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PH: sodium bicarbonate (buffering)


Nornicle

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Have a question about Sodium bicarbonate,

Here is what I know of it

a) is not a buffer

b) will adjust pH to about 7.8 but will not keep it there.

so it adjusts pH but will not keep it there without a buffer correct? I've been told that is a buffer just a weak one? or are my 2 assumptions just wrong? replies from water chemists welcome tongue.gif

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Hi Nornicle

I believe both statements are incorrect sadsmiley02.gif

Bicarbonate can either accept H+ (acid), thereby acting like a base.

HCO3- + H+ > H2CO3 (carbonic acid)

or Release H+, thereby acting as a acid.

HCO3- > H+ + CO3--

It tends to buffer water to a pH of about 7.8, that is if the pH is less than 7.8 and you add bicarbonate it will tend to increase the pH. And if the pH is more than 7.8 and you add bicarbonate then it will decrease the pH.

Yes it will adjust the pH towards 7.8. Will it keep it there? Well it depends on what is added to the tank afterwards. eg If you add CO2(acid) or the fish add ammonia then the pH will change.

Clear as mud, smile.gif

Brett

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brett not quite so clear I think

moving the pH up is NOT the same as buffering the water.. two separate things, hydrogen ions and carbonate hardness not the same thing

buffer means being able to keep pH at a value without being affected by acids..

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buffer means being able to keep pH at a value without being affected by acids

This is a relative thing. Adding any acid will affect the pH, just not as much if a buffer is present.

This is what bicarbinate does, it acts as a buffer and reduces the magnitude of the pH change. smile.gif

Cheers

Brett

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i think it doesnt....

I still don't understand what you are saying though Brett,

'Yes it will adjust the pH towards 7.8. Will it keep it there? Well it depends on what is added to the tank afterwards. eg If you add CO2(acid) or the fish add ammonia then the pH will change.'

vs

'This is what bicarbinate does, it acts as a buffer and reduces the magnitude of the pH change.'

raises pH it definitely does

keeps it there well? not really in my experience

I've done a brief research on the net and no one says that sodium bicarbonate is a buffer, but there is lots saying that in conjunction with calcium carbonate it's a great way to make hard basic water...

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hi,

as per brett's post... sodium bicarbonate is a buffer in the sense that it will pull ph levels towards it's equilibrium point (somewhere around 7.8).

I think that's all it needs to qualify as a buffer from a chemical perspective.

It's a little different from a practical fishkeeping perspective perspective.... we want a ph buffer to give us stable Ph levels over a period of time. Sodium bicarbonate dissolves well and reacts well so it has an almost immediate impact on Ph levels but also dissapates fairly quickly (eg carbonic acid breaks down into water and carbon dioxide, not sure what happens on the other side of the eq point)

Calcium carbonate on the other hand dissolves more slowly and has a lower saturation point. That means it's actually a weaker buffer than sodium bicarbonate but lasts longer.

I use both: calcium carbonate as a substrate and add sodium bicarbonate after large water changes.

As to whether sodium bicarbonate increases Kh: if you use the generalization that Kh is a measure of calcium carbonates in solution then by definition it has to.

A further question that has worried me occasionally is sodium levels in the aquarium. I add sodium chloride and sodium bicarbonate to some of my tanks to some degree.... For most of the ions in there, there is a way for excess levels to dissapate generally through effervescence in some sort of gasous form but I can't think of any way the sodium leaves the system other than in water changes.

Do sodium levels ever reach levels that should be of concern?

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sodium bicarbonate, disociates in water resulting in, carbonate ions, and hydrogen ions, and sodium ions obviously, but dont worry about them at the moment.

the bicarbonate does have a buffering effect as stated earlier the H+ ions will bind with and neutrlise any negative ions, and the coo- ions will bind with any +ve ions.

buffers are solutions that are able to resist a change in pH when acid or base is added, so the water in the tank containing these disociated ions is a buffer as it has buffering capacity.

As stated a buuffer has a specific range it can work in for the carbonate buffer i think it is about 7.4-7.9 pretty narrow range, also temp can effect the buffering capacity of the solution..cant really use bicarbonate to get the pH much above 8

To aid the buffering capacity I also have calcium carbonate in the filters as crushed coral and shell, or as a substrate such as marble chips.. limestone chips can also be used.

