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Dosing Calcs KH2PO4

Thanks to Paul Sears for correcting my mistake yesterday re pH swings
and KH.

>>>If the KH is anything other than _very_ low, then KH has no effect on the
size of the pH swing resulting from a given CO2 concentration change
(ratio change, not difference).  Check the table and see.>>>  Yes, I
see it. <g>

I received a jar of potassium phosphate monobasic (KH2PO4) from Hach
yesterday and got a vivid reminder of my own 'appalling lack of
knowledge' as I tried to work out how to dose my tank.  Maybe you
science guys will tell me if I got it right!

BTW, I'm interested in phosphate as a possible nutrient to add to my
aquarium because I've been having a number of the symptoms described by
Tim Mullins and Paul Sears last week that appear to be affected by the
addition of phosphate.  And in the two years that I've had Hach's low
range phosphate test kit I've never had any measurable reading of
phosphate in either of my tanks.  Not once.  I use a lot of light and
CO2 -- so perhaps when plant growth stops unexplainably, I have a
phosphate shortage (odd as that idea may seem).  And 'merely'
controlling nutrient inputs so that my tank wasn't nitrate limited
hasn't solved all of my problems.  And neither has adjusting levels of
Tropica MasterGrow or traditional formula PMDD.

So I'm trying to work out how to dose my 125 gal. aquarium in the range
of 0.1 ppm phosphate.

Starting with something I think I know: 1 gram of stuff dissolved in
1000 liters is 1 part per million by weight.  (1g = 1000 mg and 1000
liters = 1,000,000,000 mg of water; drop three zeros and we have 1 mg.
of stuff dissolved in 1 million mg. of water or 1 ppm.)  Right?

I figure my aquarium has 110 gals of water in it.  Since a gallon of
water is 3.785 liters, I should have about 416 liters of water in the
tank.  Dividing 1000 liters by 416 = 2.4; so 1 g of stuff dissolved in
my tank with 110 gals of water should yield a concentration of 2.4 ppm.

I found the atomic weights of KH2PO4 in the dictionary (actually I got
worried; it was 11:00 o'clock at night and I was on my 3rd dictionary
before I found it under 'periodic table' <g>) and figured that potassium
phosphate monobasic is about 70% PO4 by weight.  (95 weight of the
phospate divided by 136 the weight of the whole molecule = 70%).  So 1 g
of KH2PO4 should yield 1.68 ppm of phosphate in my tank.  Right?

Dividing by 20, 0.05 g of the stuff should yield 0.084 ppm phosphate --
which is fairly close to where I want to start dosing.

So I need to figure out how to measure 1/20 g. of KH2PO4.  (I don't have
a gram scale.)  One of my LaMotte test kits includes a little spoon
which is labeled 0.1 g.  I put 10 of these spoonfuls of KH2PO4 into a
quarter teaspoon measure and just about filled it up.  Roughly, this
would seem to confirm Neil Frank's comment that most of the stuff we
might does our aquariums with weighs between 1 - 2 g per quarter

So roughly I figure I need to add about 1/2 of the 0.1 g spoon to my
tank.  How does that sound?  Any comments on the starting dose?

Apologies for the length of the post.  Perhaps it will help some of the
other non-scientists trying to drag high school chemistry back into
their minds.

Regards, Steve Dixon