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Re: Chemistry question

Chuck Gadd wrote:
 
> So, I think I can figure out what percentage of any given compound is comprised
> of a given element.   Pretty simple.

I think you got those calculations right.

    And, I think I'm right in saying that
> 1 gram of an element dissolved in 1 liter of water will create a concentration
> of 1000ppm.    So, if I dissolved 1 gram of K2SO4 in 1 liter of water, I'd have
> a potassium concentration of 529ppm (not counting any potassium already present
> in the water..).     From there, it's simple to calculate how much a given
> dosage of that solution would add to a tank.

There is an added complication.  Water.  A lot of the chemicals we use
contain varying amounts of water and that fact is often overlooked.

Some of the water is structural water and its presence in the chemical
is pretty predictable.  The common form of calcium sulfate, for
instance, is CaSO4-2H20; there are two water molecules in the crystal
structure for each formula weight of calcium sulfate.  There are other
forms of calcium sulfate with different amounts of water.  Sometimes the
water gets left out of the formula, but it definitely has an effect.

Second, a lot of the chemicals we use adsorb water from the air. 
Calcium chloride is a great example.  A lot of different chemicals when
weighed on a sensitive analytical scale in normal humid air can be seen
to gain weight (sometimes quickly) as they take water out of the air. 
That water can be driven out of the chemical by heating at fairly low
temperatures.  It effects both the weight and the volume of the
chemical.

> BUT!  Now I'd like to be able to measure out the K2SO4 in teaspoons or some
> similar volume measurement.
> 
> Looking at that page on TheKrib dealing with dosing, there is a chart that shows
> an approximate weight per 1/4 teaspoon of various compounds.    But, it doesn't
> list K2SO4.   I know I could just weigh out a known volume of K2SO4, but is there
> a way to calculate it based on basic chemistry information?   For some elements,
> I can find "Density of solid".  But for Chlorine, Oxygen, etc (gases), it doesn't
> list the value.    I also have a Molar Volume (cm3) which I assume means one mole of
> an element would take up xx cubic centimeters.   I thought I could use that info to
> figure it out, but my numbers would not match up to any of the grams per 1/4tsp
> listed
> on the page at TheKrib.

This is a real problem, and I don't think you can calculate a number
accurately.  You're on the right track, but you need to account for the
amount of air between the grains in the sample.  You might be able to
ballpark something by approximating the actual weight of the chemical as
60% (or so) or the molar weight.

The best procedure is to get (beg/borrow/buy) a reasonably accurate
scale and measure the weight and volume of the chemicals you use under
your conditions.  That isn't a real satisfying alternative for most
people, but I think it's the only solution that will make your
calculations reasonably accurate.

An alternate approach is to mix an aqueous solution of the chemical and
then use your test kits (probably requiring a lot of dilutions) to
figure out the concentration of the solution.  Then just use the
solution for dosing instead of trying to predict doses from powdered
chemicals.
 
Yet another approach (this being something I haven't tried) might be to
mix a saturated solution of the chemical (that's a solution that will
coexist with the solid chemical without either dissolving more or
precipitating), then use the solubility of the compound to estimate the
concentration of the solution.  The saturated solution of
(approximately) known concentration can then be used for dosing.  This
wouldn't work for all chemicals, as some (like calcium carbonate) have
pH-dependent solubility and/or undergo reactions when they dissolve.


Roger Miller