Calculating KH without a KH Test

[Tong]

This was brought up in another thread. I'll place it here as to not hijack that thread.

The formula for calculating CO2 is:

CO2 (in PPM) = 3 * KH * 10^( 7-pH )

When we are not injecting CO2, ambient CO2 is ~3 ppm so we get:

3 = 3 * KH * 10^(7-pH)
1 = KH * 10^(7-pH)
1/10^(7-pH) = KH

So for example we know our pH is 7.8 and we are not injecting CO2, therefore we have:

1/10^(7-7.8) = KH
1/0.1585 = KH
KH = 6.3

Wala. You can now plug this KH value with your new PH measurement after CO2 injection back into the original CO2 formula to find your CO2. And you saved $9. Go you.

Edit: Obviously you could work the formula to give you PH if you had a KH, but I find most people have a PH while not everyone has a KH test.

 

[src]

Doesn't this theory break down in an aquarium due to the fact that the plants and fish are altering the CO2 levels constantly? You can't really count on 3 ppm.

 

[mr funktastic]

that mkaes sense!! lol i was avoiding buying a kh kit but if this is right i wont have to!! thanks tong. your the next wizzard of oz lol

 

[Purrbox]

There's one very big problem with this conclusion. If there are other buffers in the water it invalidates that formula. Without a KH kit you have no way of determining if other buffers are present or not.

 

[Tong]

Doesn't this theory break down in an aquarium due to the fact that the plants and fish are altering the CO2 levels constantly? You can't really count on 3 ppm.

Honestly, I don't know first hand. I've simply read from many sources that without CO2 injection, a planted tank will always stay at about 3 ppm CO2. I assume they knew more than I did and could test better than I could. And unfortunately...I forgot who they all are. Hehe.

But if you think about it, no matter how much CO2 your plants take up or how much CO2 your fish produce, your CO2 will always stay fairly constant because it will either gas off or re-enter through the surface. Only when we inject a huge amount of CO2 at a rate faster than can be gassed off by the surface, can we be attain a CO2 level higher than ambient. Fish don't produce enough CO2 to do this and at low levels of CO2, I don't think plants can pull more CO2 than what can be supplied by the surface.

There's one very big problem with this conclusion. If there are other buffers in the water it invalidates that formula. Without a KH kit you have no way of determining if other buffers are present or not.

It took me a second to realize what you meant. A KH test kit doesn't measure other buffers, just carbonate hardness. A GH test does. I might be wrong on that though. I think its irrelavent anyway as I believe the CO2 formula already makes the assumption that there are no other buffers in the water. So if you have other buffers in your water, the common CO2 calculation will be wrong for you even if you use a KH test.

From Chuck Gadds Page:

The pH-KH-CO2 Relationship
pH, KH, and CO2 have a fixed relationship as long as carbonate is the only buffer present (no phosphate buffers like pH-UP and- DOWN, Discus Buffer, etc). There are some parts of the country that have high levels of phosphates in their water supply. For those cases, determining CO2 levels will be difficult, as the phosphate will throw off the pH-KH-CO2 relationship, which means the CO2 charts and calculator below won't work. Note that the commercially available CO2 test kits will also be invalidated by the phosphates.

To determine your CO2 level based on the pH and KH, you can enter the values into the on-line calculator, or use the chart at the bottom of the page.

NOTE: If you aren't adding CO2 to your water, and the CO2 level based on the pH and KH indicates more than 5ppm, then it is very likely that some other buffer (such as phosphate) is present in your water. In an inhabited aquarium, the amount of CO2 produced by the fish will not have an effect on CO2 levels in the water. Any excess CO2 created by fish will dissipate into the air, leaving a fairly constant CO2 level of about 3-4ppm. If you test your pH and KH, and without adding any CO2, the chart says you've got 20ppm CO2, don't believe it.

In some case, water coming right from the tap can contain very high or very low levels of CO2. This can result in tap water with a high KH, and low pH. But, in just a few hours, that excess CO2 will dissipate from the water, leaving the normal 3-4ppm, and the pH will rise. Sometimes, the water might come from the tap with extremely little CO2, which can result in tap water with a low KH, and a very high pH. Again, after a few hours, the CO2 level will equalize, and the water will end up with 3-4ppm CO2.

I hope that answers the questions about validity.

 

[czcz]

(edited original post to remove the simley from the formula.)

Doesn't this theory break down in an aquarium due to the fact that the plants and fish are altering the CO2 levels constantly? You can't really count on 3 ppm.

If you assume ambient CO2 is 3ppm though, you can eliminate the tank variables by testing a glass of standing water after 24hours. From what I've read from experts and other hobbyists, I think 3ppm CO2 is an acceptable assumption.

Like Purrbox, more importantly in my mind is one accept the assumption that the CO2/KH/pH relationship isn't broken with their particular water before applying the formula.

