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1. Originally Posted by Kirk Keyes
So your original instructions of 100 g K2CO3 dissolved into 100 ml water is actually a 50% solution (1 part + 1 part), and not a 75% solution - even though it has a concentration of about 0.75g/ml.
This confuses me quiet a bit. I'm no chemist so I can't argue with you, in fact I don't even doubt that this is true. But I make percentage solutions so that I can quickly measure different chemicals by just pouring the correct amount into a graduated cylinder. If 0.75g/ml is really a 50% solution of K2C03, I don't want to know the correct way of doing things. The wrong way works much better for me.

2. Originally Posted by Mateo
This confuses me quiet a bit. I'm no chemist so I can't argue with you, in fact I don't even doubt that this is true. But I make percentage solutions so that I can quickly measure different chemicals by just pouring the correct amount into a graduated cylinder. If 0.75g/ml is really a 50% solution of K2C03, I don't want to know the correct way of doing things. The wrong way works much better for me.
Mateo, dont give it a second thought, Keyes is right in the sense that the formula for percent solutions is as follows.

(Wb/(Wa+Wb))x100= percent solution.

so for pot carbonate it would be,

(100 gr K2CO3/100 gr K2CO3+100gr water))x100= 50%

In the example where King mentions that he is getting about 135 ml of total solution, the mistake was not taking into account the final specific gravity of the solution.

In the end all you need to know is that for solution B you need to add 100 gr of K2CO3 to 100 ml of water and use it as you want to.

3. Thanks for the clarification Jorge.

I never bother making that almost saturated solution when I can much more easily make it half strength and double it for the working solution. But I don't make up bottles and lable them pyrocat sol B, I just write down the % of the chems in the bottles. That way I don't have notebooks with notes flying around confusing me and getting lost.

Reading this now, I see that the way I've been labling is wrong. If I were to weigh out 100 grams of something and the total volume with water came to 135 ml, I would lable that as a 74% solution. That way I know that 10ml would have 7.4 grams in it. Looks like if that is really a 50% solution it would take all the fun shortcut out of everything.

4. Originally Posted by Mateo
Thanks for the clarification Jorge.

I never bother making that almost saturated solution when I can much more easily make it half strength and double it for the working solution. But I don't make up bottles and lable them pyrocat sol B, I just write down the % of the chems in the bottles. That way I don't have notebooks with notes flying around confusing me and getting lost.

Reading this now, I see that the way I've been labling is wrong. If I were to weigh out 100 grams of something and the total volume with water came to 135 ml, I would lable that as a 74% solution. That way I know that 10ml would have 7.4 grams in it. Looks like if that is really a 50% solution it would take all the fun shortcut out of everything.
Just as long as you have a consistent method, that is all that matters. I just label sol B and always make the solution the same way.

5. Originally Posted by Kirk Keyes

So your original instructions of 100 g K2CO3 dissolved into 100 ml water is actually a 50% solution (1 part + 1 part), and not a 75% solution - even though it has a concentration of about 0.75g/ml.
I think what you describe is a percent composition by mass.

What I describe as a 75% percent solution, i.e. 100 grams of potassium carbonate + 100 grams of water to make a total of about 135 total solution, appears consistent with the description of a percent solution weight/volume as described by Grant Haist in Modern Photographic Processing, Vol. 1, p. 334. “For Photographic purposes it is the practice to dissolve the percentage quantity in grams of a chemical in a volume of water less than 100 ml, then add sufficient water to make 100 ml.”

If we follow those directions and take 75 grams of potassium carbonate, and dissolve it in water to make a total of 100 ml of total solution, we will have a 75% weight/volume solution as described by Haist. Similarly, the 100 grams of carbonate dissolved in 100 ml of water to make a total of 135 ml of total solution gives the same weight/volume % solution.

I see this as a percent solution of mass unit per volume of solution, which appears to be a perfectly acceptable way of expressing concentration. Now, maybe I am misunderstanding Haist, but that is the convention that I am following and I think it is without question the convention that most people follow in calculating percent solutions for photographic applications.

Sandy

6. Originally Posted by sanking
I think what you describe is a percent composition by mass.

What I describe as a 75% percent solution, i.e. 100 grams of potassium carbonate + 100 grams of water to make a total of about 135 total solution, appears consistent with the description of a percent solution weight/volume as described by Grant Haist in Modern Photographic Processing, Vol. 1, p. 334. “For Photographic purposes it is the practice to dissolve the percentage quantity in grams of a chemical in a volume of water less than 100 ml, then add sufficient water to make 100 ml.”

If we follow those directions and take 75 grams of potassium carbonate, and dissolve it in water to make a total of 100 ml of total solution, we will have a 75% weight/volume solution as described by Haist.

Sandy
Yes, here lies the problem with percents. They have to be define, and that's why it's safer to use other units, as per g/l. But of course we bump into metric versus imperial system (again). And this is, as you know, a big can of worms :rolleyes:.

