# CO2 Measurement - pH Drop Method



## hoppycalif (Apr 7, 2005)

The easiest method for measuring the amount of dissolved CO2 in your tank water is to measure the pH of the water with the CO2 running long enough to have a stable concentration in the water, and measure the pH of the water after the CO2 in the water has had enough time to escape from the water to the air. The difference between those two pH readings is a measure of how much CO2 was in the water. This method relies on the belief that if water sits out where it is in contact with the atmosphere, it will soon have about 3 ppm of CO2 dissolved in it. The best way to measure the pH of the tank water, with the CO2 in it degassed down to what is in the atmosphere, is to dip a cup of water from the tank, let it sit out in the open for several hours, then measure its pH. Then, measure the tank water pH after the CO2 has been on for several hours. Subtract the low reading from the high reading, and determine the tank water CO2 from this table. The equation used to make the table is:
ppm of CO2 = 3 x 10^(high pH - low pH)









I am posting this so when I want to use this method I can do it without using my calculator.


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## mistergreen (Mar 3, 2007)

You can whisk/agitate the water for a minute to degas the CO2. No need to wait for a day or hours


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## Newt (Apr 1, 2004)

This chart makes it much easier and is reasonably accurate.


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## mistergreen (Mar 3, 2007)

Newt said:


> This chart makes it much easier and is reasonably accurate.


There's a kH component which is not needed in hoppy's calculation.


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## hoppycalif (Apr 7, 2005)

mistergreen said:


> There's a kH component which is not needed in hoppy's calculation.


And, that table isn't accurate unless the only things in the water that affect pH or KH are carbonates and CO2. With normal aquarium water it usually gives you a much too high number for CO2 concentration. I agree that shaking a container of water, or otherwise sloshing it for awhile should get rid of most of the dissolved CO2 in the water. But, I haven't tried that, so I'm not sure just how long it takes. I did experiment with waiting for the CO2 to leave if the water is just sitting out in the open. It took much longer than I expected.


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## Maryland Guppy (Mar 5, 2015)

Air stone in a cup of tank water for 15 minutes and degassed sample is ready.


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## Newt (Apr 1, 2004)

hoppycalif said:


> And, that table isn't accurate unless the only things in the water that affect pH or KH are carbonates and CO2. With normal aquarium water it usually gives you a much too high number for CO2 concentration. I agree that shaking a container of water, or otherwise sloshing it for awhile should get rid of most of the dissolved CO2 in the water. But, I haven't tried that, so I'm not sure just how long it takes. I did experiment with waiting for the CO2 to leave if the water is just sitting out in the open. It took much longer than I expected.


That's not true. As stated, it is reasonable accurate.


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## mistergreen (Mar 3, 2007)

Newt said:


> That's not true. As stated, it is reasonable accurate.


It is very accurate in a controlled condition like a drop checker... but not very much in an active aquarium with lots of variables. Even with reasonable accuracy could lead to BBA or gassing the fish.


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## Newt (Apr 1, 2004)

Opinion?

The math behind it works.


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## hoppycalif (Apr 7, 2005)

Newt said:


> Opinion?
> 
> The math behind it works.


It isn't the math that is the problem. When there are carbonates and CO2 in the water, a small part of the CO2 becomes carbonic acid. There is a 3 way relationship of CO2 concentration, carbonate concentration and carbonic acid concentration. That is a chemistry relationship, not math. And, it is that relationship that makes the KH/pH/CO2 tables possible, but if the measured pH is from a combination of compounds that includes more than carbonates and CO2, then the table is no longer at all accurate. You can find many cases where this method said people had over 100 ppm of CO2, when they actually had less than 30 ppm. That makes the table unusable when used on typical aquarium water. But, if you make standard accurate KH solutions, using only sodium bicarbonate and distilled water, and use that in a drop checker, the method is accurate enough to be useful.


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## mistergreen (Mar 3, 2007)

Think of if you had a large driftwood in your tank leaching tannins/acids affecting your pH, in addition to the carbonic acid/CO2. You're not getting the exact pH from carbonic acid alone. Or if you had limestone hardscape. That'll influence the pH too.