A KH test kit doesn't measure other buffers, just carbonate hardness. A GH test does.

I may be wrong, but for the purposes of breaking the CO2/KH/pH relationship I do not think it is measurable GH (Mg, Ca) that hurts but rather other stuff difficult to test.

 

[jsoong]

czcz is right. GH does not measure buffers at all & really has little bearing on your KH/pH/CO2 relationship.

I think you should measure KH is to validate your tests, and to make sure there are no other buffers present. So, if you measure pH & KH and calculate a CO2 that is way out of wack (ie not 3 in water equilibrated with room air), then you either have secondary buffers or your tests are screwed up.

It is pretty difficult to measure the secondary buffers (esp. when you don't know what they are). In the hospital (I work with CO2 in people), we measure the CO2 and the pH, and the difference between calculated HCO3 and measured HCO3 levels gives an idea of what else is present.

 

[hashbaz]

After looking a long time at the CO2 charts, I found that no matter what your KH is, an increase in CO2 from 3 to 30ppm always dropped the pH by about 1. So if you want to go from 3ppm up to 30ppm, simply lower your pH by 1. Because of this, I would have to agree that the KH is irrelevant (or at least less than vital) for calculating CO2.

The presence of buffers other than carbonates may throw off my theory, but since the CO2 charts are invalid (with additional buffers), this theory might work better anyway.

Or am I missing something?

 

[mr funktastic]

your ph could drop only .1 ppm and your Co2 could be 30 ppm. kh measures your waters abililty to handle the co2 basically. if u have a really high kh then your ph wont change that much even if u add tons and tons of co2. but if u have a low kh then your ph could drop 1 and your co2 might still only be at 15ppm. i thinks thats how it goes any way.

 

[greenmaji]

How about that big old chunk of limestone that just mysteriouly looks like something safe in your tank?

Other then your substate and decortive rockwork not buffering your water I dont see a problem with using algebra to proof KH

 

[Tong]

The point is that when people measure CO2 in thier tanks using PH and KH, they already don't consider other buffers in the water. And every CO2 calculator out there will state that CO2 readings will be off if there are other buffers in the water. Is measuring CO2 in this way accurate then? No of course not...BUT it is still useful...seeing as I bet all of you use it. Calculating KH in this way is no more accurate...but no less useful.

After looking a long time at the CO2 charts, I found that no matter what your KH is, an increase in CO2 from 3 to 30ppm always dropped the pH by about 1.

I've found this same thing to be true as well. I've noticed that a pH drop of 0.8 will always be the result of CO2 going from 3 ppm to 19 ppm. This is not to say that KH is irrelevant though, as a higher KH will mean a higher starting pH, which means you can inject more CO2 and still be in a safe range.

 

[malkore]

There's one very big problem with this conclusion. If there are other buffers in the water it invalidates that formula. Without a KH kit you have no way of determining if other buffers are present or not.

Bingo. I can't even count the number of times I've had someone give me a pH of 6.8 and a dKh of 10.0, and they were doing NO CO2 injection.

Plus, I have 'normal' tap water with 3ppm of CO2, no weird buffers.

I've had 2 of my tanks exceed my tap water Kh due to evaporation and top offs...so you'd constantly have to be recalculating your 'estimated' Kh from reverse engineering the formula.


It was a really good idea. Unfortunately that formula already has +/-10% accuracy in determining CO2 ppm to begin with, and other buffers will throw it off further.
A Kh test kit costs $6 and will last 2 years.

 

[Tong]

So to simplify...most people accept that using pH and KH to find CO2 is acceptable...but working it backwards using CO2 and pH to find KH is unacceptable?

Isn't that like saying...

a + b = c is true

but b = c - a is not true?

It is after all...just algebra.

I've had 2 of my tanks exceed my tap water Kh due to evaporation and top offs...so you'd constantly have to be recalculating your 'estimated' Kh from reverse engineering the formula.

This point I accept. KH can always be changing and you simply can not turn off your CO2 and let it sit for a day. However, you could take a little bit of water out, let it sit for 24 hours, retest pH and then work it through the formula. And yes, I realize this is more work than just getting a test kit. My point is I think it can be done.

 

[Purrbox]

I guess the real question is whether or not all the extra work to verify your starting PH and compare to your current PH is worth saving $6 on a test kit. I think that the main reason everyone uses the formula is because it gives us a good quick estimate of the CO2 in our tanks, combined with the fact that we really don't have another good way of determining CO2. There is an alternate method for calculating CO2 in a tank that has other buffers present, but again the problem is that it involves a bunch of extra work.

