Anions in non saline waters - Determination of Anions in Natural and Treated Waters - page 205

2.62.2 Spectrofluorometric methods
All the fluorometric methods for determining nitrate ion are based on redox reactions.
These methods are not selective but they can be applied to the determination of parts per
billion levels of nitrate.

Table 2.27 Non cadmium reduction (ie direct) spectroscopic methods for the
determination of nitrate in non saline waters

D max nm Solvent extraction pre-
concentration

Chromogenic reagent Detection

limit

Ref.

<60µg L −1

Crystal violet - Chlorobenzene [415]

p -Fluorophenol - Touene - [416]

<20mg L −1

Tetraphenyl-phosphonium
chloride

269 Chloroform [417]

Salicylic acid - None - [418]

2.4-Xylenol - None - [406]

1µg L −1

9,9'bianthracene-10,
10'dione

500 None [419]

10µg L −1

Phenoldisulphonic acid - None [420,421]

10µg L −1

o -Toluidine - None [422]

6mg L −1

Indigo carmine - None [423,424]

Brucine 410 - - [425]

Chromotropic acid 420 - - [426]

50µg L −1

3,3'Dimethyl naphthidine - [427]

10µg L −1

Dihydroxy coumarin - Ethyl aceate [428]

Source: Own files

Fluorscein quenching method
Nitrate oxidises fluorescein in a concentrated sulphuric acid medium to form a non-
fluorescent product [429]. As fluorescein fluoresces and the reaction product does not,
nitrate can be determined by the measurement of the fluorescent suppression. The authors
did not determine the structure of the reaction product, but according to the Colour Index
[430] fluorescein reacts with nitrate in concentrated sulphuric acid to form Solvent
Orange 16 which has no fluorescence and is possibly the reaction product.