This is just off the top of my head, I've never worked with seaweed cultures.
Seawater might contain some organic material that continues to degrade and change the pH, or the chemicals in the seawater might react with each other over time and alter compostition, maybe also under light (transparent bottles?).
Whats the plant regulator exactly ? Depending on the chemical make up it may form or catalyze reactions with other chemicals present ?
Other than that, what Crazy Xabi said are excellent points.
You could try a test series without plant regulator to see if it's the reason for the pH change. Or filter the seawater to get rid of some organic components.
Good luck !
I'm using IAA for the hormone and I've autoclave the seawater earlier. The bottles are kept in the lab, in room temperature, so I wonder do light really do affect the pH? Anyway thank you for your opinion my friend. This is helpful
Some poorly buffered media may show a different pH when are stirred due to the CO2. Try to measure the pH both direct from the bottle and using the magnetic stirrer. If you notice a change, it's because of the air.
Did you autoclave after preparation? pH can change.
Also, some solutions, buffers and media may take a day to stabilize the pH, e.g. adjusting the pH of the Tris-HCl 1M requires initial pH adjustment and repeat it after 24h to get the desired stable pH value
Thank you for your opinion, pal. I did not autoclave the stock. Is it necessary to autoclave them?
Or just measuring inaccuracies? As in several threads here mentioned measuring the pH of water is quite tricky and small impurities (e.g. from the storage solution), differences in dissolved CO2, or calibration changes of the pH-meter lead to larger pH changes of the water or different measurements.
Not sure about the buffering capacities of seawater. Is there any?
Do CO2 or any gases will give big impact to the pH of the solution?
In my consideration there are 3 chances for this change.
1. did you really used the ddH2O? (unless cleaned apparatus during preparing solution?)
2. Proper covering of the solution as well as the Removing method(contamination of other solution).
3.The temp. of the solution where you keep them ( destroying the growth hormonesEx. indole group cleavage)
I think you used the pH meter per as std. protocol. so it is not the problem. these are just my opinions. don't hesitate to tell me the correct solution for your problem. good luck
Thank you so much for the opinion my friend. I use milliQ for the distilled water, so I guess there is no problem for that. And the solutions are prepared in reagents bottles and stored in the lab at room temperature. I can't think of any other factors that could lead to the change of pH.