Spruce,
I'm reading your question as;
You take the tankwater sample.
Immediately run the pH test and get a reading of 7.9 pH.
You set the test vial down, and let it it sit there for an hour.
Then after an hour wait - it appears as if the color now indicates a pH reading of 8.3 pH.
Couple things here.
I've never seen a pH test kit that advises to wait ANY period of time to get the most accurate result.
You want to take the immediate reading after the sample of tankwater's been pulled from the tank.
Why ?
pH is the measurement of the hydrogen ion concentration of the the tankwater.
The hydrogen ion activity can change if the sample is exposed to higher or lower levels of CO2 gas in the air.
If you leave the test tube or vial open or uncapped - it's possible for CO2 gas to either enter or leave the sample.
When ( if ) this happens over a period of time - the immediate test result can drift on you.
Another reason - The actual pH of the sample can change with temperature.
There's a very good chance that the tankwater temperature cools to ambient room temperature - that may be cooler than the tankwater's original temperature when the sample was pulled.
The change in sample tankwater temperature can and will create a change in the hydrogen ion activity in this solution, because the hydrogen ion activity in the solution is definately temperature dependent.
Simply put:
pH is the measurement of H+ hydrogen ions.
Hydrogen ion activity can be "changed" by the introduction OR escape of CO2 gas in the water sample AND can also change as the temperature changes.
If you want to try and experiment.
Do your pH test as you have.
Write down the original pH value and temperature value.
Let the sample sit for a hour as you have before.
Write down the pH value shift, and the sample temperature change.
See if the temperature has changed.
If you want to try a CO2 affect experiment.
Do your pH test as you have.
Write down the original pH value and temperature value.
Slowly blow little bubbles into the sample with a straw.
Blow the air into the sample from air from your "lungs", slow deep breaths of air bubbled into the sample.
See if the addition of CO2 causes a pH shift as well.
Or ......... just take the immediate reading as the "accurate" reading. Waiting an hour - will not give you "better" pH readings.
Note for those folks using electronic pH measuring equipment.
Most decent "electronic" pH meters that use a glass measuring / reference probe are "temperature" compensated.
There's a separate issue with electronic high impedance pH reading devices used with pH probes. This issue has to do with how the electronic readout interprets the signal from the glass electrode as it measures the number of H+ ions in the sample.
For more information on this - do a search on the net for the words " Nernst Equation"
Later
