Method 1311, Section 5.7.2 (note), states, “The pH should be checked prior to use to ensure that these fluids are made up accurately”. With using “should be”, not "must" or "shall", does a lab have to check the pH of the extraction fluid daily?
In this situation, checking the pH after the fluid is prepared, but before it is used, is mandatory. The pH is a critical parameter and must be controlled and monitored.
The note in 5.7.2 does not state that the pH of the extraction fluid should be checked daily. However, it would be a good practice to check the pH on the day of use, especially since it is very simple and quick to do.