Why Advise A 1pH Drop?

[jaypeecee]

Hi Folks,

I don't understand why a 1pH drop is advised when setting up a profile for injected CO2. For example, if the water KH is 4dH and the starting pH is 7.3, the CO2 concentration will be approximately 6 ppm. But, after a 1pH drop to 6.3, the CO2 concentration will be a staggering 60 ppm. A tenfold increase - no doubt because the pH scale is logarithmic (to base 10). It goes without saying that 60 ppm is well in excess of the 30 ppm limit that I believe is the CO2 limit for fish and other tank inhabitants.

JPC

 

[dw1305]

Hi all,

For example, if the water KH is 4dH and the starting pH is 7.3, the CO2 concentration will be approximately 6 ppm. But, after a 1pH drop to 6.3, the CO2 concentration will be a staggering 60 ppm.

We think the reason is that a one unit of pH drop will always be equivalent to about 30ppm of CO2, what ever the dKH is.

With the usual disclaimer that I'm not, or are ever likely, to be a CO2 user, there is an fairly complete explanation in <"Question about ...">. Have a look at @Jose's comments.

cheers Darrel

 

[jaypeecee]

Hi all, We think the reason is that a one unit of pH drop will always be equivalent to about 30ppm of CO2, what ever the dKH is.

With the usual disclaimer that I'm not, or are ever likely, to be a CO2 user, there is an fairly complete explanation in <"Question about ...">. Have a look at @Jose's comments.

cheers Darrel

Hi Darrel,

But the table shown in your post (the well-known pH/KH/CO2 relationship) clearly shows that the CO2 concentration will vary whilst keeping dKH constant but pH changing, or am I missing something? Just pick out a few points of intersection of pH and KH, note the figure and then drop the pH by 1 whilst keeping the KH constant. Sorry if I'm not being clear. If that's the case, I'll try again.

JPC

 

[Edvet]

pH changes can stem from a multitude of reasons, non co2 related, thus skewing the interpretation of the CO2 levels.
It has been shown the 1 point drop will get you to a healthy CO2 level ( i even got a 1,4 point drop in my soft water) It doesn't mater what the CO2 percentage is actually ( hard to measure without a solid lab) just get to a stable level. You could also do a 0.1 point drop, you just need to match up the lighting levels.

 

[dw1305]

Hi all,

clearly shows that the CO2 concentration will vary whilst keeping dKH constant but pH changing

I don't claim any proper understanding of this, but I think that is the point. The dKH doesn't remain constant because you have the pH ~ CO2 ~ carbonate equilibrium which changes as you add CO2.

Once you have more than ~4dKH of carbonate buffering the pH will always equilibrate to pH 7.8.

The pH kH Co2 chart shows that "pH drop" is independent of the kH up to where Im concerned. If you do an example with two waters with different khs you will find this out. lets try:

Water 1: KH=2, Ph at the equilibrium with air=7.6(lets suppose we have 1.5 ppm of co2 and this shouldnt matter since both water will have the same rough co2 ppm in the equilibrium with air)
water 2: KH=5, Ph at the equilibrium with air=8( again lets suppose we have 1.5 ppm of co2 and this shouldnt matter since both water will have the same rough co2 ppm in the equilibrium with air)

Now lets look at the pH for both waters once we add 30 ppms of co2.
Water 1: KH=2, pH (at 30 ppms of co2)= 6.3
water 2: KH=5, (at 30 ppms of co2)=6.7

pH drop for water 1=7.6-6.3=1.3
pH drop for water 2=8-6.7=1.3

Same pH drop for two waters with different kH as to get the same co2 dissolved in them.

Assuming that the "pH drop of 1" for ~30ppm CO2 is right, it is because it is a buffered system, and pH is a log10 scale. The drop in pH is determined only by the concentration of CO2 in the water.

In the same way a reserve of HCO3- (dKH) will maintain pH at ~pH8 at atmospheric CO2 levels, as you increase the CO2 levels you drive the equilibrium towards H2CO3 and the pH falls.

Assuming you still have a reserve of buffering then the fall in pH may appear to be linear (exponential data plotted as log10 values will form a straight line).

I think everything starts from the assumption that water in equilibrium with atmospheric CO2 levels contains ~3ppm CO2. A drop of 1 pH unit is actually an increase of 10 in the ratio of H+:O-H ions, when you have a pH drop of 1 unit you have:

3ppm x 10 (1 as log10) = 30ppm CO2.

cheers Darrel

 

[jaypeecee]

Hi Darrel,

I refer to your statement that:

"I think everything starts from the assumption that water in equilibrium with atmospheric CO2 levels contains ~3ppm CO2. A drop of 1 pH unit is actually an increase of 10 in the ratio of H+:O-H ions, when you have a pH drop of 1 unit you have:

3ppm x 10 (1 as log10) = 30ppm CO2"

The initial CO2 concentration in the tank water is very likely to be in excess of 3ppm because, overnight, the plants will have been adding CO2 to the water. How much dissolves will depend on the density of planting, water surface area, how much surface agitation, etc. In order to measure CO2 as accurately as my budget will allow, I use the following:

I have set up my Extech pH100 pH meter so that the tip of the flat electrode is immersed in a known KH liquid. Using the Salifert high resolution KH/Alkalinity test kit, the KH is 3.8. What I am doing is creating an electronic version of the drop checker. At the bottom of the assembly is a very thin permeable membrane through which CO2 from the tank water can pass. So, measurement is unaffected by solutes in the water. I'll add some pictures later.

