Drinking Water PH

"The pH of the water is not a measure of the strength of the acidic or basic solution..."

That's exactly what it IS!

Actually I think dvmdsc had it correct when he said ...

"The pH of the water is not a measure of the strength of the acidic or basic solution..."

The pH of the water is a measure of the H+ Concentration (Hydrogen ion, proton, or hydronium ion H3O+) in the water. It is a measure of the Acidity of the water, but it is not a measure of the strength of the acidic solution. http://en.wikipedia.org/wiki/PH

The strength of an acid (or acidic solutioin) is measured by the ability of the solution to donate a Hydrogen ion. This is measured by the acid dissociation constant (pKa) http://en.wikipedia.org/wiki/Acid_dissociation_constant

Lower the pKa the stronger the acid, higher the pKa weaker the acid.

Lower the pH is more acidic, higher the pH less acidic

There's an important difference, between the strength of an acidic solution and it's pH.

<...It is a measure of the Acidity of the water, but it is not a measure of the strength of the acidic solution....>

Are these two not one and the same thing?

Not quite.

pH is simply a measure of the concentration of H+ ions in the solution. Regardless of what acid has supplied them.

A solution that has very low pH only means that it has a high concentration of H+ ions. This will be an effective acid (by pure overwhelming numbers). An example is a plague of insects can do a great amount of damage.

pKa is a measure ofthe strength of the acid (how strongly it wants to give the H+ ions up). An elephant can also do alot of damage.

A weak solution with a strong acid doesn't have to have a low pH to be just as effective as a strong solution of a weak acid.

Hope this helps.

"A weak solution with a strong acid doesn't have to have a low pH to be just as effective as a strong solution of a weak acid."

Since the wording is not precise, I'll stop short of saying you are wrong, but I believe you have a common misconception that strong acids are more "effective". It is a bit complex, but let me make a statement that is true, and see if you agree.

"For any given acidic pH, the solution that contains the weaker acid will be more effective"

To define "effective", let's say that effectiveness is the ability to supply the most ions to a given reaction.

It takes a higher concentration of a weak acid to produce the same pH as a stronger acid, so it will have more ions available for reaction.

This seems to be the opposite of your statement.

NO.

Why?

The problem is the use of imprecise terminology like "strength". As others have pointed out, in chemistry the use of strong or weak when referring to an acid or base has a specific meaning; a description of how easily the compound dissociates in water.

In English, "strong" can mean several different things, such as concentration or how effective a solution is at performing a task (dissolving, reacting etc.)

pH alone does not not give an indication of concentration. A given low pH may be due to a low concentration of a strong acid or a much higher concentration of a weak acid.

Check for electrolysis. Is your chamber grounded? You can simply add a soda to bring the alkalinity up.

I agree with the comments posted so far.

It does sound like the ultrapure water (with no buffering capacity) produced by the unit is being influenced by an unknown, resulting in the pH dropping.

The unit itself shouldn't be corroding from the water (as it "should" be designed to operate in the conditions). If you have downstream piping it may be corroding, however the pH change in the water as a result will be up (not down).

If you are worried about dissolved metals, go ahead and test for it, but I would consider this unlikely to be a problem. And not in high enough concentrations to cause health issues.

The theory put forward about the sulfur dissolving and carrying over (into the distillate) in the form of SO2/SO3 does make sense. SO₃ + H₂O → H₂SO₄(g) this reaction is typically extremely slow, so very minor amounts are likely to form, but it is an extrmely strong acid in an unbuffered environment, and therefore could explain your observations (low pH direct from the unit).

If the CO2 is being dossilved to form weak Carbonic Acid (below is equilibrium data from Nalco Water Treatment Handbook), then it could be stabilising at 5-5.5 pH as you observed.

My recomendations for what it's worth is to re-mineralise the water to add come buffering capacity (stabilise the water pH at 7 - 7.5 pH). We use Calcite (Calcium Carbonate chips) to do this. Suggest getting hold of some and throwing it into the tank (won't hurt). The calcite will slowly dissolve, and add Calcium and Carbonate ions into the water. The carbonate will act to hold the pH at constant levels.

<...Presently the drinking tank water PH is not increasing more than 5.5. Is it safe to drink ...>

Comparison measurements of the pH of popular brands of cola (brand names withheld) would indicate yes, as these are of lower pH than this owing to the levels of phosphoric acid they contain, and kids drink gallons of it. The pH of freshly-squeezed orange or lemon juice would also be of interest in this regard.

It seems as though the re-hardening vessel, which should be at the back end of the processing train, is out of mineral. Inside this vessel should be a finely-divided calcium carbonate material (brand name withheld), the job of which is to raise the pH of the evaporator's outlet water and to give it some conductivity, which distilled water simply doesn't have. If the calcium ion concentration is below, oh, say 60ppm, then the water is unpleasant to drink on the grounds of palatability, and may even cause corrosion problems with downstream pipework and fittings on the way to the taps. Re-harden the water, by replenishing the re-hardening vessel's contents with fresh mineral and putting the equipment back on-line, and report back.

The fault seems to be one of not following maintenance procedures rather than anything else, at this time.