Just a little more on Formaldehyde or Formalin.
Generally when a 37% concentration is stated on the label of a chemical
solution it means 370 g/L. If you use that rate for formalin (formaldehyde)
you will end up with a different dose rate than at 996.8 g/L, which can be
very critical, as you could end up overdosing with disastrous results.
To get the correct dose rate (25 ppm) of formalin you have to use the 999.8
g/L figure in calculations like we did for the other strength eg 1031 g/L.
I think its all because formalin (which is a generic name for formaldehyde)
is an aqueous solution of formaldehyde gas generally containing not less
than 37% (by weight) of formaldehyde gas per weight of water and 6 to 13%
(12%) methanol. In solution, formaldehyde is present chiefly as HO(CH2O)H.
Its molecular weight is 30.93. (Heyden Newport Chemical Corporation, 1961).
I've checked a number of different brand name formalin solutions and checked
their recommended usage rates for 25 ppm and have come up with exactly the
same dose rate using the g/L concentration regardless of the percentage rate
stated on their labels. However, if you use the percentage figure you get a
different result (higher).
This probably applies to most gases dissolved in water. If I remember
correctly some years ago I use to handle a chemical called Vapam (a soil
fumigant) which also stated a higher g/L concentration than the percentage
listed on the label.
Its a pity we didn't have a chemist on the list :-(
Adrian.
Adrian R. Tappin
"Home of the Rainbowfish"
http://www.ecn.net.au/~atappin/home.htm