How do salts affect pH of a solution?

I am a planted aquarium enthusiast and am curious about how certain salts, when used as fertilizers, will affect pH. (Beware, the extent of my chemistry knowledge is what I half remember from high school, combined with a little reading I have done to research this.)



Take, for example, the salt Ca(NO3)2. According to what I have read, the pH of a salt depends on the relative acidity and “baseness” (whatever the term is) of the acid and base that combined to form the salt. So in this case would it be nitric acid and calcium peroxide? Or are there different ones that would work? What are the pH values for these?



Also I have read that the reason that the end result salt affects pH (in those cases where it does) is because part of the salt in solution reacts with water, splitting it, which is the mechanism behind the pH change. The site that I was reading provided tables of ions that react with water and ones that do not. Ca +2 and (NO3) –1 were ions that they said tended to not react with water; so is this to say that calcium nitrate will not affect pH?



And some final questions – What happens when one ion is depleted from water faster than the other. Plants use much more NO3 than Ca, plus I would expect a small continuous loss of NO3 from the aquarium through the activity of denitrifying bacteria forming N2 gas. This is preventable since I can add other sources of N, I am just curious about what happens when calcium ions accumulate. Would that affect pH? I have read that adding solid calcium and/or quick lime to water will form a caustic solution of Ca(OH)2; will the Ca ions from Ca(NO3)2 do something similar at a certain point?



For reference, the link to the site I used is http://www.science.uwaterloo.ca/~cchieh/cact/c123/salts.html



Please help, chemistry experts!How do salts affect pH of a solution?
That site is accurate, as far as I can see. Salts are composed of two parts (two ions). If you have a salt that's composed of two ';neutral'; ions, it will not have an impact on the pH of your aquarium. Also note that salts containing one ';acid'; ion and one ';base'; ion will -approximately- cancel each other out and leave the water at the same pH, though if you need a very particular pH, I would advise against doing this (since one ion might be stronger and predominate).



Your understanding of how this works is also correct. When added to water, salts dissociate, which means they fall apart into their two constituent ions (a cation, the positively charged one, and an anion, the negatively charged one). Depending on the acid-base chemistry of these ions, they may or may not react with the water itself to produce acids and bases. In your example, the nitrate ion (NO3 1-) that's formed would need to form nitric acid (HNO3) if it were to react with water. However, nitric acid really doesn't like existing in water, and tends not to spontaneously form, which means the NO3 ion is inert. Calcium is the same: it really tends not to mess with water molecules, which means this salt will indeed not affect pH of the water it's added to.



However, if you take, for example, sodium acetate (NaCH3COO, the salt that's formed when you react vinegar and baking soda), it will dissociate into a sodium cation (Na 1+) and an acetate anion (CH3COO 1-). The sodium ion does nothing to water, but the acetate ion will tend to rip hydrogen off water molecules, forming acetic acid (vinegar) and hydroxide, in the following reaction:



CH3COO 1- + H2O ---%26gt; CH3COOH + OH 1-



This only occurs with about 5% of the acetate ions, but it's enough to change the pH. The free hydroxide (OH 1-) will make the solution slightly basic (yes: the formation of an acid makes the solution basic. It's counter-intuitive, but it's due to the nature of acid-base chemistry, and it does make sense).



By the way, the word for ';baseness'; is ';alkalinity';.



Unfortunately, I'm more of an organic chemistry person than an inorganic chemistry person, so I can't really help you with your second question. Perhaps someone else can shed light on that. I can, however, tell you that water with high calcium ion concentrations is refered to as ';hard water'; (as opposed to ';soft water';). I know nothing about aquariums, but perhaps you can look around online for problems involving hard water and see if any other aquarium-savvy people know if that causes a problem and how to fix it.



Cheers!

PyeHow do salts affect pH of a solution?
You must be kidding. Salt is a building block to acid. Of course it will alter it towards acidity. You can counter it by putting cloride or baking soda in it. That should neutralize it or bring it back to it's previous state. (Use same amount as salt exactly or it will become too base.)