On 27/12/17 21:17, Chris Hogg wrote:
On Wed, 27 Dec 2017 20:19:16 -0000 (UTC), (Nick
Maclaren) wrote:
In article ,
Chris Hogg wrote:
Can't help you with the MSDS question, but as to adding calcium
nitrate and limestone, I was always under the impression that it's
high pH caused by lime or limestone that's the problem with ericaceous
plants, not the calcium per se. One of my books on rhodos advocates
gypsum as a soil improver for rhodos and azaleas. Proper mineral
gypsum (not the plaster of Paris commonly available from builders'
merchants these days), is naturally slightly acidic, being the salt of
a weakish base and a strong acid.
That's not the reason. There is some reason that magnesium behaves
very differently from most metals, and its salts cause what is
effectively acidity. It's not just totting up the strengths, but
I don't know the details. The effect is also used in brewing.
Regards,
Nick Maclaren.
Er...what I gave was a simple explanation.
The acidity of a salt of a weak base and a strong acid arises because
the weak base is only partially ionised in water. For example, gypsum,
calcium sulphate, dissolves in water (slightly) to give calcium ions
and sulphate ions. CaSO4 - Ca++ + SO4--.
But some of the calcium ions hydrolyse and form an equilibrium with
calcium hydroxide, as calcium is a weak base
Ca++ + 2H2O - Ca(OH)2 + 2H+
The Ca(OH)2 is un-ionised, so contributes neither acidity nor
alkalinity to the solution.
The overall equation when gypsum dissolves in water is
CaSO4 + 2H2O - Ca(OH)2 + 2H+ + SO4-- which has an excess of hydrogen
ions, i.e. is slightly acidic with a pH below 7.
The reverse occurs with a weak acid and strong base, such as sodium
carbonate; the acid is only partially ionised, resulting in an excess
of hydroxyl ions, a deficiency of hydrogen ions and a pH above 7.
The same equations could be written for magnesium, ammonium or any
weak base, but the degrees of hydrolysis will be different, as will
the resulting pH.
Strong bases, such as sodium or potassium, don't hydrolyse, so their
solutions remain neutral.
All good stuff, but after reading a bit about the problem calcifuges
have with calcium I'm even more confused!
From what I understand, the problem results from poor availability of
intracellular phosphate, in particular iron phosphate. Now just what
causes that is what confuses me. Sure, you could say that calcium reacts
with phosphate to form a pretty insoluble form of phosphate, and that's
the reason.
But calcium carbonate (as limestone) is pretty insoluble in water too,
and even though the solution is alkaline (pH around 8), how much of that
calcium is available to "mop up" phosphate? Now calcium nitrate is
extremely soluble in water, but the pH of a 5% solution is around 6. So
(like gypsum), does that pH prevent problems with calcium even though
there is so much more of it in solution?
Oh, on the msds question, I see that the date of the "uninformative" one
is a lot more recent (2012) than the one which lists the contents
(2004). Progress, eh?
--
Jeff