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Old 04-18-2005, 12:52 AM   #1 (permalink)
 
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Default Methods of altering pH

Hi everyone,

I'm new to the board, and did a quick search regarding my inquiry but couldn't find the specific info I was looking for.

As far as I know, there are multiple ways to lower your pH in an established tank, but I have some specific requests for others opinions. (Assume RO water is being used and General Hardness is around 5-8.)

From what I have learned thus far, optimizing plant growth (before considering macronutrients, micronutrients, trace elements, and lighting) has been to lower your pH using CO2 injection creating carbonic acid. I am aware of the implications of carbonate hardness and its effect on the amount of acids introduced. I am also aware that aerating the tank or surface agitation can cause CO2 to dissapte, therefore raising the pH.

I am curious about using peat filtering to create tannic and gallic acid which also will also lower the pH. I am assuming but am not sure that this method will not introduce sufficient dissolved CO2 in the tank and therefore will not be of much benefit in an 'ideal' planted tank.

I have also heard breifly about phosphouric acid (mainly phosphate buffers) to lower the pH.

My main inquiry involves the consistency of these buffering methods. I am interested in input including heavily aerated water (much surface agitation, air stones) in tanks that have been buffered without carbonic acid. My assumption is that you can aerate a tank using peat or buffered with phosphouric acid all day long with little implications on the pH.

At this point I would never reccomend maintaining a successful planted tank without using CO2 injection or a CO2 supplement such as flourish excel.

So this brings about the Alkaline buffer and Acid buffer made by SeaChem. On the bottle, they claim you can alter your pH by using these buffers in different proportions. Since these buffers used in conjuction claim to create carbonic acid in which to alter your pH, it seems to me that using these buffers to create a lower pH will be of little value if you have much surface agitation which would seem to temporarily lower your pH until the dissolved CO2 from the carbonic acid dissaptes. If this is indeed the case, using Acid and Alkaline buffer would not be of much help when attempting to maintain a stable acidic pH.

It also seems to me that phosphouric acid used without balancing disolved the other 2 macronutrients would contribute to nuisance algae growth,

Mainly as this point I don't see any benefit of using other pH altering methods as being beneficial in a planted tank. My current opinion (which I am open minded about) assumes carbonic acid and minimal aeration is necessary for optimal growing conditions, and the other methods of lowering pH should be saved for those attempting to simulate breeding conditions.

Any input would be much appreciated.

-Tim
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Old 04-18-2005, 04:20 AM   #2 (permalink)
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I'm not too sure I understand the question..

Lower pH per se isn't what's going to make the plants happy. A lower pH happens to be the result of the creation of carbonic acid in the water through the injection of CO2. What you are trying to give your plants is carbon...

I don't think any liquid commercial buffers create carbonic acid. Some of them are just phosphoric acid which I'm pretty sure does nothing for the plants anyway. Seachem Excel is not really CO2 but an alternative source of carbon for the plants.
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Old 04-18-2005, 05:11 AM   #3 (permalink)
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I learned the hard way not to use phosphate buffers or to change my pH. Have you tested your tap water after letting it sit 24 hours? Why do you want to change your pH?
Rexgrigg.com has some good info if you haven't seen that site already.
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Old 04-19-2005, 03:07 AM   #4 (permalink)
 
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Hi Tim,
I agree totally with Laith. It appears from your post that you are fixated on pH and have lost focus on the "big picture" as they say. Plants are composed of primarily four things Carbon, Nitrogen, Phosphorus and Potassium. Carbon is really quite important to the structure of the plant, that's why when we consume plant products such as grain, cereal and beer we obtain important "Carbohydrates". We are made of Carbon as well. So to make plants grow and produce their food we should offer them these four ingredients in large quantites and offer the other nutrients such as Iron in much lesser quantities. Terrestial plants have no problem accessing Carbon because the atmosphere contains sufficient concentrations of CO2. In water however the concentration of CO2 is generally much lower. To increase growth therefore we dissolve CO2 in the tank to increase it's concentration thereby making Carbon available to the plants. As the CO2 dissolves however it ionizes into Carbonic Acid and as an acid it lowers the pH.

