Technical discussion originally from this thread. JasonLion
Actually, any kind of acid should work at lowering the pH, although some work better. A "strong" acid just means that virtually 100% of the acid dissociates into solution to form H+ ions at any given time. Some of the common strong acids include HCl, HBr, HI, H2SO4, HNO3, and HClO4 (there are some other wacky ones that scientists have made too).
This is not the same as the amount that an acid will lower pH. For example, if you take a fairly basic solution, and add equal quantities (on a NORMAL basis; this refers to how many H+ ions are added, so it's basically the number of molecules * the number of H+ that will dissociate per molecule... 1 Molar HCl is also 1 Normal as there is one H per HCl, but 1 Molar H2SO4 is 2 Normal since there are 2 H's) of a weak acid like acetic acid and a strong acid like hydrochloric acid, you will end up with the same change in pH (all the acid will be used up to neutralize base, as long as you don't exceed the amount of base present). Acetic acid exists in a partially dissociated state (determined by the equilibrium constant of the dissociation reaction). Acetic acid is a carboxylic acid (these are very important in organic reactions, and are highly oxidized) and has formula CH3COOH. The last H is the one that dissociates in a carboxylic acid, which is acidic because of the resonance stabilization of the negative charge between the two oxygens when the H+ dissociates. Note that while each molecule of acetic acid has 4 H's, only ONE is acidic to any reasonable degree, so 1 molar acetic acid is 1 normal as well. The others are methyl group hydrogens and are among the least acidic hydrogens that you can have. Carbon simply is not electronegative enough to stabilize the negative charge that would result from dissociation of an H+.
The dissociation equation is:
CH3COOH <-> CH3COO- + H+
Equilibrium strongly favors the left side (hence why carboxylic acids are weak acids), and only about 0.5% of the acetic acid will exist in a dissociated form (the right side) at any given time.
However, the reason that this acid will fully neutralize a strong base is LeChatelier's principle. If there is base present, the H+ from the acetic acid will act to neutralize it (assuming an Arrhenius acid/base definition here). When that H+ is no longer there, more acetic acid will dissociate (LeChatelier's principle states that in a chemical equilibrium, if you add or remove something from either side, equilibrium will shift to counter your change). Thus, more H+ will be produced. This in turn will be used to neutralize base, and so more acetic acid will dissociate, etc. until all of it has dissociated and you have only acetate ions left.
That all being said, the basic idea stated above that using vinegar (acetic acid) or lemon juice (citric acid) to change your pool's pH is bad is absolutely correct. For one thing, you would need a lot of it, as the acids are fairly massive per molecule (high molar mass means fewer molecules to release H+ per unit mass) and not very concentrated in lemon juice or vinegar (lots of water and other stuff mixed in). Also, you don't want to be dumping all that other stuff in your pool. Vinegar and lemon juice both contain lots of complex chemicals, sugars, and other things that are not a healthy part of your pool. At best, you'll filter them out, at worst, they will serve as food for bacteria, make your water cloudy, or have some other bad effects on the pool. Putting sugar in your pool (contained in both vinegar and lemon juice) is like putting bacteria food in your pool.
As the pH changes, the amount that an acid will dissociate also changes. So as you get more and more acidic, a weak acid will essentially exist in a completely undissociated form. For example, if you took an acidic solution of hydrochloric acid (pH < 1 or so) and added acetic acid (don't try this at home... the reaction can be violent in some circumstances), the acetic acid will not dissociate because the pH is too low. Alternately, adding hydrochloric acid to an acetic acid solution will cause all of the dissociated acetate ions to pair up with H+ ions (basically, the Cl- is more stable than the acetate ion, so HCl + Acetate will form Cl- + Acetic Acid). The weaker an acid is, the higher the minimum pH you can achieve with it is (for acetic acid I think it's somewhere around 4-5). This is a number lower than you'd be trying to get your pool to, so you *could* get it there with acetic acid, but it's not a good idea. Hydrochloric acid (muriatic acid is hydrochloric) is really ideal in my opinion because it adds no TDS to your pool (the solid acid adds sulfates). It only adds H+ and Cl-, which is in your pool anyways (water produces a certain amount of H+... it's pH is 7.0, and pH is the - log (base 10) of the H+ concentration).
As a general rule, I'd suggest trying to keep anything organic out of your pool (this would include citric and acetic acids). They are much more likely to muck things up. The only carbon that should be added to a pool, imo, is carbonates for buffering (baking soda and soda ash).