My aquarium pH is always lower than my tap water and I don't have CO2 and change water every week.


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## Newt (Apr 1, 2004)

Perhaps but my local water is 1 deg KH and my CO2 injection only needed tweaking twice a year and remains very constant. If the plants are pearling and growing well I never felt the need to be anymore accurate than the chart.


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## hoppycalif (Apr 7, 2005)

Newt said:


> Perhaps but my local water is 1 deg KH and my CO2 injection only needed tweaking twice a year and remains very constant. If the plants are pearling and growing well I never felt the need to be anymore accurate than the chart.


I agree! If your plants and fish are doing well there is no reason to obsess over measuring how much CO2 you have. Just keep doing what you are succeeding with.


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## Reediculous_nanotank (Jan 12, 2019)

@Hoppycalif this is very interesting! I'll have to try it, but is the chart you have posted usable in a tank without CO2 injection? 

I ask because it would be interesting to do this on my NPT, say with pH measurements from the morning right before the lights turn on, and then the evening right after the lights turn off. Using the table, would this give me a reasonable estimate for my min and max C02 on a daily cycle?


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## hoppycalif (Apr 7, 2005)

Reediculous_nanotank said:


> @Hoppycalif this is very interesting! I'll have to try it, but is the chart you have posted usable in a tank without CO2 injection?
> 
> I ask because it would be interesting to do this on my NPT, say with pH measurements from the morning right before the lights turn on, and then the evening right after the lights turn off. Using the table, would this give me a reasonable estimate for my min and max C02 on a daily cycle?


Yes, this works fine, no matter where your CO2 comes from. The only "trick" is to get a water sample from your tank that you can be reasonably sure has only atmospheric CO2, 3 ppm, in it. If you do that, the method is foolproof. Measuring pH accurately can be a problem too, but a good pH probe, well calibrated, and used right, will give you a good answer.


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## mistergreen (Mar 3, 2007)

Oh, that's a good point. It's best to take the water outside where it is atmospheric co2. Inside the home is considerably higher.


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## hoppycalif (Apr 7, 2005)

mistergreen said:


> Oh, that's a good point. It's best to take the water outside where it is atmospheric co2. Inside the home is considerably higher.


I have no idea how much the CO2 level changes inside a house vs outside. That would be an interesting thing to try to determine. Also, I don't know if the CO2 level in water exposed to the atmosphere is 3 ppm or 3 +/-2 ppm, etc. I doubt that anyone with the equipment needed to measure CO2 in water accurately has ever tried to determine this or has published any data about it. I recall that I saw something a few years ago that convinced me that 3 ppm is a good number, but I don't remember just what it was. I think it was a calculated number?


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## mistergreen (Mar 3, 2007)

CO2 measurements are different in water (ppm by weight) and in air (ppm by volume)..
So you have to convert. Generally we use atmospheric 400 ppmv (but it's rising due to....) to convert to 0.775 ppmw actually.

So your formula should be

ppm of CO2 = 0.775 x 10^(high pH - low pH)

In the house depending on size of the room & ventilation, could be as high as 1700+ppmv but that cause only slight error in ppmw.


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## Reediculous_nanotank (Jan 12, 2019)

mistergreen said:


> CO2 measurements are different in water (ppm by weight) and in air (ppm by volume)..
> So you have to convert. Generally we use atmospheric 400 ppmv (but it's rising due to....) to convert to 0.775 ppmw actually.
> 
> So your formula should be
> ...


I'm so glad there are chemistry and math people like @mistergreen to share this kind of thing with us, haha


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## hoppycalif (Apr 7, 2005)

For many years I have questioned the assumption that aquarium water will have about 3 ppm of CO2 in it from the CO2 in the atmosphere. I have been searching the internet today to see if I can find any reference that confirms that number, with no luck at all. Then I found a table of the solubility of CO2 in water, which might be a path to determining if that 3 ppm is accurate. The table is: 









Are there any chemists here that can use this to demonstrate that the 3 ppm number is accurate? If it isn't accurate, the change in pH method for measuring CO2 isn't valid.