The sodium is solution will help increase the hardness of the water, as will the bicarbonate ions, i also add magnesium sulphate, and potasium chloride, and sodium chloride to help with the hardness,

hope this helps

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so below 7.4-7.9, sodium bicarbonate would stop working? i.e at a pH of 6 your water would have very little to stop it from dropping to 5 etc if you were just using sodium bicarbonate..

practically (from what i can understand) if i were to have an amazon tank, i wouldnt use sodium bicarbonate at all as I would want my water to stay about 6.5?

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below about 7.4 it would stop working, -- well its buffering capacity would beabout gone as it would indicate that heaps of acids were being added to the tank, and a water change and more bicarb is needed to get the pH back up.

For an american tank i would not use bicarb as the pH needs to be a bit lower,, 6.5-7.5 around neutral, or if lower pHs are neeed some peet moss in a stocking works well I have heard, For my Americans i use some salt, normal tap water, and some wood , wood will drop the pH a bit and give the water a nice tannin colour.

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I think we are agreed that bicarbonate (HCO3-) is a buffer.

CaCO3(s) + H2O <--> Ca++ + HCO3- + OH-

You can see that calcium carbonate and bicarbinate are two ends of the same system not different systems. Calcium carbonate seems to be more effective as it continues to dissolve as the bicarbinate is neutralised by acids. This is a problem in soft water tanks as the tanks carbonate hardness and pH gradually increase with time until it reaches saturation.

Chorrylan,

I think that increasing sodium (Na) ions are a potential problem, and yes they are only removed by water changes. They contribute to increased conductivity (roughly equates to the concentration of all ions), but don't increase either general or carbonate hardness. There is some evidence to suggest that spawning and general health of fish are more dependent on conductivity than hardness, although the two tend to go together. Certainly with fish that require very soft water, increased conductivity is probably a bad thing.

Cheers

Brett

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Yes it is a buffer...

Look up carbonate buffer system on the net and it will explain it for you (its a little bit complicated for me to explain), one thing you have to be aware of is that a buffer system will continue buffer the ph in a certain range, until it becomes expended. If you continue to add acid to a system - At a point the PH will "crash" very quickly. Its a very interesting little experiment to run in a test tube...

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I am not sure that "crash" is the right word, but yes, you will see a fall in the pH if you add acid. The magnitude of the fall for a given amount of acid will be greater outside the range than within the range. However bicarbinate still "buffers" the pH outside of this range, just nowhere near as effectively.

In practical terms you see this happen most with CO2 injection. When the carbonate hardness (bicarbonate concentration) is high, the pH still falls, but not to the same extent as when the carbonate hardness is low.

Cheers

Brett

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Um --- Brett, actually what i was trying to say was it will "crash" if you keep adding acid to the environment (tank).

Whilst the buffering system is fuctioning - it will hold the pH in the range, but once it is "full" it will cease buffering the water and the addition of any further acid will cause the pH to fall much more rapidly.

if your pH is not allready above 7.5 - the sodium bicarb, calcium carb, or any other carbs in your tank are not going to do much buffering for you because they have already been used up by the acids (H+'s) present in the water. If you want an alkali tank but have an acidic tank, you need to add enough carbonate ions so that they are in excess. In this regard - you add your pH-up agent (sodium bicarb).

If you want an acidic or neutral tank i would not suggest adding large amounts of sodium bicarb or putting limestone, coral or marble substrates into your tank as they will all affect the pH ballance. They will also affect the hardness of the water.

Please note that there are also acidic and neutral buffers available for your aquarium. It depends what pH you want your tank to be.... Sodium bicarbonate and calcium carbonate are great buffers for high pH tanks like for african cichlids.

ALL pH alterations where fish are allready in the tank should be done slowly. If you do them too quickly - i guarantee that your fish will die no2.gifcry2.gif

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THis is a complicated process of which I only bearly understand. THe NSWCS should find an appropriately qualified chemist of mirco biologist to give a leacture one night, even if its paid!!!

What I have observed; 1) Sodium Bicarb will raise Ph to 8.4 at least if you add enough and with no adverse reaction to the fish, Malawis anyway. 2) Sodium Bicarb with raise the Kh to what ever degree you require, just keep adding more. 3) Kh is a measure of free carbonate or bicarbonate ions in solution and nothing to do with calcium.

smile.gif)

Craig.

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