DIY alkalinity standard

Randy Holmes-Farley

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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
 
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taricha

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So now we could check those titration kits and see what the correct end color actually looks like, compared to what we think it is.

Unrelated, could an ORP standard be DIY-able?
 
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JimWelsh

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Randy Holmes-Farley

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So now we could check those titration kits and see what the correct end color actually looks like, compared to what we think it is.

Unrelated, could an ORP standard be DIY-able?

I think a better way to check the endpoint color is by pH, but that doesn’t confirm the accuracy.

An ORP standard could be made. I made one many years ago. I don’t have the recipe offhand, but it would need to combine two redox active materials, such as ferrous and ferric iron.
 
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Randy Holmes-Farley

Randy Holmes-Farley

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quick question - in the DIY testing recipe I believe you call for .1N Hydrochloric Acid - this standard solution method #2 above calls for .01N Sodium Hydroxide.

Is there a way to accurately use the same acid solution for both testing & a standard solution?

One is an acid (hydrochloric acid) and the other is a base (sodium hydroxide), so they are not interchangeable.

You can make any random alk solution (including tank water), determine the alkalinity with the acid standard, and then use the alk solution to check a kit, if you want.
 

taricha

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I'm just curious why you might think this post doesn't address your question? https://www.reef2reef.com/threads/alkalinity-titration-endpoint-ph-salifert-endpoint-color.543556/
Only because your beautiful titration analysis didn't also do every other test kit :)
The details on the seawater titration of course are the same, but the color-pH relationship for the other kit indicators hasn't been done anywhere I've seen. (when I'm too lazy to calibrate my pH meter, I use Red Sea blue to yellow with a pale green target endpoint.)

An ORP standard could be made. I made one many years ago. I don’t have the recipe offhand, but it would need to combine two redox active materials, such as ferrous and ferric iron.
Cool. It may not be relevant to many, but I'm playing around with an ORP probe that will do a 2 point calibration, but the calibration solutions are not cheap and expire fast. And I'm spoiled by @JimWelsh dirt cheap borax / cream of Tartar 2-point pH standards.
 

jimbob928

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I think I would like to make a standard of 8 dKH, since that is closer to what I keep my tank at. I'm not great at math,
but I came up with 200 ml of the LabChem Sodium Hydroxide solution mixed with 700 ml RO/DI.
Thanks Randy for writing this up. I am glad people like you share this information, so people like me (who are terrible at chemistry) can benefit.
 

JimWelsh

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I think I would like to make a standard of 8 dKH, since that is closer to what I keep my tank at. I'm not great at math,
but I came up with 200 ml of the LabChem Sodium Hydroxide solution mixed with 700 ml RO/DI.
Thanks Randy for writing this up. I am glad people like you share this information, so people like me (who are terrible at chemistry) can benefit.
What you want is 1000 (mL per liter) * 8 (your desired dKH) / 28 (dKH of undiluted hydroxide solution) = 286 mL of hydroxide + 714 mL RO/DI.

Your 200 mL + 800 mL would result in a 28 * 200 / 1000 = 5.6 dKH standard.
 
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WallyB

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Anyone have better luck than me?

I tried the best I could with accuracy. My Box of Baking Soda was Sealed, but few months old.

This is what I did, and the results I got. Using Not the Best Scale.....

2020-04-29_DIY-AlkReferenceMethod1-a.jpg

TYPO above ( Temp should be 400 Degrees Fahrenheit not Celcius)

2020-04-29_DIY-AlkReferenceMethod1-b.jpg


Funny part is if I average the Two Results I get 8.0 Dead On :)

I really wanted things to work out, to know which is closer (Salifert or Hanna).

......The diference between Hanna/Salifert using Fresh Water Reference Solution was worse than testing my Tank Water.

Did I do something wrong? (I Tripple checked things)


Is the Hanna not capable of testing Fresh Water Alk? Should be ok according to instructions.

When I test my DT Tank (Salt Water), the I get pretty close. Largest offset is 0.5 dkH.

YET....BOTH (Hanna/Salifert) ARE VERY CONSITANT on Results!!!
** I guess Consistanty is most imporant, within reason **



I'm want to try the More Accurate Method #2, but not so easy to get LabChem LC242001 Sodium Hydroxide Solution, 0.01N (0.01M) right now.


Wondering if there is another Option.
I have an Unopened Bottle of ESV B-Ionic Two Part. Could mix that down/dilute once I figure out the proportions, however still not sure on the Accuracy of the ESV Concetration either.
 
