So You Want To Alter Your Ph?

[Miss Wiggle]

you've confused me Bignose, we've always been told our RO should have a pH of 5.5 before we add any salt/buffers etc.

 

[Bignose]

Whoever told you that is wrong. Pure water has a pH of 7.0. pH is measure of the disassociation of an acid and the H+ ions. Let's just name the acid HnR. H is H+, n is the number of H+'s, R is the rest of the acid group (can be anything). For example, HCl (hydrochloric acid) has 1 H+ and R is chlorine, Cl. H2SO4 is sulfuric acid, with 2 H+'s, and SO4 the rest of the acid. H3PO4 is phosphoric acid, it has 3 H+'s. I listed them all since just because phosphoric acid has more H+'s, it does not mean it is stronger. It is in fact a weaker acid than hydrochloric or sulphuric. It is in soda and many other beverages, for example. As a simplistic definition, pH describes how many of the those H+'s come off the R part at any time. HCl and H2SO4 are considered stronger since more of the H+'s come off, even though there are more available from the phosphoric acid.

As a more precise definition, pH is the negative of the base 10 log of the concentration of H+'s. pH = -log([H+]) the bracket notation, [] is shorthand for concentration of the term inside the []'s.

Let's do an example, a concentration of 0.001 mol/L HCl. HCl is a pretty strong acid, so it almost completely disassociates into H+ and Cl-. That means that the concentration of H+ is also 0.001 mol/L. 0.001 is 10^-3. The log of 10^-3 is -3, the the negative of that is 3. Hence, 0.001 mol/L of HCl has a pH of 3.0. I hope from this example you see how pH works.

Back to pure water. Water itself disassociates. H2O <--> H+ and OH-. This happens no matter what. And since there is a concentration of H+ ions from pure water, it has a pH. Pure water will have a concentration of 10^-7 mol/L of H+, hence pure water has a pH of 7.0. This is a universal constant.

Now, measuring that in pure water can be notoriously difficult. The issue is that because of the purity of the water, any impurities accidentally added to the water will have a significant effect. For example, if the pH meter or the tube you do the testing in has any residue from previous tests in it at all, this can show up. Also, since the water is so pure, if the container it is kept in can leach out anything, it will probably find its way in the water. Just a tiny change can show up on the tests. These tiny errors are not really reflective of the solution as a whole. Most of the tiny errors are very localized (like the residue on the test tube), and won't represent the bulk of the solution.

But, if it is pure water, it has to have a pH of 7.0. That is the amount of disassociation of pure water. The amount of disassociation will change a little with temperature, but it will always be very close to 7.0. Anyone who tells you different doesn't understand the basic definition of pH, or doesn't understand the disassociation of water, or doesn't understand how just a tiny amount of impurities can introduce errors to the reading.

 

[The-Wolf]

Pure water has a pH of 7.0.

sort of off topic
Bignose
when you say pure water are you taliking pure H2O with absolutly no impurites in it.
ie H2O in its purest form will not freeze as the ice crystals have nothing to form around.

 

[Bignose]

Pure water will freeze, but usually not right at 0 Celsius. It will enter a meta-stable phase where it is waiting for an imperfection to form the crystal around. I.e. introduce a crystal and the whole thing will freeze immediately. But, once you cool it down to like -10 Celsius or so, it will freeze spontaneously. That meta-stable region is only so large. You can push the limits a little, but not too far.

The same behavior can be seen at the boiling point too. This is why you should never boil water in the microwave without some nucleation site provider. In the same way, water heated to 100*C needs a nucleation site to vaporize at, usually an imperfection in the container works well. (Next time you boil water on the stove, watch just before it starts, you should notice a stream of bubbles coming up from just a few points in the pan -- these are imperfections in the pan that are the nucleation sites. Well, you can't put a metal pan in the microwave, so you usually use glass. If the glass is well made, there won't be any imperfections to act as nucleation sites. If the water is heated above 100*C, it may not start boiling yet, and be waiting for something to push it over this meta-stable situation again. Something like the jolt from placing the container on the countertop. Then, all the water can vaporize immediately, throwing a lot of steam right into the person's face. This has happened a few times. I always just a wooded chopstick or plastic spoon whenever I heat water in the microwave, just because those objects will provide nucleation sites if the water gets to 100*C. However, again once you get about 10 degrees higher, the water won't need a nucleation site, it will boil spontaneously.

Most physical transformations have some meta-stability. But the region isn't very big. You can push the meta-stability a little, but not too far. I think the best metaphor is a round rock sitting on top of a plateau. You can push the rock right to the edge, but if you push it over the edge, the rock goes tumbling down.

 

[Tessla]

As bignose stated, impurities can quickly change the pH of pure water. RO, distilled, deionized or any other water with a low buffering capacity will quickly become acidic when exposed to air as carbon dioxide forms carbonic acid when it dissolves in water; very pure water can easily get down to a pH of 5. Heating or aerating will drive the pH back up very easily so it should be treated as pure water, but RO water that's been left to sit should show a lower pH when tested.