The chemical reactions indicate that fluosilicic acid has the largest effect on acidity, contributing one mole of hydrogen ions for every mole of fluoride. Sodium fluoride produces no theoretical change in pH. The remaining chemicals all contribute two-thirds of a mole of hydrogen ions per mole of fluoride and thus should produce two-thirds of the change in pH and acidity produced by fluosilicic acid in an unbuffered water.
Tests were conducted in the Portland Water Bureau laboratory at the Bull Run headworks to establish the actual pH change occurring in fluoridation. Fluoride was added at a rate of 1 milligram per liter using fluosilicic acid. Changes in pH that occurred are summarized in Table 3-7. An average pH of 0.68 was observed in seven separate observations, each at a dose of 1 milligram fluoride per liter. Anticipated changes in pH due to the addition of other fluoride chemicals can be calculated according to the theoretical hydrogen ion contributions of Table 3-6. For example, sodium silicofluoride would be expected to produce a pH change of 0.45.
| Table 3-7 Measured pH Compensation Requirements |
|||||
|---|---|---|---|---|---|
| Date | F (mg/L) |
Initial/ (final) pH |
change in pHa |
NaOH required (mg/L) |
Na2CO3 required (mg/L) |
| 4/10/78 | 1.0 | 7.2 (6.4) | 0.8 | 1.7 | 4.3 |
| 5/11/78 | 1.0 | 7.05 (6.25) | 0.8 | 2.2 | 4.5 |
| 6/28/78 | 1.0 | 7.10 (6.4) | 0.7 | 2.0 | 4.6 |
| 9/14/78 | 1.0 | 7.2 (6.55) | 0.65 | 1.89 | 4.21 |
| 10/13/78 | 1.0 | 7.21 (6.55) | 0.66 | 2.1 | 4.6 |
| 11/22/78 | 1.0 | 7.32 (6.81) | 0.51 | 1.6 | 3.6 |
| 1/19/79 | 1.0 | 7.16 (6.55) | 0.61 | 2.7 | 5.91 |
| Average | 1.0 | 7.18 (6.50) | 0.68 | 2.03 | 4.53 |
aFluoride source used was H2SiF6. Change in pH produced by Na2SiF6 would be 2/3 of listed values.
The amount of basic chemical required to neutralize the pH reduction from fluoride addition was also determined in experiments at the Bull Run headworks. Requirements for addition of caustic soda (NaOH) and soda ash (Na2CO3) were observed to be
aData tabulated are from Portland Water Bureau Bull Run Laboratory Tests.
Source: "Fluoridation Feasibility Study," City of Portland, Department of Public Utilities, Bureau of Water Works, Francis J. Ivancie, Commissioner, February, 1979. Report contracted to Brown and Caldwell, Consulting Engineers, 100 West Harrison Street, Seattle, Washington 98119.