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WallyB

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I tried the idea I had using ESV Bi-Ionic to make a Alk Reference Solution. Making a new Fresh Batch.

It worked!! I couldn't believe it.

2020-04-30_ALkReferenceBiOnic2.jpg


-> Made up a New Accurate 1 Gallon Batch of Part A (ESV B-Ionic)
-> Added 3.8 ml to 1 Gallon of RO DI
-----------> (used the Reef Chemistry Calculator to find out how much to raise by 8.0dkH) = 3.8ml
-----------> http://reef.diesyst.com/chemcalc/chemcalc.html

The Salifert Test TEST Gave me 8.0 dKH Dead on. (3 tests were consistantly 8.0 in a row)

The Hanna is typically off from the the Salifert by roughly 0.5 dkH.

(I did get a 6.5 on first test, so wasn't consistant)....When using Hanna on Tank Salt Water, it's very consistant.

Again, not sure if this is accurate.

I really need to get some Sodium Hydroxide Solution, 0.01N (0.01M) to do More ACCURATE Method 2
 
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Randy Holmes-Farley

Randy Holmes-Farley

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Anyone have better luck than me?

I tried the best I could with accuracy. My Box of Baking Soda was Sealed, but few months old.

This is what I did, and the results I got. Using Not the Best Scale.....

2020-04-29_DIY-AlkReferenceMethod1-a.jpg


2020-04-29_DIY-AlkReferenceMethod1-b.jpg


Funny part is if I average the Two Results I get 8.0 Dead On :)

I really wanted things to work out, to know which is closer (Salifert or Hanna).

......The diference between Hanna/Salifert using Fresh Water Reference Solution was worse than testing my Tank Water.

Did I do something wrong? (I Tripple checked things)


Is the Hanna not capable of testing Fresh Water Alk? Should be ok according to instructions.

When I test my DT Tank (Salt Water), the I get pretty close. Largest offset is 0.5 dkH.

YET....BOTH (Hanna/Salifert) ARE VERY CONSITANT on Results!!!
** I guess Consistanty is most imporant, within reason **



I'm want to try the More Accurate Method #2, but not so easy to get LabChem LC242001 Sodium Hydroxide Solution, 0.01N (0.01M) right now.


Wondering if there is another Option.
I have an Unopened Bottle of ESV B-Ionic Two Part. Could mix that down/dilute once I figure out the proportions, however still not sure on the Accuracy of the ESV Concetration either.

I do not know how accurately ESV makes the potency of their product.
 
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Randy Holmes-Farley

Randy Holmes-Farley

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FWIW, my recipe above called for sodium carbonate (washing soda) as the starting material, not sodium bicarbonate (baking soda). The results look OK, but folks doing that should bake it extra long and hot.
 
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Randy Holmes-Farley

Randy Holmes-Farley

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FWIW, Hanna does not do an ordinary alkalinity titration, but uses a slightly different method. I expect it is a one point "titration" like described here:


Essentially, they add an amount of acid similar to that needed in an ordinary alkalinity titration, and then use the color to determine the pH, and back calculate what the alkalinity would have been to give that pH with that amount of added acid.

Without seeing some data on how the dye they use responds (in terms of absorbance and also pKa), I cannot be sure this method is reliable for the Hanna, and would be wary of over interpreting the Hanna accuracy based on it.

You can get there with one additional step if you have another kit

1. Carefully check your normal kit for accuracy.
2. Use it to measure tank water or new salt water
3. use the Hanna to measure that same tank water or new salt water and compare.
 
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jimbob928

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Since I am making a liter of solution (method 2) and a typical test kit uses 10 ml of solution....
That's a lot of tests. How long will the solution stay accurate assuming it is kept in a sealed container to prevent evaporation? Can I weigh the Sodium Hydroxide solution? Is it close enough to RO/DI to be accurate enough?
 
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Randy Holmes-Farley

Randy Holmes-Farley

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Since I am making a liter of solution (method 2) and a typical test kit uses 10 ml of solution....
That's a lot of tests. How long will the solution stay accurate assuming it is kept in a sealed container to prevent evaporation? Can I weigh the Sodium Hydroxide solution? Is it close enough to RO/DI to be accurate enough?

Yes, weighing is good for that dilute NaOH since the density is approximately 0.9983 g/mL at 20 deg C (pure water at the same temp is 0.9982 g/mL), and it will stay good aside from evaporation. Anything entering it from the air is not a concern.
 

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