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## mistergreen (Mar 3, 2007)

Here's my partial pressure calculation

Using Henry's Law to find the C of CO2 in water

P is Partial pressure of 400ppm of CO2 in ATM
C=P/K
C = 0.0004/ 29.4
C = 0.0000136M (m/L)
grams = 0.0000136 * 44.01 g/m
grams = 0.0006g/L or .6mg/L or 0.6ppmw


Here ppmv to ppmw conversion
ppmw = ppmv×44.01÷(0.08205×298.15)÷1000

44.01 is mol weight of CO2
0.08205 is a gas constant
289.15 is 77F in Kelvin
1000 is density of water

400ppmv should give you 0.74200969177 ppmw


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## hoppycalif (Apr 7, 2005)

Have you used your CO2 meter to measure the CO2 in water exposed to the atmosphere? And, does it agree with your calculations? I know it wouldn't give you 3 decimal point accuracy, but it should give 0 decimal point accuracy, or 1 ppm instead of .7 or .6.


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## mistergreen (Mar 3, 2007)

yeah, I measured a long time ago and got 1648ppmv / 2.96479846316 ppmw in water... This was indoor CO2, 1701 ppmv in air.

So your 3ppm is close to my indoors measurement.


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## hoppycalif (Apr 7, 2005)

Is the CO2 concentration indoors always so much higher than outdoors? Or, is the 400 ppm number for the atmosphere as a whole just not applicable for specific locations, which makes some sense to me? In any case, if the 3 ppm number is that close to being accurate, that is indeed good news!


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## mistergreen (Mar 3, 2007)

Yes, indoors is generally higher indoors because of people expiring CO2 in an enclosed environment but I suspect the ppm will vary due to room size, ventilation and people.


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## hoppycalif (Apr 7, 2005)

mistergreen said:


> Yes, indoors is generally higher indoors because of people expiring CO2 in an enclosed environment but I suspect the ppm will vary due to room size, ventilation and people.


If it varies significantly the use of 3 ppm as the constant in the equation to determine ppm of CO2 in the tank water will also vary significantly. So, I'm going to guess that the variation is closer to +/-200 ppm than +/- 800 ppm. So, the 3 ppm would be about +/-10%. There is some serious science behind my guess.


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## mistergreen (Mar 3, 2007)

Why not use the outdoor co2, 400 ppmv?
People have to take the water outside to degass.


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## hoppycalif (Apr 7, 2005)

mistergreen said:


> Why not use the outdoor co2, 400 ppmv?
> People have to take the water outside to degass.


So many people have figured out that 3 ppm is about right for how much CO2 is in ordinary, average aquarium water, with no other source of CO2, that I suspect it is the right number for most of our aquarium settings. They didn't all measure the amount of CO2, but derived it somehow. That convinces me that we might as well continue using that number, at least until someone comes up with a very good reason to use another number. We do need to keep in mind that 3 is not the same as 3.00. It means the "real" value is between 2.5 and 3.5. And, the ppm of CO2 we get from this method is good to only one significant figure. So, it isn't 23 ppm, but 20, and it isn't 14 ppm, it is 10. So, it is a rough estimate only.


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## Reediculous_nanotank (Jan 12, 2019)

I heard from supposedly academic sources that 400ppm atmospheric CO2 has recently gone out the window, and today it is more like 600. Is anyone else familiar with reputable science on a number higher than 400 ppm? I'll try to find a peer reviewed article on this to share here.

But, maybe the point is, like @Hoppycalif said, 3 ppm in aquarium water seems accurate from multiple sources, and nitpicking isn't necessary.


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## mistergreen (Mar 3, 2007)

Reediculous_nanotank said:


> I heard from supposedly academic sources that 400ppm atmospheric CO2 has recently gone out the window, and today it is more like 600. Is anyone else familiar with reputable science on a number higher than 400 ppm? I'll try to find a peer reviewed article on this to share here.
> 
> But, maybe the point is, like @Hoppycalif said, 3 ppm in aquarium water seems accurate from multiple sources, and nitpicking isn't necessary.


