Anions in non saline waters - Determination of Anions in Natural and Treated Waters - page 109

under the spectrophotometer cover to position I (inverse). The CAL control is set to
position 2.0 and locked, and the Display Rotary Wwitch to Damp I. The reagents are fed
through the system. Reaction will occur, and iodine will be extracted into the carbon
tetrachloride, resulting in deflection of the recorder from full scale to the base line. The
base line is adjusted to read approximately 5 chart divisions and allowed to stabilise
(approximately 20min). A complete set of standards covering a concentration range from
0.01 to 0.20 mg L −1 is placed in the first positions of the sample tray followed by a blank.
Individual standards of different concentrations are placed in several positions of the
remainder of the tray. The oxalic acid solution (SO 4 H 2 C 2 O 4 .2H 2 OL −1 ) is placed in the
38th position, followed by two blanks. The remainder of the tray is filled with unknown
samples and the analysis begun.
A calibration graph is prepared by plotting the height of each standard peak against its
respective bromide concentration. With the CAL control in position 2,0, the peak height
reading for 0.20mg L −1 of bromide using a Technicon recorder was about 30 chart units.
Calculate the bromide concentration of each sample by comparing its peak height with
the calibration graph. Any baseline drift that may occur must be taken into account when
calculating the height of a sample or standard peak.
Spiking experiments carried out by this method gave recoveries in the range 94-110%.
A method based on the use of methyl orange [123] involves conversion of bromide to
bromate, catalysed decomposition of bromate in acetic acid solution and
spectrophotometric determination of the reduction in the optical density of methyl orange
by the bromide.
Various other spectrophotometric methods have been described for the determination
of low concentrations of bromide in non saline waters. These include methods based on
the formation of coloured products with Chromotrope 2B (CI Acid Red), acid [119]
roseaniline [124] (0.4mg L −1 ), phenol red [125,126] (200µg L −1 ) and o -toluidine [127]
(detection limits in parentheses). A further method is based on oxidation of bromide to
bromate by hypochlorite and subsequent decoloration of methyl orange by bromine
formed by interaction of the bromate with bromide [128]. This method will detect 8µg
L −1 bromide in non saline water.
Yonehara et al. [129] developed a kinetic-spectrophotometric method for determining
bromide (0.004-0.3mg L −1 in non saline water) based on its catalysis of the oxidation of
pyrocatechol violet by hydrogen peroxide in hydrochloric acid/sulphuric acid. They state
that the effect of bromide is greatly increased in the presence of large amounts of
chloride, but that most ions commonly occurring in non saline waters do not interfere
except for iodide. The relative standard deviations ( n =10) were 6.4% for 0.034mg of
bromide L −1 and 13% for 0.010mg of bromide L −1 . These workers have also reported a
flow injection spectrophotometric determination of bromide, based on the same reaction.
In this method the sampling rate employed was 45 samples/h with 500µL injection, the
detection limit was 10µg L −1 and relative standard deviations were 2.5% and 1.4% for
20µg L −1 of bromide L −1 and 105µg of bromide L −1 respectively.
2.12.3 Spectrofluorometric method
Bromide has been determined by a method based on the quenching by bromine of the