First of all, it is clear that for the titration of polybasic acids, the accuracy is not high and the error may be large. For example, at the first metering point, all phosphoric acid is theoretically converted into sodium dihydrogen phosphate. In fact, only 99.7% of phosphoric acid has become sodium dihydrogen phosphate, 0.3% of phosphoric acid has not been reflected, and 0.3% of sodium dihydrogen phosphate has become sodium dihydrogen phosphate. Also at the second metering point, there is a similar situation. Therefore, if strictly required, polybasic acid phosphoric acid cannot be titrated step by step. However, if the requirement is not too high, even if the error is slightly large, the analysis requirement can be met, so phosphoric acid can also be titrated step by step.

The principle of indicator selection is to make the indicator discoloration point as consistent as possible with the measurement point, or to make the indicator discoloration range partially or completely within the titration jump range.

If 65438 0% error is allowed in the discriminant of polybasic acid distribution titration,

Then ckai ≥ 10- 10, kai/kai+ 1 ≥ 10 4.

From phosphoric acid titration to sodium dihydrogen phosphate, cka1>10, KA 1/KA2 > 10 4 can be titrated accurately without cross reaction. The metering point is pH=4.7, and methyl orange (color change point 3.4) is used as the metering point.

From sodium dihydrogen phosphate to disodium hydrogen phosphate, CK A2 > 10- 10, KA2/KA3 > 10 4 can be titrated accurately without cross reaction. The pH of the metering point is 9.8, with phenolphthalein (color change point 9. 1) as the indicator, and the color varies from colorless to pink.

From disodium hydrogen phosphate to sodium phosphate, because the acidity of disodium hydrogen phosphate is too weak, CK A2