To be continued...

JPC

 

[jaypeecee]

Attached are photos of my setup.

 

[jaypeecee]

Me again!

I should add that, at 38 bubbles of CO2 per minute in a 120 litre tank, the pH in my setup finally settled at 6.39. This, in conjunction with a KH of 3.8, equates to 45 ppm of CO2. None of my fish were harmed in any way whatsoever. In fact, none of them even came to the water surface. But I'm definitely not advocating an upper threshold of 45 ppm CO2. The starting CO2 concentration by this measurement method equated to 6 ppm.

One of the major advantages of this proposed setup is the speed at which it responds. Measurable pH changes can be seen after just one minute and, after just seven minutes, the pH can change by 0.1 pH. It is essential to use a pH probe/meter with two decimal place resolution, i.e +/- 0.01pH.

JPC

 

[dw1305]

Hi all,

I think everything starts from the assumption that water in equilibrium with atmospheric CO2 levels contains ~3ppm CO2.

I don't think we ever found out where the "3 ppm" came from. The theoretical value is <"~about 0.5 ppm">, which depends upon the 400 ppm of CO2 in the atmosphere and Henry's Law.

The initial CO2 concentration in the tank water is very likely to be in excess of 3ppm because, overnight, the plants will have been adding CO2 to the water. How much dissolves will depend on the density of planting, water surface area, how much surface agitation, etc.

That is where the drop checker comes in, as long as it returns to blue you can be fairly sure you are out-gassing all the "spare" CO2.

I have set up my Extech pH100 pH meter so that the tip of the flat electrode is immersed in a known KH liquid. Using the Salifert high resolution KH/Alkalinity test kit, the KH is 3.8. What I am doing is creating an electronic version of the drop checker. At the bottom of the assembly is a very thin permeable membrane through which CO2 from the tank water can pass.

I don't see why it shouldn't work. You would need some-one who has used CO2 to advise you.

cheers Darrel

 

[jaypeecee]

That is where the drop checker comes in, as long as it returns to blue you can be fairly sure you are out-gassing all the "spare" CO2.

cheers Darrel

Hi Darrel,

I wouldn't put any trust in a drop checker being able to reliably indicate the difference between 0 and 10ppm CO2 but I'll run some experiments with my drop checker to see if I prove myself wrong.

JPC

 

[X3NiTH]

I did similar with a ph controller probe. I used a sandwich bagging machine to make a sleeve for the probe out of an air breathing fish transport bag, filled it with drop checker fluid (4dKH) and bromothymol blue for visual checking correlation and stuck the probe in the sleeve and did my best to try and seal it all up, unfortunately I failed miserably at the time because I couldn't adjust the bagging machine sealing temperature and the sleeve kept failing.

The idea is sound though as a gas permeable membrane should be much faster as a transfer interface between the tank water and the liquid sample for quicker readings than the air gap in a drop checker.

I haven't revisited the idea but did some extra thinking on it, one idea was to obtain good accuracy by overcoming the limitations of taking pH measurements in very low conductivity liquids and that was to fortify the drop checker fluid which generally comprises RO/DI and Potassium Bicarbonate added to 4dKH which is very low on the conductivity scale, the thought was to add Potasium Chloride (probe storage solution) to bump up the conductivity quite a bit without affecting the carbonate chemistry of the fluid (haven't tested this yet to see if it works) but it not being a disaster if the fluid ends up in the tank.

I had another idea to use the gas permeable fish transport bags made up like bubble wrap (filled with DC fluid instead of air) and attach it to the inside glass of the aquarium to see visually any stratification or low flow spots.

 

[dw1305]

Hi all,

I don't think we ever found out where the "" came from.

Not sure what happened there, that should have been "3 ppm".

I wouldn't put any trust in a drop checker being able to reliably indicate the difference between 0 and 10ppm CO2

I'm not sure about this either, bromothymol blue is a narrow range pH indicator, but I don't know at what CO2 level it reverts to blue (in a 4dKH solution). According to the chart it is definitely blue at 5 ppm CO2 (pH 7.4).

the thought was to add Potassium Chloride (probe storage solution) to bump up the conductivity quite a bit without affecting the carbonate chemistry of the fluid (haven't tested this yet to see if it works) but it not being a disaster if the fluid ends up in the tank.