Therefore a lower pH is a side effect of our efforts to provide Carbon in this form. It is not the goal to "...add Carbonic Acid...", There is absolutely no reason to use other acids to lower your tank's pH as a primary plant goal. Your primary goal should be to supply these four elements in whatever form available to you as well as to provide the micronutrients such as iron, magnesium etc. These nutrients should be supplied at a high enough level in the tank to sustain growth and to suppress algae.

For your info, many have reported excellent results using Excel as opposed to CO2 injection. This product provides Carbon in a different form and its concentration in the tank is unaffected by aeration therefore it can be recommended for those who do not wish to implement a CO2 injection scheme. Their success was not based on Excel alone, they supplied the other nutrients and used the Excel to satisfy the plants Carbon requirements.

If one wishes to add Carbon and to maintain a specific lower pH value (for example fish that require a low pH for breeding) then this seems more easily acheived via low kH and CO2 injection, as the acid buffers are unreliable in maintaining a specific pH value.

So to summarize, it is not the fact that you acidify the water with Carbonic acid that is important but that this particular acid contains Carbon which can be assimilated by the plants. Any method of adding plant accessible Carbon should be considered, whether it be Excel, CO2 injection, Carbon block electrohydrolysis - whatever. The pH is a separate factor to consider based on other goals.

Cheers,
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Old 04-23-2005, 09:22 PM   #5 (permalink)
 
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Thanks for caring to reply, I appreciate it.

I suppose I am trying to understand pH more than just creating an ideal planted tank at this point.

In reference to Flourish Excel, "This product provides Carbon in a different form" - according to Seachem, Excel is that important carbohyrdrate the plants need to photosynthesize.

But what do you think about this... that increasing the acidity of the water will cause some bicarbonates to be converted into CO2?.. So theoretically although maybe not as efficient as CO2 supplementation these buffers although not ideal are still promoting available CO2?

back to plant growth, if you are injection CO2 into a tank with a very high KH (10+) it would seem useless because that carbonic acid is being neutralized by those high levels of carbonates and bicarbonates. Therefore it seems necessary to add an adequate level of 'CO2' depending on your KH readings which would imply more carbonic acid per higher KH to promte maximum CO2 absorption therefore lowering the pH.

Any opinions appreciated

thanks for the time
Tim
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Old 04-24-2005, 02:07 PM   #6 (permalink)
 
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Hi Tim,
I hear what you're saying about trying to understand pH although I still think you've got the cart before the horse. What I was trying to get across in my last post was that plants are doing what they can to obtain Carbon (C) in whatever form possible. I believe that only a small percentage of the CO2 we inject actually forms Carbonic acid. The rest stays as CO2. Whatever form of Carbon is in the water the plant will use however, there are priorities in the use of the forms because most likely the energy required to break down the CO2 is significantly less than the energy expenditure to extract Carbon from, say Bicarbonate. In the absence of, or with low concentration of CO2 the next easiest is Bicarbonate or Excel if added.

Your proposal to acidify water which is high in HCO3 in order to convert that HCO3 to CO2 sounds like a great idea but I'm not sufficiently versed in chemistry to know what specific acid would accomplish that. In fact I believe it to be unlikely with any of the acids we typically use (someone correct me if I'm wrong). You said it yourself when you stated that water high in Carbonates and Bicarbonates (which essentially means high kH) is water with high alkalinity. Therefore adding acids, any acid, whether it be Carbonic acid or even Hydrochloric acid to water high in kH will simply result in absorbsion of the acid until the Carbonates/Bicarbonates are used up and then adding more acid will result in a fall in pH. This is the definition of buffering, so it's inconceivable to me that adding acid will somehow break down HCO3 to CO2.

