Unit conversion (and what do small teeth have to do with it)

[DonJasper]

Ok I was trying to understand nutrient levels. It seems that the units used for ocean observations is a micromolar (uM) ??

So how does that translate into our test kit readings
-OR- (as I suspect)
does it mean "it's so low that hobby test kits can't touch it"?

[jfinch]

There are 1,000,000 micromoles per mole.

To convert a weight concentration to molar concentration you divide the weight concentration by it's molecular weight.

So, as an example, for calcium:

410 mg/l / (40 mg/mmol) = 10.25 milimole (mM) x 1000 mM/uM = 10,250 uM

[Boomer]

:lol: I just did that on another forum, ppm vs ppb and milimoles and micromoles for Al.

or milimoles x MW =ppm and micromoles x MW = ppb

also 10.25 mmol is 10.25 mmol / Kg

and 1 mole of Ca weighs 40 mg

What is a Mole, no not the animal with the funny face like Randy :lol:

1 mole of anything has exactly 602,000,000,000,000,000,000,000 'things' (6.02 x 10^23) . 1 mole of Ca has that many atoms (ions) of Ca and weighs 40 grams (actually 40.08). 1 mole of Mg has the same number of atoms/ions and weighs 24 grams ( actually 24.305). We round them off to 40 and 24. Same for things like H20 or PO4. I mole of H2O or PO4 has 6.02 x 10^23 molecules / ions. 1 mole of PO4 is, P + 4 0 's = 31 +16 + 16 +16 +16 = weighs 95 grams, so 1 mmol of PO4 is 1/ 1000 of 95 = .095 grams or 95 ppm or 95 mg / l

[DonJasper]

So Salifert says:
... if the phosphate level is higher than 0.04 mg/L .... then all kinds of bad things happen.

So if I grind out the math, hoping against hope that the decimal place doesn't start to wander, I get .04 mg/L to work out to be something like 4 millimole/L.?

So Salifert claims:
The test kit’s range goes from a very low to a very high nitrate concentration (approx. 0.2 - 100 mg/L as total nitrate).

And I never thought I'd ever need to look up the molecular weight of anything ... (Next I'll have to cough up the molecular bonds just to raise fish :rolleyes: ? )

Nitrate weighs 62 g/mol. So a reading of .2mg/L grinds out to 30 millimole/L?

Nitrate weighs nearly half that of Phosphate, and the reading about 5 times that of Phosphate - so maybe that's corrent?

So when I see the notation uM - is that milli or micro mole - or something entirely different?

[Boomer]

I get .04 mg/L to work out to be something like 4 millimole/L.?

Nope, as Jon pointed out you divide and PO4, which is the phosphate you are measuring, is 1 mole = 95 g (1 mole of PO4 is, P + 4 0 's = 31 +16 + 16 +16 +16 = weighs 95 grams), so .04 mg / l / 95 = .000421 mmol / Kg = .421 uM / Kg

So a reading of .2mg/L grinds out to 30 millimole/L?

Nope :D

.2 / 62 = .0032258 mmol /Kg

30 mmol = 30 x 62 = 1,860 mg / l PO4

Look at Ca, it is 10.3 mmol / kg in seawater and Ca MW = 40 , so 10.3 x 40 = 412 mg / l, the other way 412 / 40 = 10.3. If your Ca test kit reads 390 mg / l Ca, then 390 / 40 = 9.5 mmol / Kg


uM = Micromoles and mmol x 1,000 = uM