determination of Anions in Natural and Treated Waters - page 573

9.3 Cyanide (free)
9.3.1 Spectrophotometric method
Kodura and Lada [4] determined cyanide in sewage spectrophotometrically using ferroin.
Iron, copper sulphide, acrylonitrile, phenol, methanol, formaldehyde, urea, thiourea,
caprolactam or hexamine did not interfere at concentrations up to 1g L −1 .
9.3.2 Atomic absorption spectrometry
In an indirect atomic absorption procedure [5] for determining down to 20µM cyanide the
sample is treated with sodium carbonate and a known excess of cupric sulphate to
precipitate cupric cyanide. Excess copper in the filtrate is then determined by atomic
absorption spectrometry and hence the cyanide content of the sample calculated. Iron(III)
does not interfere except at low cyanide concentrations. Quantitative recovery of cyanide
from sewage was obtained by this procedure.
9.3.3 Miscellaneous
See section 14.12.6.1.
9.4 Fluoride
9.4.1 Spectrophotometric method
Devine and Partington [6] have shown that errors in the determination of fluoride in
sewage by the SPADNS colorimetric method are due to sulphate carried over during the
preliminary distillation step. It is suggested that the colorimetric method should be
replaced by the fluoride ion electrode method following distillation.
9.4.2 Ion selective electrodes
Rea [7] used an ion selective electrode to determine fluoride in sewage sludge—a
trisodium nitrate buffer at pH 5.4 was used. Calibration was achieved by the standard
addition procedure.
S tandard deviations achieved range from 17-27mg kg −1 at the 244mg kg −1 fluoride
level for low sludges to 830-2100mg kg −1 at the 32300mg kg −1 fluoride level for high
sludges. Interference effects are negligible as indicated in Table 9.1.