600 seems high but yes, the number is climbing every year. Last I measured, it was 405. A decade ago, it was 370.

https://www.climate.gov/news-featur...ate/climate-change-atmospheric-carbon-dioxide


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## mistergreen (Mar 3, 2007)

hmmm, I thought of Using ppmv in your calculations and then convert to ppmw but the numbers doesn't work against know data.


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## mistergreen (Mar 3, 2007)

I was looking the the kH/pH/CO2 chart.
Let's assume a drop in 1pH. So the CO2 level is different depending on the kH.
Your formula fit a 4kH but not other kH?


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## hoppycalif (Apr 7, 2005)

I am having a senior moment and can't remember the equation that relates CO2 to KH and pH. I do remember that if you convert the equation by taking the log of each number in the equation, then differentiate it, you get the equation that can be arranged into the equation I used. I may do some more searching for that first equation, but not at the moment.


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## mistergreen (Mar 3, 2007)

I found something
https://www.researchgate.net/post/D...or_pH_estimation_from_rising_level_of_CO2_ppm



> Remi is right. The pH depends on the CO2 partial pressure, the composition of the water, and the temperature. The following formula gives an estimate of the pH:
> pH = log[Alk] - logKa1 - logKH - logPCO2
> [Alk] is alkalinity, Ka1 is the first dissociation constant of carbonic acid, KH is the Henry's law constant for CO2, and PCO2 is the partial pressure in atmospheres. The estimate is better if Ka1 and KH are adjusted for temperature and ionic strength. This estimate is for a beaker of water. A real water hole probably has some calcium carbonate in its sediment and the alkalinity will change as some of the carbonate dissolves in response to the higher PCO2. See chapters 4 and 5 of "Aquatic Chemistry" by W. Stumm and J. J. Morgan.


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## Maryland Guppy (Mar 5, 2015)

The math to solve the poor CO2 chart.


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## hoppycalif (Apr 7, 2005)

That is the equation I was trying to recall! Thank you!

ppmCO2 = P

P = 3xdKHx10^(7-pH)

P1 = 3xdKHx10^(7-pH1)
P2 = tank water ppm CO2 = 3xdKHx10^(7-pH2)

P2/P1 = [3xdKHx10^(7-pH2)]/[3xdKHx10(7-pH1)]
divide out the 3xdKH, because KH is the same for both samples.

P2 = P1 x 10^(7 - pH2 + pH1 - 7) = P1 x 10^(pH1-pH2) =3 x 10^(pH1-pH2)

Therefore, the KH of the water doesn't matter.


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## mistergreen (Mar 3, 2007)

From what I'm reading, the kH does matter. As CO2 leave the water, the kH changes (carbonate & bicarbonate), so you can't have a balanced equation.

This is reflected in the kH/pH/CO2 chart.


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## hoppycalif (Apr 7, 2005)

Years ago this was discussed in aquarium forums, and all of the "experts" agreed that the bicarbonate ions in the water were not changed by the dissolved CO2. Some of us even measured the KH of the water, with CO2 and without CO2, and no difference was seen. The equation that was developed to relate CO2, HCO3-, and H2CO3, did not include any change in the KH (HCO3-). I'm not able to repeat the development of that equation, because I don't have the needed education. It might be that those "experts" from that time were all wrong, but I haven't seen anything yet to convince me that they were. Is there a more up-to-date discussion of this that we can study?


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## mistergreen (Mar 3, 2007)

3 x 10^(pH1-pH2) can't be right. Say you have 1kH water. Look at the kH/pH/CO2 chart. It doesn't fit this equation.

I don't have the education either but the post above might give us clues https://www.aquaticplantcentral.com...-measurement-ph-drop-method-4.html#post997091

chapters 4 and 5 of "Aquatic Chemistry" by W. Stumm and J. J. Morgan might need serious study.

**
Actually, I was reading on carbonic acid the other day,
https://en.wikipedia.org/wiki/Carbonic_acid



> Carbonic acid, which is a weak acid, forms two kinds of salts: the carbonates and the bicarbonates
> H2CO3* ⇌ HCO3− + H+
> HCO3- <=> CO32− + H+


They're intertwined it looks like.


