Hanna pH/EC/TDS combo meter strange readings?

[jgallone1801]

I recently purchased a Hanna pH/EC/TDS/Temp meter (HI98129) and I'm not sure if I can trust its pH measurements.

I dual calibrated it using the 10.01/7.01 solutions and then measured the pH of my aquarium water. It gave me a reading of 8.0; this is a little lower than I expected since I did the same test with a new Salifert liquid kit and it was clearly 8.3.

To further make me question the Hanna readings, is that I tested both my tap water and fresh ro/di water (room temp) with the Hanna and it gave me a reading of 9.0. The Salifert kit read 7.0. It seems that 7.0 makes more sense for fresh ro/di water, or am I way off base here?

So, which do I trust? Any ideas how I can verify the readings? Should I just get more callibration solution and calibrate again...maybe this time test the solution with the Salifert kit?

I appreciate any insight that anyone can give.

Thank you,

Jason

[CarmieJo]

Hi Jason,

In general I would trust the meter over the test kit but... that is presuming the meter is properly calibrated. I recommend this article A Comparison of pH Calibration Buffers by Randy Holmes-Farley - Reefkeeping.com. It also includes a link to an article on using borax to make a test solution. Here is a link to a science fair project about the pH of drinking water. Science Project 2004. The pH of the municipal water in Raleigh runs between 7.9 - 8.8!

[jgallone1801]

Hi Carmie,

Thanks for the informatio, I'll check out the links you gave me.

I think my biggest concern was that the meter was measuring pH of the pure RO/DI water as 9.0 when in reality it should be 7.0 (based on the info I've seen online as well as from talking to a couple of the chemists at work).

I've found information stating that sometimes these meters have a difficult time measuring the pH of pure water because of the water's low ionic strength combined with other factors (that I do not understand).

Thanks again,
Jason

[CarmieJo]

Jason, are you using one the recommended brands of calibration standards?

That does make sense about the low ionic strength of pure water. Essentially it means that there are no ions (atoms or molecules that have a positive or negative charge because they have gained or lost an electron), which would be impurities, in the water. That is after all what we are aiming for! Let me know if you want a chemistry lesson and I will be happy to go into more detail.

[jgallone1801]

Carmie,

Yes, I am using the 7.01 and 10.01 pH calibration solutions that are recommended. The calibration process is pretty simple so I'm pretty sure I did that properly.

Thank you for the offer, but I don't want to take up any more of your time. Plus, after a little more research on my own, I think I have a basic understanding of the ionic strength concept.

Thanks again!

Jason

[CarmieJo]

Jason,

Great! Calibration is very straightforward so if you are using the recommended buffers I think you are good to go with the meter. Let me know if I can be of further help.