Adding a neutral salt (like KCl) definitely works to stabilise the reading on a pH meter.

cheers Darrel

 

[jaypeecee]

I haven't revisited the idea but did some extra thinking on it, one idea was to obtain good accuracy by overcoming the limitations of taking pH measurements in very low conductivity liquids and that was to fortify the drop checker fluid which generally comprises RO/DI and Potassium Bicarbonate added to 4dKH which is very low on the conductivity scale, the thought was to add Potasium Chloride (probe storage solution) to bump up the conductivity quite a bit without affecting the carbonate chemistry of the fluid (haven't tested this yet to see if it works) but it not being a disaster if the fluid ends up in the tank.

Hi X3NiTH,

Thanks for the feedback. I'm pleased that I'm not alone with having tried this method. The conductivity of my dKH4 KHCO3 and NaHCO3 solutions are 187 microS/cm and 128 microS/cm, respectively. I didn't consider those to be low conductivity. It will depend on the pH meter as to whether or not it can accurately take a measurement. Your idea of bumping up the conductivity with KCl solution sounds good. I really like this method of measuring CO2. I'm not a fan of drop checkers - so slow to respond and very subjective being a colorimetric test. I should add that I don't leave my setup permanently immersed in the tank water - at the moment. Build-up of any bio film on the membrane (as can happen at the entrance of DCs) could possibly interfere with the results. Instead, I have attached a hook on the underside of the tank lid. From this, I can hang the pH meter with attachment so that it is immersed in the tank water (as shown in the photos).

JPC

 

[jaypeecee]

...bromothymol blue is a narrow range pH indicator, but I don't know at what CO2 level it reverts to blue (in a 4dKH solution). According to the chart it is definitely blue at 5 ppm CO2 (pH 7.4)

Hi Darrel,

Yes, I agree with you. I think I would be correct in saying that various shades of blue exist from pH 7.2 to pH 8.1 in which the CO2 concentration varies from 7.6 to 1.0 ppm (for a 4dKH solution).

JPC

 

[Zeus.]

I find the DC a very quick way off checking that the CO2 profile is still acceptable without the need to calibrate pH probes and multiple pH readings.
A quick glance before CO2 on is it blue and then mid CO2 period is it green ( or yellowish green in my case) then all is good. Obviously for those reading who havent used CO2 before my quick glance method for checking is after doing pH profiles many times and in a tank that has algae under relative control in an establish tank without changing light periods and intensities . pH profiles aren't always needed esp with experience scrapers and soft water, however for folk new to CO2 and hard water I would strongly advise them

 

[jaypeecee]

You could also do a 0.1 point drop, you just need to match up the lighting levels.

Easier said than done!

JPC

 

[jaypeecee]

Hi Folks,

I'd just like to add a few more of my thoughts about dissolved CO2 concentration:

[1] The 1pH drop when profiling seems to have become the de facto pH change to aim for. I think this figure is too high.

[2] It is important to measure the pH accurately. Could I suggest to +/- 0.05pH or better? And, here I mean accuracy - not resolution. So, that means checking the calibration regularly with pH buffer solutions.

[3] CO2 toxicity to fish is dependent on dissolved oxygen concentration. I don't go along with the idea of waiting until the fish gasp at the water surface so I measure dissolved O2 concentration.

[4] Now, here's the difficult one. I have been unable to find any scientific data that quantifies CO2 toxicity to fish or other critters. I'm not sure where the recommendation of 30ppm maximum comes from. Perhaps someone can enlighten me.

JPC

 

[zozo]

I have been unable to find any scientific data that quantifies CO2 toxicity to fish

Etiology of this disease is still uncertain and it is believed that it is related to dietary factors and or alteration in water physical and chemical parameters (Ferguson1989). Bruno (1996) presented that there are two circumstances which are considered to cause or exacerbate nephrocalcinosis. Firstly, a prolonged exposure of fish to high levels of carbon dioxide at levels greater than 10 to 20 mg/L

http://www.bioflux.com.ro/docs/2016.574-579.pdf

 

[dw1305]

Hi all,

[3] CO2 toxicity to fish is dependent on dissolved oxygen concentration. I don't go along with the idea of waiting until the fish gasp at the water surface so I measure dissolved O2 concentration.

[4] Now, here's the difficult one. I have been unable to find any scientific data that quantifies CO2 toxicity to fish or other critters. I'm not sure where the recommendation of 30ppm maximum comes from. Perhaps someone can enlighten me.

We have some <"older threads"> where Tom Barr @plantbrain <"talked about this">.

I'm not a CO2 user, but the <"sub-lethal effects of elevated CO2"> would worry me if I was.

You can use 2dKH solution in your drop checker to give you a light green response at 15ppm CO2.

cheers Darrel

 

[zozo]

I'm not sure where the recommendation of 30ppm maximum comes from.

I guess nobody is and it depends highly on the fish sp. there is scientific data to be found that certain tropical fish sp. can take up to 60ppm without issues. In these sp. it is found that 80ppm had narcotic effect and above was deadly. Fish do also slowly acclimatize to higher CO² concentrations. Never add new fish from the LFS to a High tech tank. You might kill them while acclimatized fish show no problems. This can have an effect on newly introduced fish far bellow 30ppm.

Simmilar to if we would be dropped on top of mount everest in a split second. It might kill you..