Need help checking my concentration math

[chadfish]

If I take 1tsp of powder that's 8.06% Calcium and dilute it in 100mL RODI water, then dose 10mL into a 50 gallon tank, shouldn't expect the Ca ppm to rise by a small fraction? Like 0.04ppm?

That's clearly wrong though because my Ca went from 380ppm to 430ppm

What am I missing?

notes:
Hanna Ca checker
Ca in the form of Calcium Sulfate
The powder was Seachem Equilibrium which I dosed for the potash and iron because my chaeto isn't thriving. I figured it could help boost my Ca and Mag a little bit,, but I didn't expect this..

 

[BranchingHammer]

Do you know what the mass of one teaspoon of the compound is? That would help with calculating.

 

[chadfish]

Do you know what the mass of one teaspoon of the compound is? That would help with calculating.

I don’t have a scale

 

[Dan_P]

If I take 1tsp of powder that's 8.06% Calcium and dilute it in 100mL RODI water, then dose 10mL into a 50 gallon tank, shouldn't expect the Ca ppm to rise by a small fraction? Like 0.04ppm?

That's clearly wrong though because my Ca went from 380ppm to 430ppm

What am I missing?

notes:
Hanna Ca checker
Ca in the form of Calcium Sulfate
The powder was Seachem Equilibrium which I dosed for the potash and iron because my chaeto isn't thriving. I figured it could help boost my Ca and Mag a little bit,, but I didn't expect this..

Something went wrong. The instructions say 1 TBS is 16 g, 8% of that is about 430 mg. That amount diluted by 181 L (50 gal) does not account for the observec 50 ppm increase in Ca.

What type of fresh water are you using to dilute the first reagent in the Hanna calcium test? If it contains a trace of Ca, it will throw off the test results.

 

[BranchingHammer]

Ok, I did some calculations with information from the bottle (@Randy Holmes-Farley, please check them lol). It’s important whether the calcium sulfate is in it’s most common hydrate form (calcium sulfate dihydrate, gypsum) or it’s anhydrous form. I did two sets of calculations: one with the anhydrous form and one with the dihydrate. I think that in a tank with an actual water volume of 50g the calcium level will rise like so:
If the compound is anhydrous: 0.227ppm increase in Ca concentration
If the compound is the dihydrate: 0.18ppm Increase in Ca.
The powder is probably the dihydrate form, so exposure to water in the air won’t hydrate it much more, but if it’s anhydrous the powder will be a mixture of anhydrous and dihydrate calcium sulfate.

In addition, your water volume is not your tank volume. You can use this equation that @Jay Hemdal taught me to find actual water volume:

Get close to the actual tank volume by measuring the water depth, tank width and tank length in inches, multiply them together and divide by 231. Then, multiple that number by 0.85 to remove the typical 15% rock displacement. Don’t forget to add in the volume of the sump.

Please use this equation to find the actual water volume of your tank. Even if the actual volume of the tank is 25 gal for example, you are still only raising the Ca concentration by 0.36ppm or 0.454ppm.

I agree with @Dan_P that something went wrong. Either there is a lot of calcium in the water that you dissolved it in, or something else went wrong. Do you dose anything else for calcium? There might have been a test error as well: do you clean your cuvettes during each test for the Hanna checker? Does all the reagent get added to the cuvette?

 

[taricha]

Just wanted to point out that 380 to 430 is not actually outside the kit error.
6% error at 400ppm is +-24 so it's possible the increase from the additive is actually near zero.

 

[chadfish]

1) I used 0 TDS RODI to perform the test
2) the 430 result was the second data point. The first was over 500! I had to re-do the test.
2a) the colors between the two tests actually seemed different to my naked eye
3) I bought the Hanna secondhand
4) I rinse everything with RODI three times before and after tests
5) I didn’t dose anything prior to the equilibrium
6) thanks for finding that weight info for me. That was the missing link to find my concentration
7) it has to be the test, right?

edit: 8) I have a 40gal breeder with a 15 gal sump, I assume 50gal water- could be less I suppose

 

[chadfish]

Ok, I did some calculations with information from the bottle (@Randy Holmes-Farley, please check them lol). It’s important whether the calcium sulfate is in it’s most common hydrate form (calcium sulfate dihydrate, gypsum) or it’s anhydrous form. I did two sets of calculations: one with the anhydrous form and one with the dihydrate. I think that in a tank with an actual water volume of 50g the calcium level will rise like so:
If the compound is anhydrous: 0.227ppm increase in Ca concentration
If the compound is the dihydrate: 0.18ppm Increase in Ca.
The powder is probably the dihydrate form, so exposure to water in the air won’t hydrate it much more, but if it’s anhydrous the powder will be a mixture of anhydrous and dihydrate calcium sulfate.

In addition, your water volume is not your tank volume. You can use this equation that @Jay Hemdal taught me to find actual water volume:

Please use this equation to find the actual water volume of your tank. Even if the actual volume of the tank is 25 gal for example, you are still only raising the Ca concentration by 0.36ppm or 0.454ppm.

I agree with @Dan_P that something went wrong. Either there is a lot of calcium in the water that you dissolved it in, or something else went wrong. Do you dose anything else for calcium? There might have been a test error as well: do you clean your cuvettes during each test for the Hanna checker? Does all the reagent get added to the cuvette?

Looks like I’m running the calcium test two more times and checking the expiration date on the reagents

 

[BranchingHammer]

Looks like I’m running the calcium test two more times and checking the expiration date on the reagents

Do you wipe your cuvettes with a microfiber cloth before each test? I use Kim wipes which leave no lint or fingerprints left on the cuvette. Any smudges will definitely impact the reading.

Just wanted to point out that 380 to 430 is not actually outside the kit error.
6% error at 400ppm is +-24 so it's possible the increase from the additive is actually near zero.

This here is definitely an important point.

 

[chadfish]

Yes, I use the soft lens cloths for eyeglasses. Not Kim wipes, but still good. And I always place the cuvette in the same orientation every reading.

And that’s crazy about the % error! I didn’t realize this. I mean that’s the difference between a good concentration and a low concentration. I wonder if Salifert has the same %error. Looks like BRS-TV tonight...

 

[taricha]

And that’s crazy about the % error! I didn’t realize this. I mean that’s the difference between a good concentration and a low concentration.

If the accuracy of the colorimetric kit is less than you'd like, then use a titration kit. For Ca & Alk, titrations are more accurate and precise than colorimetry.