Actually, any kind of acid should work at lowering the pH, although some work better. A "strong" acid just means that virtually 100% of the acid dissociates into solution to form H+ ions at any given time. Some of the common strong acids include HCl, HBr, HI, H2SO4, HNO3, and HClO4 (there are some other wacky ones that scientists have made too).
This is not the same as the amount that an acid will lower pH. For example, if you take a fairly basic solution, and add equal quantities (on a NORMAL basis; this refers to how many H+ ions are added, so it's basically the number of molecules * the number of H+ that will dissociate per molecule... 1 Molar HCl is also 1 Normal as there is one H per HCl, but 1 Molar H2SO4 is 2 Normal since there are 2 H's) of a weak acid like acetic acid and a strong acid like hydrochloric acid, you will end up with the same change in pH (all the acid will be used up to neutralize base, as long as you don't exceed the amount of base present). Acetic acid exists in a partially dissociated state (determined by the equilibrium constant of the dissociation reaction). Acetic acid is a carboxylic acid (these are very important in organic reactions, and are highly oxidized) and has formula CH3COOH. The last H is the one that dissociates in a carboxylic acid, which is acidic because of the resonance stabilization of the negative charge between the two oxygens when the H+ dissociates. Note that while each molecule of acetic acid has 4 H's, only ONE is acidic to any reasonable degree, so 1 molar acetic acid is 1 normal as well. The others are methyl group hydrogens and are among the least acidic hydrogens that you can have. Carbon simply is not electronegative enough to stabilize the negative charge that would result from dissociation of an H+.
The dissociation equation is:
CH3COOH <-> CH3COO- + H+
Equilibrium strongly favors the left side (hence why carboxylic acids are weak acids), and only about 0.5% of the acetic acid will exist in a dissociated form (the right side) at any given time.
However, the reason that this acid will fully neutralize a strong base is LeChatelier's principle. If there is base present, the H+ from the acetic acid will act to neutralize it (assuming an Arrhenius acid/base definition here). When that H+ is no longer there, more acetic acid will dissociate (LeChatelier's principle states that in a chemical equilibrium, if you add or remove something from either side, equilibrium will shift to counter your change). Thus, more H+ will be produced. This in turn will be used to neutralize base, and so more acetic acid will dissociate, etc. until all of it has dissociated and you have only acetate ions left.
That all being said, the basic idea stated above that using vinegar (acetic acid) or lemon juice (citric acid) to change your pool's pH is bad is absolutely correct. For one thing, you would need a lot of it, as the acids are fairly massive per molecule (high molar mass means fewer molecules to release H+ per unit mass) and not very concentrated in lemon juice or vinegar (lots of water and other stuff mixed in). Also, you don't want to be dumping all that other stuff in your pool. Vinegar and lemon juice both contain lots of complex chemicals, sugars, and other things that are not a healthy part of your pool. At best, you'll filter them out, at worst, they will serve as food for bacteria, make your water cloudy, or have some other bad effects on the pool. Putting sugar in your pool (contained in both vinegar and lemon juice) is like putting bacteria food in your pool.
As the pH changes, the amount that an acid will dissociate also changes. So as you get more and more acidic, a weak acid will essentially exist in a completely undissociated form. For example, if you took an acidic solution of hydrochloric acid (pH < 1 or so) and added acetic acid (don't try this at home... the reaction can be violent in some circumstances), the acetic acid will not dissociate because the pH is too low. Alternately, adding hydrochloric acid to an acetic acid solution will cause all of the dissociated acetate ions to pair up with H+ ions (basically, the Cl- is more stable than the acetate ion, so HCl + Acetate will form Cl- + Acetic Acid). The weaker an acid is, the higher the minimum pH you can achieve with it is (for acetic acid I think it's somewhere around 4-5). This is a number lower than you'd be trying to get your pool to, so you *could* get it there with acetic acid, but it's not a good idea. Hydrochloric acid (muriatic acid is hydrochloric) is really ideal in my opinion because it adds no TDS to your pool (the solid acid adds sulfates). It only adds H+ and Cl-, which is in your pool anyways (water produces a certain amount of H+... it's pH is 7.0, and pH is the - log (base 10) of the H+ concentration).
As a general rule, I'd suggest trying to keep anything organic out of your pool (this would include citric and acetic acids). They are much more likely to muck things up. The only carbon that should be added to a pool, imo, is carbonates for buffering (baking soda and soda ash).
Even when I can't fully wrap my pea brain around a technical chemistry discussion, I like to read them - whatever small % that I can grasp makes me more confident in doing my job of servicing pools - and sometimes when a customer is asking just TOO many questions, I'll quote some of the advanced chemistry I've read to shut them up 
If just a small amount gets into your lungs it can kill you! 
