Anions in non saline waters - Determination of Anions in Natural and Treated Waters - page 96

A large number of fluorometric reactions are known for the determination of borate
anion. Some of them involve its previous distillation as methylborate [46,47] to isolate it
from interfering species.
In spite of the fact that benzoin is a widely used fluorometric reagent for the
determination of boron, special precautions must be taken with this method as benzoin in
alkaline solution is oxidised by the oxygen of the air and nitrogen must be bubbled
through the solution [48]. Also, benzoin undergoes photodecomposition when exposed to
radiation.
2-hydroxy-4-methoxy-4 ′ -chlorobenzophenone is a sensitive fluorometric reagent for
borate and has been applied satisfactorily in the

Table 2.2 Effect of foreign ions on boron determination

Foreign ion Maximum concentration tested without giving

interference mol L −1

Spectrophotometric method a Fluorometric method b

Cl −

7.2 7.2

SO 4 2−

2.5 2.5

NH 4+

1.0 0.8

Na +

0.2 0.2

K + , Ca 2+ , Al 3-

0.5 0.4

NO 3 − , NO 3 − , Cr 3+

0.2 0.2

Mg 2+ , HCO 3 − , CO 3 2-

0.05 0.05

Mn 2+

0.05 0.01

F −

0.02 0.01

Cu 2+ , Zn 2+ · Ni 2+ 4×10 -3 4×10 -3

Sr 2+ , PO 4 3− , SIO 3 2− , Br − 3×10 -3 3×10 -3

Fe 3+ 7×10 -5c 7×10 -5c

Fe 3+ (by elimination with three
10ml IBMK washes)

0.1 0.01

a Determination of 58.3µg of boron by the spectrophotometric method.
b Determination of 2.38µg of boron by the fluorometric method.
c Tolerance limit (M) as the concentration level at which the interferent causes an error of not
more than ±2% (spectrophotometric method) or ±3% (fluorometric method)
Source: Reproduced with permission from the Royal Society of Chemistry [23]

determination of this anion in several samples. Monier et al. [49] studied the action of 41
ions on the fluorescence of the complex and none interfered in the determination of
borate. The fluoride ion interfered only at ratios of fluorine to boron of 100:1 (m/m) or