 

[czcz]

most people accept that using pH and KH to find CO2 is acceptable

Usually it is recommended one validate the relationship by testing KH and pH of standing water (3ppm CO2) first to validate the results. Because so many folks have a broken relationship, modern methods focus more on the pH drop, the alternate method Purrbox posted, and others, or simply observing the plants and fish (Barr's method/EI). Many also no longer care about exact KH, only that it stay stable for their fish and the hard plants.

FWIW I like the idea. Just needs the qualification that it assumes KH/pH/CO2 works.

 

[fish_4_all]

I think that if you are willing to accept that pH and KH calculations are absolutely correct then you can also assume the reverse formula is also absolutely correct. If you assume that, like my tanks, the pH and KH calculation is acceptable but not absolutely correct then you can not reverse the calculations because you don't have an answer, you have an acceptable value. Remember, being off by .2 in an equation where you use whole numbers and addition won't change your results, exponents will.

My regular pH before I started injecting CO2 was 7.2 and KH was 7. After I started injecting CO2 my pH went to 6.6. Which means my CO2 levels went from 3, if you assume that it is NOT 15 from Chuck’s calculator using KH and pH. It then went up 72 ppm because of CO2 injection. This with passive diffusion in my AC HOB which is almost impossible, or it is supposed to be. Being the calculation is assuming a perfect environment; I think it is simply more useful to know that once you add injected CO2 you can then get an idea of what your CO2 could be.

I know why my readings are off some. I have snail shells in my tank from old snails and I now have MTS in my tank. The shells and any new snails that die will add more shells that will dissolve and throw my KH off resulting in high CO2 readings.

All that matters to me right now is I can grow any plant I want to and the CO2 levels are high enough that my plants like it but my fish don't die from it. Did my CO2 ppm go up, of course they did. Do I assume I have 75ppm, no way! More like 30-50ppm because I doubt my CO2ppm went up 72 ppm from passive injection in my AC HOB.

BTW, I don't know why there is an assumption of 3 ppm in any tank without injected CO2 if the KH is higher than 1dKH. That is a pH of 7.0 and a KH of 1dKH to get 3ppm CO2. Can anyone explain why I would assume that I only have 3ppm CO2 if my KH is 10 and my pH is 7.2 when the calculator says I should have 19ppm? Maybe this is why some people have some really sweet growing tanks and have never injected CO2 where some can't grow anything until they get power diffusion and compressed CO2. Of course this doesn't take into account their light levels but that has been beaten into the ground already in other topics.

 

[Purrbox]

Basically 3ppm of CO2 is the ambient level of CO2 in water. If you aren't doing anything to the water that would affect the CO2, this is how much CO2 should be present give or take a smidge. I'm sure there's lots of scientific research out there somewhere to prove this, I just don't happen to know where. The other thing that you need to keep in mind is that KH and pH move together (except in the case of CO2 injection). This means that if you increase your KH by 1 degree there would be a similar change in pH leaving you with the same resulting amount of CO2.

The reason that the calculator says you have more CO2 than you actually do is there are other buffers in the water that throw off the pH/KH relationship. The formula that the calculator is based on assumes that KH is the only buffer, so in your case it is invalid. This is why you need to test some rested tap water (or whatever water you are using in your tank) to ensure that you get a result of approximately 3ppm CO2 from the formula. It's how you determine whether or not there are other buffers present throwing off the results of the CO2 formula.

Hope this clears things up some.

 

[malkore]

Kh shouldn't change.
and the formula for CO2 from Kh and pH has a 10% variance.
so reversing it, your Kh result would have a 10% variance. no good. you need to know the REAL NUMBER. the formula uses real numbers...your formula uses assumptions of numbers.

and reversing it will give you what your Kh would normally be. if you have screwed up tap water, you'll think your Kh is 4, when it's really 9, and either end up with too little CO2 and dead plants, or wasting too much, and killing CO2/pH sensitive fish.

at least if you have the Kh test kit, you know your water is screwed up and the normal formula won't work.

 

[mr funktastic]

ok so if i do the formula that u have posted tong then i get
1/10^(7-7.9) = 7.94 KH

which is a pretty high KH. i also have a high ph though. and based on the amount of calcium i get in my tanks and everything else(u can really feel if u have hard water when u have a shower) i would assume that this could be accurate. but if that is true then according to chucks co2 calculator i am getting about 75 ppm of co2!! which is pretty scarry high. but nothing in my tank is showing stress of anykind and everything looks good. my ph is at around 6.5 - 6.6 right now with injection. i am running 2 2litre bottles that are diffused by a hagen ladder then any bubble that goes out of the top is sucked into my AC and diffused the rest of the way. should i be worried? maybe disconnect a bottle? its a 20 gal by the way.

 

[Lepomis]

So I tested two of my tanks. Neither have Co2 injection. Tank one pH 8.4 so equation equals 25.1 (tested 20). Tank two, pH 7.6: equation 4.0 tested (7).