Hanna ph tester. Hi98107. Any problems?

[legrunt]

Just got a hanna hi98107 ph tester. Soaked it in rodi for an hour, did the calibration using the supplied sachets, 2 point calibration, then got about testing it.
My tank tests at 8.0 consistently, but i can't get consistent results when i dunk it back into pure rodi... It goes all the way from 6 to 9ish...
Anyone notice anything similar? It's almost rodi is too "pure" for it to ... "sense" anything? How do they work anyway?

 

[Lasse]

RODI water is a water with zero in ionic strength. pH probes - especially those adapted to saltwater (a heck of ionic strength) do not manage to measure in water with very low ionic strength like soft freshwater. there is pH probes developed for low ionic strength but even thes are more or less worthless in RODI water - this was the long answer - the short answer is that you can´t use a pH probe in RODI water.

A pH probe measure the mV difference between the HO minus ion and the H3O plus ion. If it is zero - as much HO minus ion as H3O plus ion - pH is be definition 7. The step between the different pH is between 56 - 60 mV. As an example between pH 7 and pH 8 it is difference of around minus 56-60 mV and between 7 and 6 - a difference around plus 56-60 mV. Between pH 8 and pH 9 - it is also a difference of around minus 56 - 60 mV. So if you probe report minus 112-120 mV - the computer in the pH meter transfer that to ph 9

If there is not background noise of other ions - the reading will be very erratic (as you discover)

Sincerely Lasse

 

[Randy Holmes-Farley]

Just got a hanna hi98107 ph tester. Soaked it in rodi for an hour, did the calibration using the supplied sachets, 2 point calibration, then got about testing it.
My tank tests at 8.0 consistently, but i can't get consistent results when i dunk it back into pure rodi... It goes all the way from 6 to 9ish...
Anyone notice anything similar? It's almost rodi is too "pure" for it to ... "sense" anything? How do they work anyway?

That is exactly what one should get from RO/DI water. It will swing wildly for a variety of reasons, and is not accurate.

 

[legrunt]

Great! Thanks guys! I thought i had a defective tester...

 

[legrunt]

Also, ... Instructions say to store the tester in hi70300 storage solution and not deionized water, which apparently will shorten the life of the probe. Is that true? Couldn't i just cap it after rinsing in rodi without shaking off the excess? Is the 70300 solution really necessary?

 

[Lasse]

Probably have your probe a little glass membran and the reference solution (inside the probe) is KCl If you store the probe in RODI water - KCl will make its way out from the inside of the probe into the RODI water because of osmosis. the storage solution is probably KCL and you will not miss anything from the internal reference solution.

Sincerely Lasse

 

[legrunt]

I see... So, a good idea to get the storage solution then i guess?

 

[Lasse]

Yes

Sincerely Lasse

 

[legrunt]

Thank you. I went out during lunch just now to pick up a bottle.