Calcium test oddities - Preparing a calibration solution

amps · Nov 24, 2017

I recently bought a new Salifert calcium test that's giving me issues. My old kit would consistantly read around 400-430ppm, right in the range that I expected. The new kit is giving me readings above 500ppm. Of course that planted the seed of doubt in my mind. I had been using the old kit for 6+ months and my dosing regimen had been based on it's results and hence would have been skewed by it's results. What if the new kit was right and my old kit was wrong?

I decided to try and prepare a calibration solution that was in the range of our tanks. I used 5.5g of calcium chloride (bulk from BRS) and mixed it in 1 gallon of 0 TDS RODI water. According to a reef calculator (http://reef.diesyst.com/chemcalc/chemcalc.html) that should make a solution at 400ppm.

When I used the new test kit, it gave me a reading of 530ppm. Unfortunately, I had used every last drop of my old kit so I had nothing to confirm my expected readings with.

I guess my big question is: Did I prepare the calibration solution properly? I don't know if it should have been buffered, had some other salts added to stabilize it or anything else.

jbrady429 · Nov 24, 2017

amps said:
I recently bought a new Salifert calcium test that's giving me issues. My old kit would consistantly read around 400-430ppm, right in the range that I expected. The new kit is giving me readings above 500ppm. Of course that planted the seed of doubt in my mind. I had been using the old kit for 6+ months and my dosing regimen had been based on it's results and hence would have been skewed by it's results. What if the new kit was right and my old kit was wrong?

I decided to try and prepare a calibration solution that was in the range of our tanks. I used 5.5g of calcium chloride (bulk from BRS) and mixed it in 1 gallon of 0 TDS RODI water. According to a reef calculator (http://reef.diesyst.com/chemcalc/chemcalc.html) that should make a solution at 400ppm.

When I used the new test kit, it gave me a reading of 530ppm. Unfortunately, I had used every last drop of my old kit so I had nothing to confirm my expected readings with.

I guess my big question is: Did I prepare the calibration solution properly? I don't know if it should have been buffered, had some other salts added to stabilize it or anything else.

I have been very interested in the same thing, as I have also had problems with inconsistent test kit readings. There has been a lot of discussion about diy calcium references, and if I remember correctly the problem with the recipe you propose is that there are other elements in salt water that impact the results we get from our test kits. Calcium chloride in RO water will not give the same result as the same concentration of calcium in salt water, unfortunately.

So far the best “reference” solution I have found is the Accurasea product from Two Little Fishies. I use it as a reference for calcium and alkalinity (I also use a diy alkalinity reference of sodium carbonate in RO water). Other than that, I have also found occasional ICP testing to work as a good “reference” point to check my more frequent test kit results against.

Randy Holmes-Farley · Nov 24, 2017

Without knowing the exact hydration level of the calcium chloride used, it is a poor standard.

Also, the other attributes of seawater can impact such kits. For example, how well they distinguish magnesium and strontium from calcium in seawater.

amps · Nov 24, 2017

Randy Holmes-Farley said:
Without knowing the exact hydration level of the calcium chloride used, it is a poor standard.

Also, the other attributes of seawater can impact such kits. For example, how well they distinguish magnesium and strontium from calcium in seawater.

So it seems like making a standardized solution might not work.

In my case though, does it make sense that the 'bad' test kit is reading high? I figure that 5.5g of perfect calcium in a perfect world should give a perfect amount of 400ppm. If my kit is reading high (530ppm) and the test water was made with RODI, there should be zero chance that other elements would be causing the test to return an elevated results. That should let me surmise that the kit needs to be replaced?