# Aquasoil pH increase



## herezor (30 May 2016)

Hi

Quick question and maybe a silly one. I had my ADA Aquasoil decreasing my pH during about 7-8 moths from 7.8 (tap water) to around 6.6-6.8 at the beginning. That is, as you all know, quite normal. That value started to go up 2 or 3 months ago and now my pH is 7.8 (same as tap water). The  conclusion I reach to is that the buffering capacity of my Aquasoil is over. The question is;

_Does this lack of buffering capacity have any implications in my plant growth?. Does it matter at all?. Can I keep using it?_

FYI, I have been dosing EI all the time so if Aquasoil nutrients have just run out, it is not a problem.  And I do not care much about KH or pH values

Cheers

Pedro


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## rebel (31 May 2016)

The only implication for plant growth is the resultant kH. Generally plants grow better at lower kH (4-6) so if you can maintain that, then you should be able to grow anything. For most 90% of plants out there, even that wouldn't matter. So yes, you can keep using the same aquasoil with EI.

Do you use CO2?


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## herezor (31 May 2016)

Yes I do use CO2. And KH is around 5 (crappy test kit...).

OK thanks. I thought that wouldn´t make any difference, but just to make sure

Thanks again


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## ian_m (31 May 2016)

rebel said:


> Generally plants grow better at lower kH (4-6)


Er no. Some plants may grow better, but in general plants don't care about water hardness.

See the pictures here of fine green plants on rock hard water.
http://www.ukaps.org/forum/threads/high-cec-with-hard-water-pointless.38582/#post-418921
http://www.ukaps.org/forum/threads/wanted-pictures-of-tanks-kh-related.36141/page-2#post-398380


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## rebel (31 May 2016)

ian_m said:


> Er no. Some plants may grow better, but in general plants don't care about water hardness.
> 
> See the pictures here of fine green plants on rock hard water.
> http://www.ukaps.org/forum/threads/high-cec-with-hard-water-pointless.38582/#post-418921
> http://www.ukaps.org/forum/threads/wanted-pictures-of-tanks-kh-related.36141/page-2#post-398380


Hi Ian, that's why I said generally.  Also can you show me Tonina fluviatilis (must be alive and thriving) in 'rock' hard water?


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## EdwinK (31 May 2016)

I guess that there is a misunderstanding how low kH benefits the plant grow. Generally speaking in low kH water you are able to dissolve more CO2 in natural or artificial way.


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## rebel (31 May 2016)

EdwinK said:


> I guess that there is a misunderstanding how low kH benefits the plant grow. Generally speaking in low kH water you are able to dissolve more CO2 in natural or artificial way.


Hence the reason I casually asked about whether the OP uses CO2...  there is method to the madness.

Simple questions can have nuanced answers.


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## ian_m (31 May 2016)

EdwinK said:


> Generally speaking in low kH water you are able to dissolve more CO2 in natural or artificial way.


Er no again. Hardness (or any other salt) does not affect the solubility of the CO2. Thus if one adds say 10 grams of CO2 to 200litres volume of water to get 30ppm (for instance) you will always get 30ppm regardless of the hardness of the water.

What hardness does do is alter the pH changes for the addition of your CO2.


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## EdwinK (31 May 2016)

Can't agree with that. kH reading has nothing to do with solid salts it states how much of hidrocarbonate ions are dissolved in the water.


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## dw1305 (31 May 2016)

Hi all, 





EdwinK said:


> Can't agree with that. kH reading has nothing to do with solid salts it states how much of hidrocarbonate ions are dissolved in the water.


It is a bit strange, but adding CO2 doesn't change the dKH (the amount of HCO3- ions) in the water. 

When you add CO2 (so that the rate of addition exceeds the degassing rate) you push the HCO3- ~CO2 equilibrium towards CO2. The pH falls because the 0.15% of the dissolved CO2 that becomes H2CO3 is now a larger proportion than it was at the stable equilibrium point.

More CO2 in the water = more H2CO3 and the additional H+ ion cause the pH to fall. When you add 30ppm of CO2 you get a drop of about 1pH unit, it doesn't matter how many HCO3- ions you have (pH drop is independent of dKH, because pH is a log10 scale).

