Tuesday, January 18, 2011

Calibrating a Hanna HI 98129

A few posts ago, I talked about a combined pH and EC meter (Hanna HI 98129) I recently acquired from RS Components (who have an excellent overnight service on anything they have in stock - even to a backend Dorpie like Grahamstown)!

Of course, like any scientific instrument, it needs a baseline from which to work in order to ensure accuracy. For this, you need to calibrate your instrumentation. Some instruments drift a *lot* over time - so much so that they can become dangerously misleading if you rely on them and don't realise that the values can drift! So, in the interests of accurate readings, I ordered some calibration and storage solutions:

To calibrate pH, you need two solutions:
Hanna HI-7007/1L, a pH 7.01 buffer
Hanna HI-7004/1L a pH 4.01 buffer

To calibrate EC, you need one solution:
Hanna HI-7031, a solution with an EC of 1,413μS

Hanna recommend storing their pH electrodes in a particular storage solution,
Hanna HI-70300

I'm still waiting for the electrode cleaning solution,
Hanna HI-7061

Interestingly, these solutions cost about the same as the instrument itself... Science works, but the toys can get pricey, fast!

Here's a random shot of the calibration and storage solutions around the instrument which is sitting in pH 4.01 buffer. Note the pH had already drifted out by .01 in the time it took me to assemble the shot!
A few tips:
  • Make sure you rinse out the container you're doing the measurements a couple of times with the reagents you want to use for calibration (and ideally the instrument too) - this should remove any contamination which might affect the readings. This includes previous buffers and solutions!
  • When you do a two point pH calibration, don't rinse the electrode with pH 4.01 buffer, it assumes this is immersing it in the buffer - instead, add slightly more to the container you're doing the calibrations in and swish the instrument back and forth a couple of times. If you don't, I find the calibration goes waaaay out. 
  • When you're trying to do the EC calibration, you have to start in EC measurement mode (NOT pH) - the manual isn't explicit here, so I then ended up having to redo my pH calibration. Again.
  • Storing a pH sensor in the recommended solution is usually a good idea; the manual specifically warns against storing it dry (requiring a one hour rehydration in storage solution) or in distilled/tap water, which (I assume) causes the delicate ionic balance inside the electrode to go awry.  I'm storing mine with a few drops of the solution in the pH electrode "cup" inside the lid, standing upright in a glass, hopefully preventing leaks. The EC of this solution is higher than the measurement range of this instrument, so I'm not sure flooding the entire cap is a good idea. 
  • Ideally, calibrations should be performed at STP (Sea level, 25ºC), although these meters can theoretically cope with deviations from this as they have a temperature correction factor built in. Temperature affects both the pH of buffers and EC values of salt solutions (in fact the labels have the correct values at a range of temperatures printed on them). This evening, it was about 27 degrees in my house when I did this (and very humid!).