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Posted by SteveB on June 16, 2008, 12:01 pm
bent wrote:
> Ok, anybody want to attempt to splain to me why there is both a "pH"
> (6.2-8.4) and a "Total Alkalinity" (0-240 ppm) scale on a pool water
> chemical test strip kit . I have been using the strips that have the
> 4 square foam pads on the ~1/4" x 4" plastic strip that you dip in
> the water, wait and then compare the colours change on the pads to
> the reference colours on the label of the strips' bottle. The strips
> have a colour range for Total Bromine (0-20 ppm), Free Chlorine (0-10
> ppm), pH, and Alkalinity. Granted, it is likely that the strips I
> currently have are technically past the expiration date, and I will
> get some new ones soon, but firstly just to compare the results of
> the test between the old strips and new strips, so I cna finish them
> off. Any comments on how off they can get with age?
>
> This is my problem - ngQ(uestion):
>
> I was taut in skool that pH and alkalinity was the same scale. The
> number is determined from the definition regarding the hydronium ion
> concentration, and math or something like that. The pH of neutral
> water is about 7. That it, it is neither acidic nor base in its
> chemical character. If the number is low, like 3, it is "acidic" and
> if its high, like 9, it is alkaline (base). But if it is acidic (low
> number on the pH scale), by definiton it CANNOT be alkaline at the
> same time, or vice versa: it cannot be alkaline and be acidic too.
>
> So why are there two scales on these (pool) water test strips, which
> read from low to high for both pH and Alkaline? There is a pH and a
> Alkaline scale!? So according to the pool companies you can have a
> low pH and a low alkaline pool at the same time, or high & high, or
> low & high, or high & low.
>
> Because I go to the chemical cupboard for the right chemical to fix
> the problem (assuming all other parameters are being considerately
> moderated too) and I find chemicals labelled "pH Up", "pH Down",
> "Alkalinity Up", and "Alkalinity Down"! And I don't want to do
> anything, (a) because the strips may be wrong (via expiration) or (b)
> the pool chemical companies may be full of crud.
In poolspeak, Total Alkalinity (TA) refers to the concentration of
sodium bicarbonate. See rant below.
What you're remembering from chemistry class is pH versus pOH. Those
are scales for measuring acid / base. To simplify, pH is the
concentration of the hydrogen ion (H+), and pOH is the concentration of
the hydroxyl ion (OH-). They're complementary:
pH - 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
pOH - 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
Rant:
Non-technical people keep making up terms that conflict with scientific
definitions. My least favorite is "natural", which in strict terms
means "obeying the laws of nature". That makes it a totally useless
definition, since anything which doesn't obey the laws of nature
doesn't exist.
--
Steve Bell
New Life Home Improvement
Arlington, TX
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