OK, with this in mind, as you say, back to plant growth - Let's say we are injecting CO2 in a tank with kH of 10 which is just another way of saying the water is high in CO3 and HCO3. As you say the Carbonic acid formed by the CO2 injection is in fact being neutralized by the Carbonates/Bicarbonates holding the pH steady, but there is still CO2 present because only a small amount of the injected CO2 is changed to Carbonic acid therefore it's far from being wasted. The amount of CO2 that is changed to Carbonic acid is in proportion to the unconverted CO2 and the amount of drop in pH as a result of this acid is proportionally related to the amount of kH in the water and that's why the the relationship between the pH, kH and the amount of disolved CO2 is expressed in the equation CO2 = 3.0 * KH * 10^(7.00 - pH).

So if the kH is very high the resultant drop in pH is very small even if you inject large amounts of CO2 but you are still disolving large amounts of CO2 and this large amout of CO2 is available to the plants. At low kH an identical drop in pH is an indication that less CO2 is disolved.

So apart from any other benefits of a given pH value (I'm sure that plant physiology is pH dependant) the ph "delta" is only used as an indication of the exisiting levels of disolved CO2. The pH is a means to an end. In fact it's possible to have a high pH and still have oodles of available disolved CO2 depending on what buffers are in the water, but if the equation assumes that the only buffers are Carbonate and Bicarbonate so if there other types of buffers in the water such as "pH Down" the equation is useless, but all that means is that the measurable relationship between the pH, kH and dissolved CO2 is broken with the addition of the buffers.

So really,chasing pH in the context you are suggesting won't really buy you anything. pH is simply a measure of the ratio of H+ to OH- ions. Acids behave in such a way that when they disolve in water there is a net increase in H+ ions. Strong acids such as HCl disassociate the chemical bond almost completely so a large amount of H+ is contributed but weak acids such as acetic and carbonic do not disassociate nearly as much so they reult in only a small net increase in H+ and most of the compund is disolved in water more or less intact.

Hope that helps,

Cheers,
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Old 04-24-2005, 05:05 PM   #7 (permalink)
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Ardvark is exactly right. Basically, changing th epH of your water willnot affect the Co2 concentration unless you lower it with CO2. The kH of your water is basically a measurment of the CO3 in the water. higher kh means less room for CO2 to form CO3 which is why waters with higher KH exhibit smaller pH drops with the addition of CO2. If you use any buffers to adjust your water, then you cannot use the Kh/pH relationship to test for CO2 levels. You can though, allow a sample to sit over night so that all CO2 evaporates. Test that sample for pH and kH. You will know that your CO2 levels on the sample are 3-5ppm because it will stapilize at the air equiblirium. Check your findings against a ph/Kh chart, like found at Chuck Gadd's site. If your readings say more than 3-5ppm, then you have water with a buffer other than CO3, probably a phosphate based buffer. Example, youtest your water for kH and Ph, look it up in a chart and it says you have 10ppm CO2. You know this is wrong so look in the same column as your kH reading and see what pH should have a CO2 level of 3-5 ppm. That is the approxomite number of points you pH test is off. When you inject CO2 and test you pH, you will have the correct amount of CO2 in your water when the reading you get is that many points below the actual concentration you should have. For instance, you find your sample is off by .3 degres ph. THe kh Chart says that for a kH of 3 you need a pH of 6.5(that is a fake number, check the CO2 chart for actual figures) to have 30ppm of CO2 in your aquarium. You will have enough CO2 only when you ph test reads 6.2, or vise a verse, depending on your original sample that sat overnight.

Really the only affect pH plays on your plants(as long as you are in the 5.6-7-5 range) is redox potential and cation binding in the substrate. This is really more important in natural enviroments than in our planted tank.. To learna bit more about this, check out a post in the Fertalizing froum about pH and plant growth, from a week or so ago.

HTH
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