Can this be correct? Dilution ratio confusion...

[Trask]

I recently visited a German language website wherein the blogger suggests several film developer formulae, including one which calls for "Potassium sulfite - solution 45% 35 ml ." In a side note, the blogger states (according to Google Translate): "A 45% potassium sulfite solution does not contain 45g in 100ml solution, but 45g plus 55g water (with water in a good approximation g = ml). The concentration in g / l cannot be easily deduced from this!" Here it is in the original German: "Eine 45%ige Kaliumsulfit-Lösung enthält nicht 45g in 100ml Lösung, sondern 45g plus 55g Wasser (bei Wasser in guter Näherung g = ml). Auf die Konzentration in g/l lässt sich daraus nicht ohne weiteres schließen!"

While I mix a fair number of developers from scratch, I rarely work by diluting one chemical in a volume of water and then using a small portion of that solution. In instances where I've seen similar, it's often stated that, for example, a 1% solution of XX may be made by diluting 10 grams of XX in 1 liter of water. I realize that the volume of XX is so small in comparison to a liter of water that the German fellow's assertion is approximately correct, but what about higher percentages in smaller volumes of water -- could it be correct that a 45% solution would be 45 grams dissolved in 55 grams of water, with the idea being that when mixed the solution will weigh 100 grams? In my innocence, I would have dissolved 45g in 100ml of water and thought myself correct. Thoughts?

 

[koraks]

When it comes to % solutions, there are a few ways to specify them:
w/v = weight/volume. This is the standard way that is used for virtually all photographic purposes, AFAIK. A 45% w/v solution would be 45g in a total volume of 100ml. The total weight of the solution will be more than 100g, but less than 145g, since there will be less than 100ml water added to the 45g of dry material.
w/w = weight/weight. This is the approach the German author uses that you refer to. A 45% solution will be 45% of the coumpound + 55g (=55ml) of water. Total weight will be 100g. The volume will be more than 55ml (because the dry compound adds to the volume), but less than 100ml (because the solution is essentially more space-efficient).
v/v = volume/volume. Commonly used when discussing solutions of one fluid in another. E.g. when making a 45% alcohol solution (for whatever purpose...), this could be done by specifying 45ml of alcohol being made up to 100ml with water. The resulting volume of the solution will thus be 100ml (by definition), but *I think* the amount of water added will likely be marginally more than 55ml since the solution will again be very slightly more space-efficient than the separate fluids. However, this difference is probably much less than when dissolving a solid in a fluid and in my experience can be ignored in practice (e.g. when making salted collodion solutions for wet plate photography!)
v/w = volume/weight; I haven't come across this one as it's a bit counterintuitive.
Neither v/v and v/w are generally used for specifying solutions of a solid compound for the obvious reason that the volume of a solid depends very much on the physical state of the solid - i.e. fine powder vs. large crystals/rocks, and how loosely or densely packed the compound is (i.e. how much air is between the particles).

Personally I find it confusing that the German source uses the (in my experience) non-standard notation of w/w instead of w/v. Technically there's nothing wrong with it however, particularly since the author is explicit on what (s)he means.

 

[Anon Ymous]

We, as hobbyists, like to use w/v solutions, it's just more convenient. We calculate the volume needed and just use a graduate. In the industry, the use of w/w solutions is much more prevalent as far as I can tell. And the German guy you mentioned probably buys the 45% w/w solution from Suvatlar, he doesn't make it himself.

 

[Trask]

Thank you both for your thoughts on this. All this opens the door to wondering about other formulae I’ve seen and/or used where one component was a solid diluted in water — I’wonder If I got it right! Live and learn...

 

[koraks]

In nearly all formulas for photo chemistry I've come across, w/v is used. If it's not specified, in the context of photo chemistry I always assume w/v.

 

[john_s]

We, as hobbyists, like to use w/v solutions, it's just more convenient. We calculate the volume needed and just use a graduate. In the industry, the use of w/w solutions is much more prevalent as far as I can tell. And the German guy you mentioned probably buys the 45% w/w solution from Suvatlar, he doesn't make it himself.

And to minimize confusion, I label such solutions as g/L, so for example, my KBr solution is labelled 100g/L. If I need 2g I just measure out 20mL.