How do the alkalinity methods measure hydroxides, carbonates, and bicarbonates?

Document ID

Document ID TE112

Version

Version 8.0

Status

Status Published

Published Date

Published Date 01/08/2019
Question
How do the alkalinity methods measure hydroxides, carbonates, and bicarbonates?
Summary
Understanding the relationship between the contributors of alkalinity
Answer
Alkalinity is expressed as phenolphthalein alkalinity or total alkalinity. Phenolphthalein alkalinity measures the hydroxides and half the carbonates at a pH 8.3. Total alkalinity measures all carbonate, bicarbonate, and hydroxide alkalinity at a pH 4.5 (approximately).

Phenolphthalein alkalinity is measured by titration of the sample to an end point pH of 8.3 which corresponds to the end point of the phenolphthalein indicator. Total alkalinity is measured by titration of the sample to an endpoint pH of 4.5 corresponding to the end point of the bromcresol green-methyl red indicator. When doing a sequential titration to measure both phenolphthalein and total alkalinity, total alkalinity measurements include all of the titrant added from the beginning of the titration (including the titrant used to get to the Phenolphthalein end point). Phenolphthalein alkalinity is a subset of total alkalinity.

Individual concentrations of hydroxide, carbonate and bicarbonate can be estimated by the phenolphthalein and total alkalinity measurements and use of the "Alkalinity Relationship Table" in alkalinity titration methods 8203 or 8221.

Alkalinity, Phenolphthalein and Total Method 8203
Alkalinity, Phenolphthalein and Total Method 8221 BT



As an example, consider a theoretically pure solution of water and sodium hydroxide. All of the hydroxide would be measured by titration to a pH of 8.3. It would take very little additional acid to get to a pH of 4.5 since there are very little to no carbonates or bicarbonates (theoretically). Therefore the phenolphthalein alkalinity would equal the total alkalinity and all of it would be hydroxide.
 
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