From: Christina Z. Anderson (zphoto@montana.net)
Date: 04/21/03-09:22:04 AM Z
Alberto,
Thank heavens there are scientists on this list such as yourself who
can set me straight so I don't have to go back and take a chemistry course.
This makes sense, and you and Katharine are right, we do add a certain
percent and then fill up to a liter, so it stays a 30% weight/volume.
Demachy is not mixing 300g of gum into 750ml of water and then adding
water to make 1000ml. He is mixing 1 part gum to 2 parts water--300g to
600ml and boom. I will do that this week and then get a total end ml count
to therefore see the weight to volume ratio that I end up with. I don't
think it will be 900ml total volume because (bear with me, you scientists)
the little particles of gum powder will be filled in with the water and
compact. So my guess is, it is a stronger than 30-35% dilution as far as
weight/volume goes.
Henney and Dudley also had the formula of 1 +2 which was 500g of gum to
1000ml water, not the 300g to 1000 that is common today. My only interest
boils down to testing this side by side with regular gum, and tho I can't
get the baume of the two since I don't have a...is it a hygrometer....and
tho it seems I am laboring a really picky point and you all will have to
bear with me, I will be "sentimental" as Judy says and test the thicker gum
against the thinner gum we use today and see if there is any benefit to
doing so, with a pigment I have found to stain terribly--q violet. In
powder form in powder gum it does not stain a bit. In tube form I have had
problems with even low dilutions.
In practice, if the thicker gum has uses, it may just boil down to
having a powdered supply on hand to mix in a teaspoon of it here and there
to thicken a solution when lower pigment powder/thinner gum solutions are
used. Another option perhaps in a gum printer's arsenal (see, Jack, I have
abandoned the term gummist per your request!).
Chris
> 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
>
>
>
>
This archive was generated by hypermail 2.1.5 : 05/01/03-11:59:54 AM Z CST