Is "dissolvability" cumulative

I decided to add Potassium and Magnesium to the last batch of "PMDD" I mixed
up to save me the trouble of adding it separately. But the amounts I needed
to add to my stock solution to reach Chuck Gadd's recommended dosages were
above the amount of each compound that would dissolve in my stock solution.
So I reduced the amounts to levels that should dissolve in my stock
solution, but they still did not stay in solution.

I gather these "dissolvability" numbers are cumulative and not specific to
each chemical? Is the cumulative effect straight line or does it depend on
the chemicals being dissolved.

Just curious.

Bill Stock wrote:
I decided to add Potassium and Magnesium to the last batch of "PMDD" I mixed
up to save me the trouble of adding it separately. But the amounts I needed
to add to my stock solution to reach Chuck Gadd's recommended dosages were
above the amount of each compound that would dissolve in my stock solution.
So I reduced the amounts to levels that should dissolve in my stock
solution, but they still did not stay in solution.

I gather these "dissolvability" numbers are cumulative and not specific to
each chemical? Is the cumulative effect straight line or does it depend on
the chemicals being dissolved.

Just curious.



Yes, the amount of solids you can add to a solution is cumulative. The
effect depends on the chemicals being dissolved and some even
precipitate out in combination, like calcium and phosphate.

--
Elaine T __
http://eethomp.com/fish.html '__
rec.aquaria.* FAQ http://faq.thekrib.com

"Elaine T" wrote in message
...
Bill Stock wrote:
I decided to add Potassium and Magnesium to the last batch of "PMDD" I
mixed up to save me the trouble of adding it separately. But the amounts
I needed to add to my stock solution to reach Chuck Gadd's recommended
dosages were above the amount of each compound that would dissolve in my
stock solution. So I reduced the amounts to levels that should dissolve
in my stock solution, but they still did not stay in solution.

I gather these "dissolvability" numbers are cumulative and not specific
to each chemical? Is the cumulative effect straight line or does it
depend on the chemicals being dissolved.

Just curious.



Yes, the amount of solids you can add to a solution is cumulative. The
effect depends on the chemicals being dissolved and some even precipitate
out in combination, like calcium and phosphate.

Thanks Elaine, makes sense based on what I'm seeing.

I was hoping to increase K and Mg above the standard PMDD strength to avoid
adding extra doses separately. I guess I could increase the size of my stock
container and just add more solution each week to accomplish the same thing,
but a 10 gallon stock container might be a little impractical.

Bill Stock wrote:
"Elaine T" wrote in message
...

Bill Stock wrote:

I decided to add Potassium and Magnesium to the last batch of "PMDD" I
mixed up to save me the trouble of adding it separately. But the amounts
I needed to add to my stock solution to reach Chuck Gadd's recommended
dosages were above the amount of each compound that would dissolve in my
stock solution. So I reduced the amounts to levels that should dissolve
in my stock solution, but they still did not stay in solution.

I gather these "dissolvability" numbers are cumulative and not specific
to each chemical? Is the cumulative effect straight line or does it
depend on the chemicals being dissolved.

Just curious.




Yes, the amount of solids you can add to a solution is cumulative. The
effect depends on the chemicals being dissolved and some even precipitate
out in combination, like calcium and phosphate.


Thanks Elaine, makes sense based on what I'm seeing.

I was hoping to increase K and Mg above the standard PMDD strength to avoid
adding extra doses separately. I guess I could increase the size of my stock
container and just add more solution each week to accomplish the same thing,
but a 10 gallon stock container might be a little impractical.



ROFL! I was stirring forever trying to get a Plantex and potash mix to
dissolve this afternoon. I kept adding more and more water - it's half
the strength that I wanted. *sigh* Is it just me or does it seem like
you have to add incredible volumes of PMDD to get decent amounts of
ferts in the water? I'm looking at 1.5 ml per 10 gal per day of my brew
to try Tom Barr's Estimative Index dosing.

