DIY Reference Solution for Alkalinity tests that use pH Titration

[louisvillereefer]

Randy and Co,

Can you please recommend the best way to create a 8 dKH alkalinity reference solution that could be used to verify the accuracy of alkalinity tests such as Salifert and the KH Guardian?

I have a small jewelers scale and I can get some graduated cylinders if that's what you recommend.

Ideally, the solution would be accurate within 1%.

Thanks!
Brent

 

[Randy Holmes-Farley Verified Community Expert]

Randy and Co,

Can you please recommend the best way to create a 8 dKH alkalinity reference solution that could be used to verify the accuracy of alkalinity tests such as Salifert and the KH Guardian?

I have a small jewelers scale and I can get some graduated cylinders if that's what you recommend.

Ideally, the solution would be accurate within 1%.

Thanks!
Brent

Sure, but 1% is tricky since the pH endpoint of an alk titration is not necessarily the same in seawater and freshwater, and in both situations it changes a little with the actual alk being tested. But it will be reasonably close.

Do you have sodium bicarbonate (baking soda) or sodium carbonate (washing soda) on hand?

 

[louisvillereefer]

Sure, but 1% is tricky since the pH endpoint of an alk titration is not necessarily the same in seawater and freshwater, and in both situations it changes a little with the actual alk being tested. But it will be reasonably close.

Do you have sodium bicarbonate (baking soda) or sodium carbonate (washing soda) on hand?

Yes, I have both on hand. Thanks!

 

[Randy Holmes-Farley Verified Community Expert]

Sodium carbonate has a molecular weight of 106 grams per mole.

An 8 dKH standard will contain 2.88 meq/L of sodium carbonate, and since each carbonate carries two units of alkalinity, we want half that, or 1.44 mM sodium carbonate, or 1.44 millimoles per liter.

1.44 millimoles is 1.44 millimoles x 106 mg/mmole = 152.6 milligrams per liter.

So dissolving 152.5 milligrams in a liter (1.00 kg) of water will give an 8 dKH standard.

If that is not able to be weighed out accurately (or the 1 L measured accurately), you can make a higher concentration and dilute it down, but that too potentially introduces errors, in the volume measurements.

I might also suggest baking the sodium carbonate first to be sure it has no moisture in it and it is all present as carbonate and very little as bicarbonate. 1 h at 400 deg C should be sufficient.

 

[louisvillereefer]

Sodium carbonate has a molecular weight of 106 grams per mole.

An 8 dKH standard will contain 2.88 meq/L of sodium carbonate, and since each carbonate carries two units of alkalinity, we want half that, or 1.44 mM sodium carbonate, or 1.44 millimoles per liter.

1.44 millimoles is 1.44 millimoles x 106 mg/mmole = 152.6 milligrams per liter.

So dissolving 152.5 milligrams in a liter (1.00 kg) of water will give an 8 dKH standard.

If that is not able to be weighed out accurately (or the 1 L measured accurately), you can make a higher concentration and dilute it down, but that too potentially introduces errors, in the volume measurements.

I might also suggest baking the sodium carbonate first to be sure it has no moisture in it and it is all present as carbonate and very little as bicarbonate. 1 h at 400 deg C should be sufficient.

Thanks so much Randy! That is really helpful.

What type of measuring device would you recommend to obtain an accurate volume measurement? Graduated Cylinder?

Also, I'm not sure that my oven reaches 400 degrees C but I can turn it up all of the way.

After making the solution, assuming it is capped in an airtight bottle how long will it be good for?

 

[Randy Holmes-Farley Verified Community Expert]

Thanks so much Randy! That is really helpful.

What type of measuring device would you recommend to obtain an accurate volume measurement? Graduated Cylinder?

Also, I'm not sure that my oven reaches 400 degrees C but I can turn it up all of the way.

After making the solution, assuming it is capped in an airtight bottle how long will it be good for?

Top bet is a volumetric flask. Second to that is either mass on a scale, or a graduated cylinder.

What is the range of your scale?

 

[Randy Holmes-Farley Verified Community Expert]

Oops, my error, I meant 400 F.

 

[Randy Holmes-Farley Verified Community Expert]

It is good for as long as evaporation does not change the liquid volume significantly. The entry or exit of CO2 does not alter the actual alkalinity, just how much of it is carbonate and how much is bicarbonate.

