Role of Epsom Salts in swimming pool chlorination?

[Steve Turner]

Peabody <waybackK...@yahoo.com> wrote:

>The folks who make "HTH" brand calcium hypochlorite pool water
>sanitizer have introduced a new formula which they say is
>"buffered". It is a mixture of their normal 65% cal-hypo product
>with magnesium sulfate heptahydrate, aka Epsom Salts, the latter
>making up about 30% of the total mixture.
>
>I'm trying to figure out exactly what role the Epsom Salts would
>have in this situation, but so far have drawn a complete blank.

Good question. What was the balance of the mass in their original
formula? IOW, if it was 65% calcium hypochlorite, what was the other
35%?

I'm guessing that they may be using the term "buffered" in a more
generic sense which may have little or nothing to do with pH.

Steve Turner

Real address contains worldnet instead of spamnet

[Oscar Lanzi III]

Does HTH possibly produce hydroxide ions which might make things a
little too basic? Myabe the MgSO4 scoops them up?

--OL

[ebeveridge]

Dear me, has everyone forgotten the use of magnesium salts to fix the pH of
a solution? It should give you pH 9.5 or so. See Charlot & Bézier -
Quantitative Inorganic Analysis, English translation by R C Murray of 3rd
French edition, p.117 (London: Methuen, 1957)

I find this effect convenient for coprecipitating hydroxides with Mg(OH)2

DB

Oscar Lanzi III <o...@webtv.net> wrote in message
news:15080-3E...@storefull-2317.public.lawson.webtv.net...

[ebeveridge]

What pH do you get with just calcium hypochlorite in water? Could be quite
high - I don't know, it's ages since I last had to mess with real Bleaching
Powder (i.e. commercial calcium hypochlorite).

Adding a soluble magnesium salt will stop the pH rising above that which is
required to precipitate magnesium hydroxide, and therefore the pH is
controlled.

You have to wonder how much this matters! I suppose if you make up a
concentrated solution and add that to the water, then that solution could be
quite nasty to handle in the absence of the magnesium salt.

DB

> I asked the question originally, and now it appears that my
> cover has been blown. It's true. I am not a chemist.
>
> I did dissolve a spoonful of epsom salts in distilled water
> and tested it in my pool test kit. Roughly, it has a ph of
> 7.0.
>
> Could I prevail upon you to explain what "fix" means, as
> well as "coprecipitating hydroxides"?
>
> Basically, I would just like to understand what effect the
> magnesium sulfate would have, either on the ph or on the
> compounds produced when the "HTH" is mixed with water. HTH
> used alone tends to increase ph and produce scaling.
>
>

[ebeveridge]

Further thought - I rather doubt that keeping the pH down to 9.5 ish will
have much difference on scale formation. Calcium salt solutions always tend
to pick up carbon dioxide from the air - worse at high pH, it's true, but
still happens quite extensively at lower pH. Scale inhibitors exist but I'm
not familiar with the technology.

[Oscar Lanzi III]

When you put Epsom salt into water and find the pH is 7, that means it's
not acting as an acid or base towards water. But it could act as an
acid or a base towards OTHER THINGS. For example, if you add HCl to a
solution of Epsom salt, you will find that the solution is less acidic
than HCl in just water (you formed bisulfate ions). If you add NaOH to
Epsom salt solution, you get a precipitate (Mg(OH)2, of course) and find
that the solution pH goes up only by a limited amount. What started as
a neutral compound has thus been found to act as a base in one case and
an acid in the other.

Sometimes the acid-base character is actually reversed by adding a
reagent to a salt solution. Dissolve some NaHCO3 in water. Measure the
pH. Basic, right? Now add a little NaOH. Which solution is more basic
NOW?

There's a word for compounds that can act either as acids or as bases.
It's "amphoteric".

--OL