--
Elaine T __
http://eethomp.com/fish.html '__
rec.aquaria.* FAQ http://faq.thekrib.com

Elaine T wrote:

ROFL! I was stirring forever trying to get a Plantex and potash mix to
dissolve this afternoon. I kept adding more and more water - it's half
the strength that I wanted. *sigh* Is it just me or does it seem like
you have to add incredible volumes of PMDD to get decent amounts of
ferts in the water? I'm looking at 1.5 ml per 10 gal per day of my brew
to try Tom Barr's Estimative Index dosing.


Elaine, I made up two potions to solve this problem. My "tea" is
Plantex and magnesium together. I think it is 1 tablespoon each to
500ml water. My "pink stuff" is KNO3, KCI and a touch of KH2P04. I've
been talking to Greg Watson and at his suggestion I mix separate
batches to solve the solubility issue.

For what it's worth, I add 15ml of tea every other day and 20 ml pink
stuff every other day. The pink stuff is (I think) 4 TBL KCI, 1 TBL
KN03 and 1 tsp KH2P04 in 500 ml water. 55 gallon tank with C02 @ 30ppm
and full of plants.

steve

Bill Stock wrote:

I gather these "dissolvability" numbers are cumulative and not specific to
each chemical? Is the cumulative effect straight line or does it depend on
the chemicals being dissolved.

In chemistry the amount of a salt that can dissolve is sometimes
expressed as a solubility product constant. That is, at saturation,
multiplying the concentration of the negative ion by the concentration
of the positive ion gives a constant value. (What the value is depends
on which salt you're talking about.)

To a first approximation, the value of the constant does not depend on
what other salts are dissolved in solution. For example, the amount of
K+ in a saturated KNO3 + NaCl solution is the same as the amount of K+
in a saturated KNO3 solution without the sodium chloride.

However, if the two salts share the same ion, you get what's called the
common ion effect - you increase the total amount of the shared ion,
which reduces the amount of the other ions which can be dissolved,
relative to dissolving each one individually. I've seen it happen with
concentrated HCl added to concentrated NaCl - the Cl concentration got
too high, and NaCl crashed out of solution.

On 2005-08-22, Elaine T wrote:
ROFL! I was stirring forever trying to get a Plantex and potash mix to
dissolve this afternoon. I kept adding more and more water - it's half
the strength that I wanted. *sigh* Is it just me or does it seem like
you have to add incredible volumes of PMDD to get decent amounts of
ferts in the water? I'm looking at 1.5 ml per 10 gal per day of my brew
to try Tom Barr's Estimative Index dosing.

Interesting - I've experienced the opposite. I've had to turn down my
concentration because it was too rich. I suspect it actually has to do
with the water in my current area. It's very rich in nitrate, and so
I've cut that out of my PMDD concoction. As a result, everything
dissolves easily ::-)

--
Ross Vandegrift

"The good Christian should beware of mathematicians, and all those who
make empty prophecies. The danger already exists that the mathematicians
have made a covenant with the devil to darken the spirit and to confine
man in the bonds of Hell."
--St. Augustine, De Genesi ad Litteram, Book II, xviii, 37

steve wrote:
Elaine T wrote:


ROFL! I was stirring forever trying to get a Plantex and potash mix to
dissolve this afternoon. I kept adding more and more water - it's half
the strength that I wanted. *sigh* Is it just me or does it seem like
you have to add incredible volumes of PMDD to get decent amounts of
ferts in the water? I'm looking at 1.5 ml per 10 gal per day of my brew
to try Tom Barr's Estimative Index dosing.



Elaine, I made up two potions to solve this problem. My "tea" is
Plantex and magnesium together. I think it is 1 tablespoon each to
500ml water. My "pink stuff" is KNO3, KCI and a touch of KH2P04. I've
been talking to Greg Watson and at his suggestion I mix separate
batches to solve the solubility issue.

For what it's worth, I add 15ml of tea every other day and 20 ml pink
stuff every other day. The pink stuff is (I think) 4 TBL KCI, 1 TBL
KN03 and 1 tsp KH2P04 in 500 ml water. 55 gallon tank with C02 @ 30ppm
and full of plants.

steve

Thanks! I'm not too far off your ranges then. I did something similar,
but I made a brew with Plantex and K, and a second with everything
because some of my tanks don't need extra N and P.

