DIY alkalinity standard

[taricha]

Very interesting, thanks for doing it!

I presume you know it, but others may not. The temperature of the kalkwasser is important, and it is critical to have no cloudiness or solids enter the test sample. :)

Yeah it's not a great choice of standard - temperature, solids, instability... Even diluted in the distilled water, it starts to form a hint of a crust on the top from reaction with CO2 within an hour.
Only really makes sense to use as a standard if you keep a bucket of limewater around anyway. Otherwise the other suggested DIY's are definitely less trouble.

 

[Robert Minett]

For folks who may want to evaluate the accuracy of their alkalinity test kits, here are two suggested ways to do it. One is DIY and one is a mostly commercial standard with one DIY step.

Method 1
Standard solutions could be made with sodium bicarbonate, but it can be hard to be sure it hasn't begin to absorb moisture and/or decompose into sodium carbonate.

Thus my suggestion is to start with sodium carbonate, and bake it yourself to be sure it is dry. If not dry, the weight of the water will make the standard less potent than you expect.

To fully dry sodium carbonate before use we will bake it in a home oven.

Preheat the oven to about 400 deg F.

Spread about a quarter cup of sodium carbonate on a pyrex casserole dish or a roasting pan. You want CO2 and H2O gas to readily escape from it, hence the spreading out.

Place it in the oven and leave it there for 60 minutes.

Remove and cool the dish. Don't place it right near a pot of boiling water, you do not water it reabsorbing moisture.

Weight out 10.0 grams of the dry solid powder (scale the whole recipe down if your scale cannot weigh 10 grams). This 10 grams contains 0.0943 moles (94.3 mmoles) of sodium carbonate. Each carbonate contributes 2 units of alkalinity, so this 10 grams contains 2 x 94.3 mmoles = 189 milliequivalents of alkalinity.

Dissolve the 10 grams of powder in 1.0 liter (1000 g) of RO/DI or distilled water. The more accurate you are with this measurement of water, the more accurate the standard will be.

This solution has an alkalinity of 189 meq/L, which equals 528 dKH. Call this stock solution #1.

Remove 15.1 grams (or 15.1 mL) of Stock Solution #1 and combine the 15.1 g with Ro/DI or distilled water to a final total mass of 1000 g or a volume of 1000 mL.

This final solution will contain 189 meq/L x 0.0151 L = 2.86 meq in one liter = 8.0 dKH.

This final solution can be used to test a titration test kit for total alkalinity.

Method 2.
For those who do not want a pure DIY, but want a highly accurate standard, you can start with a premade solution of 0.01 N sodium hydroxide, such as this one from amazon for $16.45:

LabChem LC242001 Sodium Hydroxide Solution, 0.01N (0.01M), 500 mL Volume

As it arrives, this solution contains 10 meq/L of alkalinity, or 28 dKH. You could use that without dilution with a test kit, but will typically need a lot of titrant to reach the endpoint, wasting the titrant.

If you take 250 mL of this fluid and 750 mL of RO/DI water (or any volumes that are a 1:3 ratio), you will have a fluid that is 7 dKH and is ready for titration kit testing.

If anyone notes a math error in either recipe, please let me know :)

Happy Reefing

 

[Randy Holmes-Farley]

Were you trying to post something?

 

[Robert Minett]

Were you trying to post something?

Yes..lol. I think I found what I was looking for though. Using your method 2, I wanted to know how to make a 9dkh reference solution. Saw another post by you....this is what I came up with.

1000ml x 9dkh/28dkh (sh) = 321ml of sodium Hydroxide and 679ml of rodi. Correct?

 

[Randy Holmes-Farley]

Yes, that is correct. :)

Just remember it works well for any titration test kit or method, but may not give accurate results with a method like the Hanna alk checker..

 

[Robert Minett]

Yes, that is correct. :)

Just remember it works well for any titration test kit or method, but may not give accurate results with a method like the Hanna alk checker..

Ok. Using it to calibrate the Kamoer SPA automatic tester.

 

[Randy Holmes-Farley]

I do not know and cannot tell from the web site how it is measuring alk.

Do you know what it does?

 

[Robert Minett]

I do not know and cannot tell from the web site how it is measuring alk.

Do you know what it does?

Normal tests use a reagent. Calibration is a single solution.

 

[Randy Holmes-Farley]

Normal tests use a reagent. Calibration is a single solution.

that doesn’t tell me if it is an alk titration, or the sort of spectrometric alk determination Hanna uses. Either way it may be ok, I’m just not as sure.

 

[Robert Minett]

that doesn’t tell me if it is an alk titration, or the sort of spectrometric alk determination Hanna uses. Either way it may be ok, I’m just not as sure.

Oh....do it has a little test tube with a mini magnet stirrer. Just like the Trident.