Calcium carbonate is insoluble in water, but in water with carbonates present the small amount of CO2 (that goes into solution as H2CO3) is in equilibrium with the HCO3- to give a stable value of ~pH8 at atmospheric CO2 levels (400ppm CO2) and standard barometric pressure (1013mb). Carbonic acid (H2CO3) and bi-carbonate (HCO3-) are the weak acid and weak base pair in carbonate buffering.

When we add CO2 above atmospheric levels we drive the H2CO3 ~ HCO3- equilibrium towards H2CO3. We know that pH is a ratio, and that an acid is defined as a H+ ion donor and we've added extra H+ (from H2CO3), so the pH falls. How much the pH falls depends upon the reserve of carbonate buffering, we usually measure this as "dKH".

We use a 4dKH solution and narrow range pH indicator (bromothymol blue) in a drop checker because this combination allows us to estimate CO2 addition using the experimentally defined pH/dKH/CO2 chart.



 
cheers Darrel


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## EdwinK (1 Jun 2016)

Thanks. Then explain me one more thing. Lets say we have two tanks with same water in and we are using same CO2 dissolving device with the same CO2 amount. One tank has a carbonate buffer of 20 KH and other 2 KH. Now lets add 10 g of CO2 to both tanks. Do we get the same pH drop?


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## EdwinK (1 Jun 2016)

With the same initial pH.


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## ian_m (1 Jun 2016)

EdwinK said:


> With the same initial pH


That is impossible. Differing KH will give differing pH values.


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## dw1305 (1 Jun 2016)

Hi all, 





EdwinK said:


> One tank has a carbonate buffer of 20 KH and other 2 KH. Now lets add 10 g of CO2 to both tanks. Do we get the same pH drop?





EdwinK said:


> With the same initial pH.


No, I don't think we do, the relationship will break down at low dKH values, and that is to do with the reserve of carbonate buffering. If you have a small reserve of buffering ("base" H+ ion acceptors) you will need to add a smaller amount of acid (H+ ion donors) to change the pH. That is quantified in the pH/dKH/CO2 chart.

If we want to measure the total reserve of carbonate buffer in the water you would use titration with an acid of known strength to an end point marked with the change of colour of a pH indicator. This is basically what you do with a drop checker when you add CO2 (the acid) to a solution of known dKH (the base), using bromothymol blue as the narrow range pH indicator.

The actual chemistry is quite complicated (I'm not a chemist) because dissolved CO2 is a <"diprotic acid">. If you start with pure water (H2O), at standard temperature and pressure, the pH will be <"~pH5.7">, due to the dissolved CO2 (based on 400ppm CO2 in the atmosphere).

If you add a source of carbonates (but not any other <"non-neutral salts">) the rise in pH is entirely due to the carbon dioxide ~ carbonate equilibrium and the addition of HCO3- ions will raise the pH to pH~8.3, whether we have 2dKH or 20dKH. Usually however we would have other acids, or bases, present. In natural water with very low dKH the small addition of acids from humic compounds will lower the pH, often down to ~pH4.   

The detail is in <"http://www.chem1.com/acad/webtext/pdf/c3carb.pdf">. 





> Photosynthesis and respiration by algae correspond to opposite directions of the reaction. Addition, or removal, of CO2 have no direct effect on the alkalinity, but these processes do effect the pH and are responsible for the significant diurnal changes in pH that can be observed in ponds and small lakes.



cheers Darrel


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## herezor (1 Jun 2016)

EdwinK said:


> Do we get the same pH drop?



Well, obviously not. Adding the same amount of acid to a solution with different buffer capacity will generate different pH drop. But, I think, that is exactly what Darrel is saying here...



dw1305 said:


> How much the pH falls depends upon the reserve of carbonate buffering, we usually measure this as "dKH".





EdwinK said:


> Can't agree with that. kH reading has nothing to do with solid salts it states how much of hidrocarbonate ions are dissolved in the water.