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## hoppycalif (Apr 7, 2005)

I did some checking of the CO2 ppm vs KH/pH chart: https://barrreport.com/attachments/co2_graph_zps9c124ef0-gif.6317/

Pick a KH, 1.0, for example. Look for the pH that coincides with 3 ppm of CO2 at that KH. It is 7.0. Now look at a drop of 1 in pH, or 6.0, and you see that you have 30 ppm of CO2.

Now try KH of 2.0. The pH at 3 ppm of CO2 is 7.3. Look at pH of 6.3, and you get 30.1 ppm.

Now try KH of 3.0. The pH at 3 ppm of CO2 is a little less than 7.5 Look at pH of a little less than 6.5, and you get 30 ppm.

I don't see any problem???


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## mistergreen (Mar 3, 2007)

Oh, I see.. I was picking arbitrary pH, not the CO2. Make note the 3ppm will vary and definitely don't apply to real bodies of water.
I have a feeling the 3 in 3 x 10^(pH1-pH2) doesn't represent CO2 ppm but some constant.


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## hoppycalif (Apr 7, 2005)

mistergreen said:


> Oh, I see.. I was picking arbitrary pH, not the CO2. Make note the 3ppm will vary and definitely don't apply to real bodies of water.
> I have a feeling the 3 in 3 x 10^(pH1-pH2) doesn't represent CO2 ppm but some constant.


The "3" in ppm CO2 = 3xdKHx10^(7-pH) is just a constant that is made up of various constants, but the "3" in ppm CO2 = 3 x 10^(pH1-pH2) is the ppm of CO2 when the water is "out gassed". I have no proof that outgassed tank water, in a typical home, has 3ppm of CO2 in it. For years it has been accepted as a good number, but I wasn't able to verify it. In fact, when I did some experimenting I was able to get lower amounts of CO2 by letting the sample sit on the kitchen counter for 24+ hours. I don't recall how low I was able to get, but I think it was around 1.5 ppm. And, I used to argue that this method of measuring the CO2 in the aquarium was very inaccurate because of that. No one ever agreed with me, so I decided to go along with the consensus.


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## mistergreen (Mar 3, 2007)

In our previous posts, I've establish that indoor CO2 is different that outdoor CO2 levels and the indoor will vary. I think it could be as varied as 0.6ppmw - 3+ppmw or 400ppmv - 1700+ppmv.

Also you derived CO2 = 3 x 10^(pH1-pH2), from CO2 = 3xdKHx10^(7-pH)... The 3 constant should be a constant and not become something else unless you cancel it out too  so all you have left is 10^(pH1-pH2).


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## hoppycalif (Apr 7, 2005)

mistergreen said:


> In our previous posts, I've establish that indoor CO2 is different that outdoor CO2 levels and the indoor will vary. I think it could be as varied as 0.6ppmw - 3+ppmw or 400ppmv - 1700+ppmv.
> 
> Also you derived CO2 = 3 x 10^(pH1-pH2), from CO2 = 3xdKHx10^(7-pH)... The 3 constant should be a constant and not become something else unless you cancel it out too  so all you have left is 10^(pH1-pH2).


No, I didn't directly derive the pH change equation from the original equation. I used the original equation to get a relationship between the original ppm of CO2 and the pH at that time, and a relationship between the degassed ppm of CO2 and the pH at that time. Then I divided one equation by the other and rearranged it to make it the relationship between the two pH readings and CO2 ppm's. That let me cancel the constants that were common to both of the relationships. Then I substituted 3 ppm for the degassed CO2 ppm. I could have used any other number there - it is a coincidence that "3" shows up twice.


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## Maryland Guppy (Mar 5, 2015)

If carbonic acid forms two types of salts shall we say, carbonate & bicarbonate, alkalinity reads the total of both.
Why would the dKH change during CO2 injection?
I've never seen it change and it doesn't matter what time of photoperiod I test it.


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## mistergreen (Mar 3, 2007)

I did some asking around and they say carbonate & bicarbonate produced by carbonic acid is very minor. You won't be able to measure it your typical chemical drop kH test kit. They also said, the only thing the leaves the water is CO2, and the balance of carbonic acid, bicarbonate, and carbonate shift as CO2 leaves.


