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Discussion Starter · #1 ·
I have a TDS meter. Could I just mix distilled water with baking soda and use my TDS meter to get the 4dKH reference solution? What ppm should I be look for 30ppm or 45ppm?

Also, a CO2 indicator is kinda just like a ph indicator correct? It changes color when the PH drops, the lower it drops the greener it turns and higher the CO2concentration correct?

Thanks, I'm still trying to put all this together.

206 Posts
With a TDS conductivity meter you're going to have something like +/- 10% accuracy. You can do better with diluting and an accurate scale. +/- 10% alkalinity at this range is going to throw your drop checker off by +/- 2.5ppm CO2.

And yes, drop checkers work on pH. Bromothymol blue is a pH indicator between 6.0 and 7.6. As the pH drops from diffusion of the air/CO2, your drop checker changes color.


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Here is one method for making the 4 dKH (30ppm CO2) solution. It is the method that I used. There is also a little easier method posted somewhere on this forum.
KH standards

Here is the information needed to make a KH standard. When I started using the drop checker method for measuring CO2 there was a lot of confusion as to the proper way to make a KH standard. After a lot of searching and asking questions I found the correct way to make a good KH standard.

I have made the KH standards with the following.

  • baking soda (sodium bicarbonate)
  • sodium carbonate anhydrous
  • a lab certified Alkalinity Standard Solution as NA2CO3 in 10mL Ampules
How accurate you want the standard is up to you. I think the sodium bicarbonate and the sodium carbonate standards are very accurate if they are made right.

If you want a more accurate standard than that you could use the alkalinity standard solution. The reason for the increase in accuracy is because all that needs to be done is the dilution part, the weighing process is removed, which eliminates any errors from moisture in the sample or an inaccurate scale.

To make a KH standard using baking soda you would need the following.

  • distilled water
  • baking soda (new and unopened would be best)
  • .01 gram scale (a calibration weight)
  • 500 ml graduated cylinder
  • 50 ml graduated cylinder

1. Measure 3000 ml of distilled water using the 500 ml graduated cylinder and put it in a clean container. The container could be an empty 1 gallon distilled water bottle. (I found it easier to use 3000 ml, instead of the often suggested 5000 ml because you can make your standard with 1 gallon of distilled water instead of having to have a larger container or multiple containers)

To read the precise water level in the graduated cylinder.

-Place the cylinder on a flat surface.
-Make sure your eye is on a level plane with the of the meniscus.
-The meniscus is the half-moon curve formed at the surface of liquid.
-Water should be read from the bottom of the meniscus

2. Weigh 3.60 grams of baking soda, add it to the 3000 ml of distilled water and mix well, this will make a 40 dKH / KH standard.

3. Pour 450 ml of distilled water into the 500 ml graduated cylinder.

4. Pour 50 ml of the 40 dKH standard into the 50 ml graduated cylinder

5. Pour the 50 ml of 40 dKH standard into the 500 ml graduated cylinder and mix well, this will give you 500 ml of a 4 dKH standard.

6. If you want a 5 dKH standard instead of a 4 dKH - change step 3 from 450 ml to 400 ml and you will end up with 450 ml of a 5 dKH standard.

A few addition items:

1. Don’t heat your baking soda to dry it out or it will change from sodium bicarbonate into sodium carbonate. You could then end up with a mixture of sodium bicarbonate and sodium carbonate, this would change the sample weight needed because it takes less sodium carbonate to make the KH standard than sodium bicarbonate.

2. You can use sodium carbonate anhydrous instead of baking soda. When I found out that heating baking soda turns it to sodium carbonate I thought this might be a good alternative to using baking soda. I bought Lab grade sodium carbonate that is 99.95% pure, thinking it might make a more accurate standard. While in theory it should, I really can’t say for sure that it does because when I follow the above instructions both the baking soda and sodium carbonate standards come out correct.

3. Not all of the calculators on the web are correct for calculating how much baking soda or sodium carbonate to use.

If you want to calculate your own weights here is the calculator to use.

'Reef Chemistry Calculator FV'

This calculator is correct, it was off when calculating sodium carbonate but Jose
changed it in the last couple of weeks and now it’s right.

This calculator rounds everything up because it was made for calculating
aquarium water not small samples. To get precise weights just add some zero’s to the sample size.
For example: if you wanted to know how much baking soda to add to 3 liters of water to get a standard that was 40 dkh the calculator would give you a weight of 3.6 grams to add. We want to be accurate to at least .01 grams so instead of putting in 3 liters of water you can put in 300 liters, the calculator will then give you a weight of 360 grams to add which you know would be 3.60 grams.

It has been said that there are a lot of assumptions built into these calculators but that’s not correct statement for this calculator when calculating how much sodium bicarbonate or sodium carbonate to use for KH. This calculator will give you the same answer you would get if you calculated long hand, except that it rounds up the numbers (the solution for that is in the above paragraph).

Although there is one assumption in this calculator when using sodium carbonate, it assumes that the sodium carbonate is 100%. You will need to calculate the difference between your sodium carbonate and 100%
For example:
I purchased lab grade sodium carbonate anhydrous and it is 99.95% pure. After I calculate how much to use I have then multiply the answer given by the calculator by .5% and add that to the weight given by the calculator.

4. I have made these KH standards several times with sodium bicarbonate and sodium carbonate. When I follow the instructions listed above the standards has always came out right.

I have also made the KH standards using a lab certified Alkalinity Standard Solution as NA2CO3 in 10mL Ampules. As long as the accurate dilutions are done this should make the most accurate standard. This is what the EPA uses as an alkalinity standard when they are monitoring and assessing water quality.


I hope this helps when others when they are making their KH standards and that they won’t have to do all of the research I had to do just to get a good KH standard.

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