No and Yes. No to the sentence where you say that kH reading has nothing to do with solid salts. Hydrocarbonate salt are composed of solid ions. HCO3-, CO3-2 are ions derived from salts like KHCO3, NaHCO3, Na2CO3, etc. They are not normally present in tap/drinking water. The only carbonated salt usually present in tab or drinking water is CaCO3. And they are all solid salts. In solution you have 1 mol K+ or Ca+2 and 1 mol of HCO3-1 and 1 mol of CO3-2  from KHCO3 and CaCO3 respectively. The salt dissociates in positive and negative ions and that is why they get dissolved or surrounded by water molecules interacting with them. If your tap water has only CaCO3 as carbonate and you have a reading from your local water company, you could calculate your máximum KH (those report give the GH value normally and not the KH) using the Ca+2 concentration (it will normally be lower as there are other compounds using Ca+2)



ian_m said:


> Er no again. Hardness (or any other salt) does not affect the solubility of the CO2. Thus if one adds say 10 grams of CO2 to 200litres volume of water to get 30ppm (for instance) you will always get 30ppm regardless of the hardness of the water.
> 
> What hardness does do is alter the pH changes for the addition of your CO2.



I agree and disagree here. The KH does not affect the solubility of CO2. If you add 10 g, then you add 10 g and the 10 g are dissolved (or not, depending on pressure and temperature, for how long they will remain dissolved will depend on the equilibrium). The pH drop will depend on the KH. Higher KH, lower pH drop. Lower KH, higher pH drop. Adding the same amount of CO2, of course. I desagree where you say that if you add 10g of CO2 and get 30 ppm, you will get that independent of the KH valeu. That is not correct. To get the same amount of CO2 in two tanks with different KH, you will need to provide different amounts of CO2. The higher the KH the higher the amount of CO2 to be provided to achieve a 1 unit pH drop. But you will have 30 ppm still. What varies is the proportions of the different components of the equilibrium equation:

CO2(gas) + H2O <---> H2CO3 (sol) <--->  HCO3-(sol) + H+ <---> CO3-2(sol) + H+

Let´s build an hypothetical case. Without any addition (CO2), one can have 30 ppm H2CO3 and 300 ppm HCO3 and the ratio HCO3/H2CO3 is 10 or one can have 3 ppm H2CO3 and 30 ppm HCO3 and ratio is also 10. First case is high KH and second is low KH, and both are at equilibrium and at whatever pH. Let´s say we add 30 ppm CO2. It is more difficult to increase the proportion in solution from 30 to 60 ppm of H2CO3 (high HCO3 concentration of 300 ppm pushes equilibrium to the left, avoiding the CO2 dissolution, and that is what biochemist call _buffer effect or the resistance of a acid/base pair to allow a pH change_) than from 3 to 33 (lower HCO3- concentration will be displaced to the right towards CO3-2) using CO2. When they reach equilibrium, in the first situation you will have maybe 35 ppm H2CO3 and 305 ppm HCO3 (almost no change in HCO3/H2CO3 ratio or pH as HCO3- is pushing equilibrium to the left). In the second situation you will have around 20 ppm H2CO3 and around 40 ppm HCO3 (easier to move equilibrium to the right and a big change in HCO3/H2CO3 ratio or big pH change). In first case you will have say 5 ppm (35 ppm final minus 30 ppm initial) and in the second you will have say 17 ppm CO2 (20 ppm final minus 3 ppm initial). If you add again 30 ppm CO2, you will reach say 10 ppm in the first case and close to say 30 in the second. In first case your pH drop has been low (conserved HCO3/H2CO3 ratio) and in the second your pH drop has been high (shifted HCO3/H2CO3 ratio). In the end, in the first situation, to reach 30 ppm HCO3- you will have to add a lot more CO2 but at the end the HCO3/H2CO3 ratio will have to be exactly the same as in the second situation, maybe a ratio of 1...  but same pH drop (same ratio). And still 30 ppm.

