From: Alberto Novo (alnovo@inwind.it)
Date: 04/20/03-07:50:32 AM Z
Christina,
> But then why do we call a 30% solution mixing 300g in 1000 ml water?
> Because it seems here Demachy is mixing 300g gum in 600ml water, correct?
The replies from Katherine are absolutely correct, this above is a wrong way
to prepare the right concentration.
I must ppoint out that there are two ways to define a concentration: by
weight and by volume.
The first is "a defined weight of a certain substance in a total weight of
solution" (w/w, weight/weight);
the second is "a defined weight of a certain substance in a total volume of
solution" (w/v, weight/volume).
The conversion from the first to the second unit is made multiplying w/w by
the density. For extremely concentrated solutions it may happen that the w/v
is higher than 100%, and this is not wrong: 100 mls of solution may contain
more than 100 g of solute.
Many solutions used in the darkroom are prepared by w/v: in fact, you add
different weights to a certain amount of water, then you fill up the
solution to 100 or 1000 ml.
As for gum... I don't know. Surely it is simpler, due to the viscosity of
the solution, to prepare a solution by weight: a certain amount of gum plus
a certain amount of water, disregarding the information about the final
volume.
> So then tell me--if I mix 300g of gum in 1000ml can I call that a 30%
> solution or am I mistaken?
It is a 300/(300+1000)=23% solution (weight/weight)
> I have absolutely no clue what baume is or what a difference of 14-18
baume even
> means to a layperson
Baume degrees are defined in two ways, for densities above and below 1. The
original Baume densimeter for density above 1 has the zero deg. mark when
put in pure water, and the 15 deg. mark when put in a 15% sodium chloride
solution.
For densities below 1 the zero mark is when a 10% sodium chloride solution,
and the 10 mark when in pure water.
Baume densities should not be used anymore; the conversion formulae are (D=
density, B= Baume):
for D>1 D = 144.32/(144.32 - B) B = (144.32 D - 144.32)/D
for D<1 D = 144.32/(134.32 + B) B = (144.32 - 134.32 D)/D
(note that 144.32 and 134.32 are correct)
So 12 Baume = 1.091 kg/dm3
14 " = 1.107 "
18 " = 1.142 "
You cannot predict the concentration simply from its density; you can do
this only knowing the relationship existing between density and
concentration (for a well defined substance).
In practice, you should prepare some gum solutions with different
concentrations, then measure their densities, then plot the results against
concentration and draw a curve to interpolate the results.
Alberto
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