Anions in seawater - Determination of Anions in Natural and Treated Waters - page 368

The first result in Table 3.6 is the value found directly. The next set of three results
shows that recovery of fluoride after deliberate addition to the spiked samples is
excellent. The fifth result is that obtained on an acidified sample. The sixth, seventh,
eighth and ninth sets of data show that the expected fluoride concentration is still
obtained after deliberate

Table 3.6 Determination of fluoride in seawater under a variety of conditions to
demonstrate the reliability of the method

Sample {F] found

(mg L −1 )

Conditions

Point Lonsdale,
Victoria,
Australia

1 1.40±0.05

1.35±0.05*

pH=8

1.30±0.05 1.5ppm spike added. [F] T determined=2.80±0.05ppm

2

1.40±0.05 4.0ppm spike added. [F] T determined=5.40±0.05ppm 3

1.30±0.05 8.6ppm spike added. [F] T determined=9.90±0.05ppm 4

5 1.28±0.05 pH 5.50 (HCl added)

6 1.36±0.05 5mg potassium alum added/100 ml seawater ≈ 2.7ppm

Al3 +

7 1.43±0.10 50mg potassium alum added/100ml seawater ≈ 27ppm

Al 3+ . Equilibration time of electrode very long

8 1.39±0.05

1.37±0.05*

50mg potassium alum and 3g ammonium citrate
added/100ml seawater

9 1.30±0.05 37mg ferric alum added to 100ml seawater and pH

adjusted to 3 by adding few drops of glacial acetic acid.
Soln ≈ 40ppn Fe 3+

10 1.37±0.05 50ml TISAB added/100ml seawater

11 1.28±0.10 Calculated via addition of TISAB and calibration vs.

synthetic seawater

Synthetic 0.0068±0.0008

0.0053±0.0008

pH5.8
pH4.3

*This determination was performed approximately three weeks after the first
Source: Reproduced with permission from the American Chemical Society [60]

addition of aluminium or iron in the form of their alums. Aluminium(III) and iron(III)
form very strong fluoride complexes [73] and, provided that sufficient time is allowed for
equilibration (as also noted by Baumann [67] for very low fluoride concentrations), total
fluoride could still be determined accurately by the method of standard additions even
though the artificial concentration levels of aluminium and iron were three orders of