 

[louisvillereefer]

It is good for as long as evaporation does not change the liquid volume significantly. The entry or exit of CO2 does not alter the actual alkalinity, just how much of it is carbonate and how much is bicarbonate.

Great. Thanks again. I'm surprised that no one seems to sell a standard reference solution.

Oops, my error, I meant 400 F.

Thanks for clarifying.

Top bet is a volumetric flask. Second to that is either mass on a scale, or a graduated cylinder.

What is the range of your scale?

My scale ranges up to 500g in 0.01g increments and is calibrated with a 500g weight

I will probably get another scale that 0 - 20g in 0.001g increments with a 10g calibration weight since my other scale will not be accurate for mgs.

 

[Randy Holmes-Farley Verified Community Expert]

My scale ranges up to 500g in 0.01g increments and is calibrated with a 500g weight

I will probably get another scale that 0 - 20g in 0.001g increments with a 10g calibration weight since my other scale will not be accurate for mgs.

2 batches of 500 g is a good way to get 1 L of water pretty accurately.

 

[louisvillereefer]

2 batches of 500 g is a good way to get 1 L of water pretty accurately.

Thanks so much. I really do appreciate the help!

 

[Randy Holmes-Farley Verified Community Expert]

Alternatively, here's a better method using the scale you have.

EDITED DUE TO A MISTAKE

Dissolve 1.52 g in 498.5 g of water.
Take out 100 g and add to 500 g of water. Conc ~ 0.304 grams in 600 grams total,

Take out 89.5 g and add to 210.5 g of water. Conc ~ 0.0453 grams in 300 grams total, ~0.1512 grams/L, ~ 1.426 mM (8.01 dKH)

 

[louisvillereefer]

Thanks Randy. That's a great option.

 

[Randy Holmes-Farley Verified Community Expert]

Oops, made a mistake in the last protocol. I am editing it.

 

[Randy Holmes-Farley Verified Community Expert]

Fixed now. Forgot the factor of 2 in mM to dKH conversion due to 2 units of alk for each carbonate.

 

[louisvillereefer]

Ok thanks.

 

[louisvillereefer]

Hi Randy,

The carbonate solution worked perfectly on the Salifert test. However, when I tried it out on the KH Guardian, the KH Guardian would not test the solution because the pH was too high. I'm guessing that a baking soda solution would have a lower pH.

Can you please tell me how much baking soda is need for the 8 dHK solution? Also, do you have an idea what pH the baking soda solution would be (approximately)? Is there anything special I need to do to ensure that the baking soda is almost pure bicarbonate?

Thanks again!
Brent

 

[Randy Holmes-Farley Verified Community Expert]

Hi Randy,

The carbonate solution worked perfectly on the Salifert test. However, when I tried it out on the KH Guardian, the KH Guardian would not test the solution because the pH was too high. I'm guessing that a baking soda solution would have a lower pH.

Can you please tell me how much baking soda is need for the 8 dHK solution? Also, do you have an idea what pH the baking soda solution would be (approximately)? Is there anything special I need to do to ensure that the baking soda is almost pure bicarbonate?

Thanks again!
Brent

We can try a modification of the last recipe using sodium bicarbonate instead of sodium carbonate, but it is hard to ensure sodium bicarbonate is pure sodium bicarbonate without added water or loss of CO2:

Dissolve 2.41 g sodium bicarbonate in 497.6 g of water.

Take out 100 g and add to 500 g of water. Conc ~ 0.482 grams sodium bicarbonate in 600 grams total

Take out 89.5 g and add to 210.5 g of water. Conc ~ 0.0719 grams in 300 grams total, ~0.2397 grams/L, ~ 2.853 mM ( 7.99 dKH)

 

[louisvillereefer]

Thanks Randy. I'll give that a try. I have that 20g scale now that read in milligrams. So, I'll just add 240mg of sodium bicarbonate to 1 L of water.

I also found a good source of 0.1N HCl for your DIY alkalinity test, which I can use to test the solution. For other people's reference, Hanna Instruments sells a 1L bottle of 0.1N HCl for $20 plus shipping.

 

[Randy Holmes-Farley Verified Community Expert]

Thanks Randy. I'll give that a try. I have that 20g scale now that read in milligrams. So, I'll just add 240mg of sodium bicarbonate to 1 L of water.

I also found a good source of 0.1N HCl for your DIY alkalinity test, which I can use to test the solution. For other people's reference, Hanna Instruments sells a 1L bottle of 0.1N HCl for $20 plus shipping.

Sounds good!