Anions in non saline waters - Determination of Anions in Natural and Treated Waters - page 93

number of samples, but better accuracy is achieved when the intervals are relatively
close.
The results obtained from the study of interference from fluoride, magnesium, calcium
and nitrate ions show clearly that there is no significant interference from any of these
ions.
Aznarez et al. [23] have described a sensitive spectrophotometric method based on the
curcumin complex. In this method the boron is extracted into isobutyl methyl ketone with
methylpentanediol.
The absorption spectrum of the boron-curcumin compound in isobutyl methyl ketone
obtained following the above procedure exhibits maximum absorbance at 510nm when
measured against a reagent blank solution, as shown in Fig. 2.3.
The calibration graph at 510nm obtained in the curcumin method is a straight line and
Beer's law is obeyed from 0.5 to 5µg mL −1 of boron in the final measured solution
(corresponding to 10-110µg of boron in the aqueous phase).
In this method 50ml of non saline water (containing 10-100µg boron) with
hydrochloric acid (1+1) is transferred to a 100ml calibrated flask and the solution
extracted with three 10ml volumes of methyl isobutyl ketone to eliminate iron
interference, 10ml of 20% volume methyl pentane diol in methyl isobutyl ketone is added
and the mixture shaken for 5min. The organic phase is then dried with 1g anyhdrous
sodium sulphate.
3ml of the organic phase is pipetted into a polyethylene test tube with a hermetic cap,
and 2ml of curcumin solution (0.1% w/v in glacial acetic acid), 2ml of concentrated
phosphoric acid are added. The sealed test tube is shaken for 2min and heated at 70±3°C
for 1h in a thermostated bath. After rapid external cooling to room temperature, the
absorbance of the solution is measured at 510nm against a reagent blank solution within
45min.
Prepare a calibration graph as follows. To different volumes of standard solution
containing 10-100µg of boron add an equal volume of hydrochloric acid (1+1) and
extract with 10ml of extraction solution according to the above extraction procedure.
Pipette 3ml of the organic phase and carry out the same spectrophotometric procedure.
The Department of the Environment UK [28] have discussed four alternative methods
for the determination of boron comprising the curcumin method, automated and manual
methods based on azomethine H, with measurements of absorbance of the complexes
formed, and a mannitol titration method suitable for a wide range of boron concentrations
from 0.5 to 1000mg L −1 . Interference by nitrate is discussed.
A further chromogenic reagent that has been used for the determination of borate is
azomethine [29-31] which forms a yellow complex with borate. Randow [30] evaluated
the Azomethine H method for several non saline water samples and also for model
solutions containing humic acids with standard amounts of boron added to the sample.
The effects of illumination, temperature and time on the colour development were
examined, together with other factors such as the initial colour of the sample and the
container materials. Over the temperature 12-24°C the absorbance maximum was
reached within 1h and remained stable for 1h afterwards. The sample colour intensity
necessitated a proportionate correction to the observed value. Recoveries of added boron
ranged from 86 to 104% for concentrations between 20 and 200µg L −1 . The limit point of