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!).

New Gadget: Combo pH & EC meter

Yesterday I received a Hanna Instruments HI 98129 combo pH and EC meter. Grahamstown has a chequered history of water quality; debate has been raging over the last few years whether it's even fit for human consumption, let alone raising fish or fairly delicate plants like orchids.

Most orchids like their water to be slightly on the acidic side; a pH meter lets you see just how acidic or alkaline your water is.

Electrical conductivity of water gives you an approximation of the amount of salts dissolved in the water; pure water hardly conducts electricity at all, whilst water with a lot of salts and minerals in it conducts electricity quite well (which means you can use distilled water to put out electrical fires in server rooms, but spilling salt water on electronics usually results in them not ever working again). Most orchids are only ever watered by rainfall, which is pretty close to distilled water in its composition - hardly any mineral salts and a slightly acidic pH.

One generally expects that replicating the conditions orchids usually grow in in the wild will result in success at home; long experience has shown people that this is indeed the case.

There is of course another source of salts which we generally consider "good" for our plants - fertilisers, which are made up of various salts and minerals, which of course affect the composition of the water. Years of advice has suggested that fairly weak concentrations of fertilisers on a fairly frequent basis ("weakly, weekly") are the best bet for orchid cultivation. Too much fertiliser can be worse that too little (just as more orchids are killed by over-watering than drying out!).

I hauled the meter out of its box this morning and quickly did some tests. As I expected, Grahamstown water did not fare well:

Grahamstown Tapwater: pH 8.20, EC 669μS/cm  - quite alkaline and rather salty (for orchids).
Aquelle spring water: pH 7.2, EC 72  μS/cm - not too bad!
Distilled water: pH 7.05, EC 3μS/cm - pretty good, as you'd expect. 

Experience has shown that different kinds of orchids will tolerate different levels of dissolved salts in their water before it essentially has the same effect as you drinking seawater - i.e. it dehydrates you and is no good at all, despite your best intentions.

I found a reference on the Eastern Province Orchid Society's website which suggests the following thresholds:

400μS/cm: Cattleya, Paphiopedilum
600μS/cm: Odontoglossum, Coelogyne, Lycaste, Oncidium, Masdevallia, Miltonia
800μS/cm: Phalaenopsis, Cymbidium

That means that even without fertiliser added, Grahamstown tapwater is unsuitable for all but the most commonly and widely grown orchids (Phalaenopsis and Cymbidium). And I suspect Disa are real pansies and need a lot lower than that! Buying distilled water and/or spring water to water your plants seems a little excessive, so I'm going to test some other possible sources, particularly spring water. Given that we live in a rented second story flat, there isn't really any feasible way of harvesting rainwater, and Reverse Osmosis units require plumbing modifications, unfortunately!

Next I'll have to test the various fertilisers we use to see how they affect the conductivity and pH and just how much we should be using... I've also ordered some calibration and storage solutions for the meter to make sure it's accurate and stays that way!