What can be done to get better pH results in seawater?

Document ID

Document ID TE2772

Version

Version 2.0

Status

Status Published

Published Date

Published Date 27/05/2017
Question
What can be done to get better pH results in seawater?
Summary
Procedure to get better pH results in seawater.
Answer
The salinity of seawater can vary over a wide range, depending on the geography. Pacific, Atlantic, Baltic Sea, Mediterranean Sea etc. have different salinity values. The most typical value is 35 ppt, what is 35 g salt / kg water.
The pH calibration is usually done in a buffer solution with much lower ionic strength. For example; pH buffer 4.01 = 10.2 g/L, pH buffer 10.01 = 4.73 g/L. Seawater contains 3 to 6 times more salt than a pH buffer. This fact must be considered, when doing a pH calibration in normal pH buffers and then measuring high saline samples, like seawater. Experiments have shown that the pH measurement in saline water samples can be improved by adding sea salt mixture or KCl salt to the pH buffer solution and then doing the calibration.

Preparation of solutions and calibration
The easiest way to prepare calibration solutions with additional salt is:
• Take 100ml of a pH buffer solution and add 3.5 g KCl salt. Mix until all salt is dissolved. Cover the buffer until start of calibration procedure.
• Prepare at least 2 solutions for the calibration.
• Rinse probe with tap water and before calibration with a few ml of the buffer solution. Do NOT rinse the probe with deionised water, because this willl change the pH response of the probe.
• Select the calibration routine on the pH meter and insert pH probe in first buffer with salt and start calibration. Rinse with tap water and change to next buffer. After calibration record slope and offset and the measurement temperature in the buffers.

This is the same for both glass and ISFET pH sensors.
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