Anions in aqueous precipitation - Determination of Anions in Natural and Treated Waters - page 432

5.1.4.2 Ion selective electrode
Warner and Bressan [16] carried out direct measurements of less than 1µg L −1 fluoride in
rain, fog and aerosols with an Orion lanthanum fluoride ion selective electrode.
Fucsko et al. [17] applied ion selective electrodes combined with a flow injection
system to the determination of fluoride in rainwater.
Slanina [2] has discussed the application of ion selective electrodes to the
determination of fluoride in rainwater. Detection limits of 0.005mg L −1

were achieved

with a relative accuracy of 3-5% using a sample volume of 0.5ml.

5.1.4.3 Ion chromatography
The application of this technique is discussed under multianion analysis in sections
5.1.14.3 and 12.4.1.

5.1.5 Formate

5.1.5.1 High performance liquid chromatography
Kieber et al. [18] have developed an enzymatic method to quantify formic acid to non
saline water samples at submicromolar concentrations. The method is based on the
oxidation of formate by formate dehydrogenase with corresponding reduction of β-
nicotinamide adenine dinucleotide (β-NAD + ) to reduced β-NAD + (β-NADH); β-NADH
is quantified by reversed-phase high performance liquid chromatography with
fluorometric detection. An important feature of this method is that the enzymatic reaction
occurs directly in aqueous media, even seawater, and does not require sample
pretreatment other than simple filtration. The reaction proceeds at room temperature at a
slightly alkaline pH (7.5-8.5) and is specific for formate with a detection limit of 0.5µM
( S/N =4) for a 200µL injection. The precision of the method was 4.6% relative standard
deviation ( n =6) for a 0.6µM standard addition of formate to Sargasso seawater. Average
recoveries of 2 ≤ M additions of formate to seawater were 103%. Intercalibraiton with a
Dionex ion chromatographic system showed an excellent agreement of 98%.
Concentrations of formate present in natural samples ranged from 0.4 to 10µM for Miami
rain.

5.1.6 Iodide

5.1.6.1 Spectrophotometric method
Sub-microgram elements of iodide in rainwater have been determined
spectrophotometrically [19] by its catalytic effect on the substitution reaction of the
mercury 11-4-(2-pyridyrazo) resorcinol complex with 1, 2-diaminocyclohexane
NN,N'N' -tetraacetic acid. The sample solution is first freed from cations by passage
through a column of Amberlite IR 120-B. The eluate is buffered to pH 9, then treated