Making a Standard Calcium Solution

Making a calcium standard solution is not all that easy.

PS : note that Tom's answer to this message follows the end of it so keep reading!

Calcium chloride is hygroscopic and tends to pickup moisture from the air. It is normally available in the dihydrate form but this is not precise.

The most accurate way is to dissolve 1.00 gm pure calcium carbonate in acid and dilute to 1.00 liter after bringing to a neutral pH. This gives a stangard of 400 mg/ l to test with.

We have also used both artificial seawater made precisely in the lab and natural ocean water (which varies very little in the ratio of major ions) to perform quality control tests for the SeaTesT calcium tests. This approach eliminates interferences from other ions that might alter the results.

Tom Frakes

PS: I have asked Tom for more details on how this balancing of the pH is done and how how much of the acid solution to use, etc. and will post his answer here when I receive it.

Reply from Tom Frakes

Albert,

To prepare an accurate calcium standard requires access to precise lab equipment such as an analytical balance, volumetric flasks for mixing, reagent grade chemicals and a pH meter for the neutralizing.

Most people with acces to this type of equipment would have the basic chemistry skills to make to standard on their own and there are several methods that would work.

Simplistically, 1.00gm of pure calcite (CaCO3) is added to say 700 ml distilled water in a flask. A slight excess of concentrated HCl (approx. 12M, I believe, but I don't have reference texts here) is added to dissolve the calcite.

Since 1 gm calcite is 0.01 mole, if my calculations are right you would only need about 1 ml acid although more would speed the reaction.

Once dissolved, the low pH can be checked with a pH meter and titrated up to say 7-8 range with dilute NaOH.

After neutralization, the volume is brought up to 1.00 liter with distilled water. Ideally all of this is done at 20 C since this is the temp. the glassware is calibrated for.

This prepares a Calcium standard, however it may not give accurate readings with all tests since most are designed to shift to pH to a specific range for testing. The pH of this unbuffered standard may go too high and give a low reading for Ca.

Also, tests that are calibrated with this standard won't necessarily give accurate results in seawater since some are subject to interference from magnesium (Mg) ions that could give high readings.

This could vary depending on the amount magnesium and/or the alkalinity (buffer) which can affect performance of some tests. Obviously I am biased toward SeaTesT from Aquarium Systems.

I hope this information is helpful. Thanks again for your kind support of IO.

Regards,

Tom Frakes

There are quite a few tests on the market and I really cannot take a position on which one is the best. Individual hobbyists' experiences vary. What you may want to do is join our reefkeeping mailing list and follow the threads on a multitude of topics that deal with reef tanks. Calcium is just one of them.