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In general the available toxicity studies with sodium carbonate were not conducted according to current standard guidelines. In many cases pH, buffer capacity and/or medium composition were not discussed in the publications, although this is essential information for toxicity tests with sodium carbonate. The main factor explaining the acute aquatic toxicity of sodium carbonate is most likely the increase of the pH.

The most appropriate parameter to assess the environmental effect of a sodium carbonate discharge is to determine the change in pH. To get an idea about the order of magnitude for acceptable anthropogenic additions, the acceptable sodium carbonate addition will be calculated for 2 representative cases. According to Directive 78/659/EEC, the pH of surface water for the protection of fish should be between 6 and 9. In section 5.1 it has been mentioned that the 10th-percentile and the 90th-percentile of the bicarbonate concentrations of 77 rivers were 20 and 195 mg/l, respectively. If it is assumed that only bicarbonate is responsible for the buffer capacity of the ecosystem and if it is assumed that an increase of the pH to a value of 9.0 would be the maximum accepted value then the acceptable anthropogenic addition of sodium carbonate would be 2.7 and 17 mg/L for bicarbonate concentrations of 20 and 195 mg/l, respectively (see Table 14). This gives an indication of the order of magnitude of the acceptable amount of sodium carbonate which could be discharged to an aquatic ecosystem if there was an emission of a pure sodium carbonate solution. Sodium carbonate concentrations of 2.7 and 17 mg/L are equivalent with the sodium concentrations of 1.2 and 7.4 mg/L. Sodium concentrations of 1.2 to 7.4 have no effect on aquatic organisms because sodium has a low toxicity for aquatic organisms. Reconstituted water of toxicity tests contains for example sodium concentrations which range between 3.3 and 105 mg/L.

Table 14: Concentration of sodium carbonate (mg/l) needed to increase the pH to values of 9.0, 10.0 and 11.0 (De Groot et al., 2002).

Buffer capacityA

Final pHB

 

9.0

10.0

11.0

0 mg/l HCO3 -

(distilled water)

1.1 (0.6)

16 (6.1)

603 (61)

20 mg/l HCO3-

(10th percentile of 77 rivers)

2.7 (21)

32 (26)

766 (81)

106 mg/l HCO3-

(mean value of 77 rivers)

9.7 (107)

102 (112)

1467 (167)

195 mg/l HCO3 -

(90th percentile of 77 rivers)

17 (196)

175 (201)

2192 (256)

A The initial pH of a bicarbonate solution with a concentration of 20 – 195 mg HCO3-/L is 8.3 (calculated).

B Between brackets the final concentration of bicarbonate is given.

[partly taken from OECD SIDS on sodium carbonate (2002), pg. 9 -10]