And this is why ADA aquasoil decreases KH, by removing either HCO3- or CO3-2 ions by combination with a positive charged compound or ion which generates surely an insoluble product that most likely precipitates out of solution. This moves the equilibrium to the right of the equation releasing H+ ions and decreasing pH. And all of that with the purpose of making it easier for the aquarist to achieve a good CO2 value in solution without needing a lot of CO2

Sorry for the dense post and I hope I explained myself correctly

Cheers

Pedro


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## EdwinK (1 Jun 2016)

Thanks guys for your help.


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## rebel (1 Jun 2016)

Thanks for the above discussion guys. I stand corrected!

For some reason I thought very hard water may cause co2 absorption issues for plants. Not sure where I got it from. 

Some plants don't like high kh though, regardless of co2. Almost all plants will adapt to low kh however?  is that correct?


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## herezor (1 Jun 2016)

Well, I am not an expert, and I am not really sure why that would be. I cannot see the science behind it. Only thing that comes to my mind would be the salinity increase that the the counterpart ions would provide. Na+, Ca+2, K+. As I metioned before, for each HCO3- ion there must (usually) be another Na+ or K+ or whatever and for each CO3-2 there has to be one Ca+2 or 2Na+ or 2K+. A high KH implies high content of HCO3- and/or CO3-2 and hence high Ca+2, Na+, K+ or whatever ion. Those ions increase salinity of the water which may interfere with electrochemical potential of plant cell membranes which is associated to molecule transport. Some plants may be easily affected by this than others.

Just hypothesizing. Don´t take anything I just said for granted and I will be happily stand corrected by the plant physiology gurus around here.


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## dw1305 (3 Jun 2016)

Hi all,





rebel said:


> For some reason I thought very hard water may cause co2 absorption issues for plants. Not sure where I got it from.


That is right, the total amount of Total (dissolved) Inorganic Carbon (DIC/TIC) doesn't change with pH, but the proportions of the compounds that make-up the TIC do. (From <"limited effect of CO2....">)


 

Plants that naturally occur in hard water can utilize HCO3- ions as their source of inorganic carbon source (it gives them a competitive advantage because the vast proportion of TIC is HCO3-). When the plant removes the CO2 from HCO3- you are left with OH- which causes the pH to rise (OH- is a base (or "proton acceptor"), because it will pick up a H+ ion (a "proton")) to get back to H2O. There is a description in <"Biogenic decalcification">.

Plants that naturally occur in soft water don't have the adaptation to use HCO3- because they have access to CO2, and there is a physiological cost involved in being able to utilise the CO2 from HCO3-. 





herezor said:


> A high KH implies high content of HCO3- and/or CO3-2 and hence high Ca+2, Na+, K+ or whatever ion. Those ions increase salinity of the water which may interfere with electrochemical potential of plant cell membranes which is associated to molecule transport. Some plants may be easily affected by this than others.


I think those probably are the reasons, lack of access to CO2 in plants without the adaptation to use HCO3-, and the high levels of Ca++ interfering with the uptake of other ions, particularly Fe+++, Mg++ and K+. You may also get problems with precipitation of insoluble phosphate and iron compounds. 

There is a much more thorough discussion in <"Personal experience using...">.

cheers Darrel


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## zozo (4 Jun 2016)

dw1305 said:


> <"Biogenic decalcification">.


Awsome website!! Thanks..


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## dw1305 (4 Jun 2016)

Hi all, 





zozo said:


> Awsome website!! Thanks..


There is a still a load of good stuff on their. I often refer to it even now.

cheers Darrel


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## zozo (4 Jun 2016)

dw1305 said:


> Hi all, There is a still a load of good stuff on their. I often refer to it even now.
> 
> cheers Darrel



There certainly is, very knowledgeable articles to find.. I do not mind it looking like or maybe even was an old newsgroup community.. Kinda still used to that from the early internet days..  Tho maybe not that inviting to read for many.. But a lot of info is top notch..


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## zozo (4 Jun 2016)

I see now it indeed is partialy is from usenet newsgroups.


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## herezor (13 Jun 2016)

dw1305 said:


> Hi all, There is a still a load of good stuff on their. I often refer to it even now.
> 
> cheers Darrel



For me this site is a fountain of knowledge where, every now and again, I come back to drink a sip


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