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## mistergreen (Mar 3, 2007)

Here's a table of the CO2 species relationship on the wiki page









In order to get the pH of a CO2 solution, you need to find the partial pressure of CO2. Lots of chemistry math involved but this table shows you the relationship.

* this is pure water btw.


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## hoppycalif (Apr 7, 2005)

I believe the inaccuracy in this method of measuring CO2 is the degassed concentration of CO2 in the water, and I have always believed that. Unfortunately, I haven't seen any data that supports the belief that 3 ppm is a good number to use, nor whether that is 3 or 3.0 or 3.4 or 2.6, etc. Someone who wants to contribute to the knowledge base of this hobby could make a big contribution by researching and experimenting to determine this.


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## mistergreen (Mar 3, 2007)

I think X*10^(pH1-pH2) is too simplified to work somewhat accurately.
We might need to look into 
pH = log[Alk] - logKa1 - logKH - logPCO2

Rearrange to find PCO2(partial pressure of CO2)... From there we can use Henry's law to find the CO2 in the solution.


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## mistergreen (Mar 3, 2007)

I'm reading on kH, https://en.wikipedia.org/wiki/Carbonate_hardness



> In a solution where ONLY CO2 affects the pH, carbonate hardness can be used to calculate the concentration of dissolved CO2 in the solution with the formula CO2 = 3 × KH × 10(7-pH), where KH is degrees of carbonate hardness and CO2 is given in ppm.


lol, the kH/pH/CO2 chart only works for pure water as we suspected. There's that formula.


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## hoppycalif (Apr 7, 2005)

That puts us back in the same corner: does "3" mean 3.00000 or just 3+/-0.5?


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## mistergreen (Mar 3, 2007)

At this point, I'm thinking the best way to get CO2 is stick a sensor in the water 

Go ahead and stick with your formula 3*10^(ph1-ph2) with an error margin of +- 15ppm


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## Maryland Guppy (Mar 5, 2015)

mistergreen said:


> I did some asking around and they say carbonate & bicarbonate produced by carbonic acid is very minor. You won't be able to measure it your typical chemical drop kH test kit. They also said, the only thing the leaves the water is CO2, and the balance of carbonic acid, bicarbonate, and carbonate shift as CO2 leaves.


Kind of my whole point when testing dKH we never see it change when injecting CO2.
Carbonic acid is too weak to eliminate the alkalinity.
HCl is another story.


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## hoppycalif (Apr 7, 2005)

mistergreen said:


> At this point, I'm thinking the best way to get CO2 is stick a sensor in the water
> 
> Go ahead and stick with your formula 3*10^(ph1-ph2) with an error margin of +- 15ppm


Have you ever kept your sensor in your tank, for 24 hours, and made a plot of the ppm of CO2 vs time, etc?

I'm thinking about filling a fairly large bowl with water from my tank, when I'm doing a water change, and measuring the pH vs time, just to see what it looks like. I only have a pH test kit, so the accuracy would be questionable.


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## mistergreen (Mar 3, 2007)

Yes, I've kept track of CO2 & pH... I'll have to find it. I used a pH stick meter to track the numbers.


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## mistergreen (Mar 3, 2007)

Here you go....
I injected yeast CO2 into a bucket of tap water.. My tap is 3 or 4 kH. Ignore the 'old output' column.


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## mistergreen (Mar 3, 2007)

Maryland Guppy said:


> Kind of my whole point when testing dKH we never see it change when injecting CO2.
> Carbonic acid is too weak to eliminate the alkalinity.
> HCl is another story.


Take note that the kH/pH/CO2 chart only works for pure water/distilled.


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## hoppycalif (Apr 7, 2005)

mistergreen said:


> Take note that the kH/pH/CO2 chart only works for pure water/distilled.


Pure water with nothing dissolved in it that affects the pH or KH but a carbonate/bicarbonate compound and CO2. Baking soda works fine.


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## mistergreen (Mar 3, 2007)

hoppycalif said:


> Pure water with nothing dissolved in it that affects the pH or KH but a carbonate/bicarbonate compound and CO2. Baking soda works fine.


Yup, like in a drop checker.


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