How can the accuracy of the alkalinity indicator endpoints be checked?

Document ID

Document ID TE113

Version

Version 4.0

Status

Status Published

Published Date

Published Date 01/19/2018
Question
How can the accuracy of the alkalinity indicator endpoints be checked?
Summary
Determining the accuracy of alkalinity endpoints using pH buffers
Answer
In the alkalinity titration methods, the sample is titrated with sulfuric acid to a colorimetric endpoint, using a liquid or powder indicator, corresponding to a specific pH. The endpoint can be confirmed using pH buffer solutions. The phenolphthalein alkalinity endpoint is confirmed with a 8.3 pH buffer. The total alkalinity endpoint is confirmed with a 4.5 pH buffer.

Listed below are the details for checking both the phenolphthalein and total alkalinity endpoints:
  • To confirm the phenolphthalein alkalinity end point, mix the contents of one Phenolphthalein Indicator Powder Pillow (or solution) and the contents of one pillow of the Buffer Powder Pillows, pH 8.30, 50 mL, pk/25 (Product # 89868) with 50 mL of deionized water in a 250-mL Erlenmeyer flask. The resulting color is the endpoint. The buffer solution without the indicator can be used to standardize a pH meter.
  • To confirm the total alkalinity endpoint, mix the contents of one Bromcresol Green-Methyl Red Indicator Powder Pillow (or solution) and the contents of one pillow of the Buffer Powder Pillows, pH 4.50, 50 mL, pk/25 (Product # 89568) with 50 mL of deionized water in a 250-mL Erlenmeyer flask. Titrate to a light pink color change.

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