Dissolving K2SO4 surprised me because I was expecting solubility similar
to KCl. It took me a liter to dissolve 5 tbsp of K2S04 and 1 tbsp
Plantex. Since I had trouble with that one, I made up the full PMDD
formula (minus epsoms) in a liter rather than 500 ml.

--
Elaine T __
http://eethomp.com/fish.html '__
rec.aquaria.* FAQ http://faq.thekrib.com

"Rocco Moretti" wrote in message
...
Bill Stock wrote:

I gather these "dissolvability" numbers are cumulative and not specific
to each chemical? Is the cumulative effect straight line or does it
depend on the chemicals being dissolved.

In chemistry the amount of a salt that can dissolve is sometimes expressed
as a solubility product constant. That is, at saturation, multiplying the
concentration of the negative ion by the concentration of the positive ion
gives a constant value. (What the value is depends on which salt you're
talking about.)

To a first approximation, the value of the constant does not depend on
what other salts are dissolved in solution. For example, the amount of K+
in a saturated KNO3 + NaCl solution is the same as the amount of K+ in a
saturated KNO3 solution without the sodium chloride.

However, if the two salts share the same ion, you get what's called the
common ion effect - you increase the total amount of the shared ion, which
reduces the amount of the other ions which can be dissolved, relative to
dissolving each one individually. I've seen it happen with concentrated
HCl added to concentrated NaCl - the Cl concentration got too high, and
NaCl crashed out of solution.

Thanks Rocco, so my MgSO4+7H2O should not reduce the solubility of my KCL if
I understand you correctly. This does not seem to be what I'm experiencing
though, as I'm achieving no where near the solubility numbers in Chuck's
calculator. Perhaps there is some CL in the Plantex mixture, which is
causing the KCL to precipitate.

Bill Stock wrote:

Thanks Rocco, so my MgSO4+7H2O should not reduce the solubility of my KCL if
I understand you correctly. This does not seem to be what I'm experiencing
though, as I'm achieving no where near the solubility numbers in Chuck's
calculator. Perhaps there is some CL in the Plantex mixture, which is
causing the KCL to precipitate.

Hmm.. I probably mislead you. In addition to each individual salt, you
also have to look at the double displacement salts (where you swap the
anions and cations from the two mixed salts).

The approximate solubility of various compounds in cold water, according
to the CRC handbook:

KCl - 350 g/L or 4.7 M for a Ksp of 22 (4.7*4.7)
MgSO4 - 260 g/L or 2.2 M for a Ksp of 4.8 (2.2*2.2)
MgCl2 - 543 g/L or 5.7 M for a Ksp of 740 (5.7*11.4*11.4)
K2SO4 - 120 g/L or 0.7 M for a Ksp of 1.4 (1.4*1.4*.7)

A saturated solution of KCl thus has 4.7 M K+ and 4.7 M Cl-
A saturated solution of MgSO4 thus has 2.2 M Mg++ and 2.2 M SO4--

A hypothetical "dual saturated" solution would have
4.7 M K+, 4.7 M Cl-, 2.2 M Mg++ and 2.2 M SO4--

But we also have to consider the double displacement products MgCl2 and
K2SO4.

The solubility products for the hypothetical solution are

MgCl2 - [Mg++][Cl-]^2 = 2.2*4.7*4.7 = 48.6, well under the 740 limit
K2SO4 - [K+]^2[SO4] = 4.7*4.7*2.2 = 48.6 - well *above* the limit of 1.4

So what's happening is that you dissolve the MgSO4 and KCl, and the two
dissolve fine, but when the K+ and the SO4-- find each other in
solution, they feel crowded and crash out as a precipitate of K2SO4.
That whitish "undissolved" powder in the bottom of the bottle is not KCl
or MgSO4, but K2SO4.

Note that you can probably get the precipitate to dissolve if you heat
it (The CRC handbook gives solubility of K2SO4 in hot water of twice
that in cold water), but when the solution cools the K2SO4 will just
come out of solution again.