Hi all,
I wonder if you know what the deal is with the frequent calibration of pH probes
Unfortunately they need calibrating every time you switch them on, and you are even meant to re calibrate them every so often when you are doing a continuous long run of samples.
You really need two point calibration (pH 7 & pH 4 or pH 10) as well, dependent on what you think the pH of your sample solution may be. You can <"
add a neutral salt to your sample"> and that stabilises how long the meter drifts for before stabilising, but that drift period could be ~10 minutes in a low conductivity situation, even with an expensive (~$1000) meter.
With my Hanna TDS DIST1 I "calibrate" it occasionally but when I dip it in calibration fluid before pressing the Calibrate button it usually just a couple off ppm's (+/- 1-2) off from the 1382 ppm.
Yes, and that is one reason why I like conductivity (TDS) as measurement, the meters are pretty much "
plug and play" and you can go for weeks at a time without re calibrating them, without the calibration drifting. You can also make up your <"
own calibration solutions"> easily.
We have a good test for the differences between ease of pH and conductivity measurement, we have a small spring fed water course on campus (<"
Corston Brook">), that runs through a wood into a lake.
<"
The whole, small catchment"> is on <"
190 - 210 million"> year old <"
Lias limestone and clays">,
The underlying geology is mapped as interbedded Langport Member and Blue Lias Formation mudstone and limestone (BGS 2012).
which means that the water in the brook is always fully saturated with Ca++ and 2HCO3- ions and the pH will always be ~pH 8, via the pH ~ conductivity ~atmospheric CO2 equilibrium.
Conductivity can vary dependent on how much rainfall we've had, but is always in the range of <"
300 - 750 microS"> and dissolved oxygen somewhere around 100%, when we take out readings (with the students always in winter or spring).
When the students take their readings ~99% of the conductivity and DO readings will be very, very similar, but the pH readings will be absolutely anything from pH6 to pH9, although most will be somewhere around pH8. I know it isn't the meters, they were all calibrated before they left the lab. and then re-calibrated in the field, so it is "user error", but it disproportionably effects pH meters.
cheers Darrel