Marko

7. Originally Posted by marko_trebusak
Yes, here lies the problem with percents. They have to be define, and that's why it's safer to use other units, as per g/l. But of course we bump into metric versus imperial system (again). And this is, as you know, a big can of worms :rolleyes:.

Marko

Frankly I don't see the problem. Every endeavor of human activity has a specific vocabulary and lexicon usage that needs to be known and understood. Photography is no different, and in photography the mixing of photographic solutions as percentage solutions defined by w/v (weight/volume) and v/v (volume/volume) has been standard for decades. Consult virtually any reference on mixing photographic chemistry and you will find this to be the case.

Although there are some areas of confusion that need greater clarification, such as the issue of 1:1:100 as opposed to 1+1+98 on the whole the preparation of percentage solutions is simple, easy and unambiguous. There is really no need to make the matter more complex than it really is by introducing alternative measuring systems that may incorporate specific gravity of the solute and solvent or molar weights, unless in fact the possible greater precision offers some practical advantage.

Sandy

8. One question for Sandy; what does the sodium metabisulfite do? One person mentioned it was basically the same as sulfite, but I think it differs. I know you had used metol as a variation in your formula; any comment on the difference between phenidone and metol in the P-Cat formula? Thanks,

John

9. Originally Posted by John Z.
One question for Sandy; what does the sodium metabisulfite do? One person mentioned it was basically the same as sulfite, but I think it differs. I know you had used metol as a variation in your formula; any comment on the difference between phenidone and metol in the P-Cat formula? Thanks,

John
Two things. First, sodium metabisulfite provides a small amount of sulfite that is important in the synergism of the reducers. If you were to leave it out altogether the developer would not be as energetic. You can actually add more than I recommend for more energy still, but you must be very careful as too much more kills the stain completely.

Second, sodium metabisulfite is acidic and helps in the preservation of the stock solution. If you were to mix a Pyrocat-HD stock solution in propylene glycol, as many of us are doing these days, you would not need further preservation so you could substitute regular sulfite for the metabisulfite in approximately the same amount. There is a more precise ratio that provides equivalent amount but I can not remember it right now and I don't think it will have any practical impact anyway since the difference is very small.

Metol can be substituted for phenidone in the Pyrocat-HD formula at a ratio of about 10:1 but there will be some differences in the results. First, to produce an equal energy level you will need to use a stronger dilution, say 1.5+1.5+ 97 with the metol version instead of 1+1+98 with phenidone. Second, there is a slight loss in EFS with the metol version, even when the working strength is adjusted for the same energy. And third, the metol variant gives slightly greater sharpness with continuous agitation, as in constant agitation with Jobo, because metol, though very super-additive with pyrocatechin, lacks the regenerative qualities of phenidone, and this fact makes the formation of adjacency effects slightly more likely with metol when using this type of agitation.

Sandy

10. I would suggest that being more precise in our use of terms will benefit everyone in the long term. Sure, Kodak appears to have internally standardized on certain conventions, but like the notation Kodak uses for making dilutions, if they are not consistently used by everyone in the field, then we have problems.

I'm gald Sandy mentioned two different types of percent solutions, % w/v, and % w/w. It doesn't stop there - there are actually 4 types, the two just mentioned, as well as % v/v and % v/w. Granted that last one, % v/w doesn't seem to get used that much, but the other one, % v/v is often used for mixing liquids as Sandy went on to say.

The problem is how does someone reading 75% solution ever know which one of those 4 sets of units someone else is reffereing to? They can't, without guessing. And maybe they guess wrong...

Let's take a commercially available ammonium thiosulfate solution - it's 60% ammonium thiosulfate. It's a material that is used in the photography industry for rapid fixers. So which is it, 60% w/v or 60% w/w? Or one of the other two possibilities? You just can tell with the information that's presented here. Well, it is actually 60% w/w. (See the MSDS, and it says percent by weight.)

So here's an example of a solution that is commonly used in the photographic industry and it is not a %w/v. So I don't think that we can always assume, that because Haist said this is how we should calculate % solutions, we can always assume it to be calculated that way throughout the entire field.

Sandy claims that "on the whole the preparation of percentage solutions is both easy and simple." Well, it only is when you know which set of units you are using.

So all I am suggesting is that when using percent solutions, one notate the type of percent solution that is being discussed. So for Sandy's Pyrocat Part B solution, it could be notated as 75% w/v. Or, if you were preparing that solution as the CRC Handbook notates it, then it would be notated as 50% w/w. Sure, it's the same solution, but we are being very explicit about how we are specifing the concentration of it. It makes it really easy to figure out how to prepare it that way too. And it only took 3 extra characters (w/w or w/v) to make it very clear to everyone.

"There is really no need to make the matter more complex than it really is by introducing alternative measuring systems that may incorporate specific gravity of the solute and solvent or molar weights unless in fact these systems offers some practical advantage."

My recommendation in no way makes this subject more complex. It actualy simplifies the subject by removing all ambiguity from the notation system. Nowhere have I suggested using specific gravity or molar weights, it's still the same system of notation, just a lot more precise by adding 3 extra characters to units.

Kirk

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