Accurate chemical measurements

[David Lyga]

That is not a 2% solution! A 2% solution is made by adding 2g compound to 90 g of solvent and bringing up to 100 g to make a 2% wt/wt solution. If you bring it to 100 ml it is then a wt/vol solution.

Volumes change when mixing chemicals and sometimes the change can be very large and either positive or negative.

PE

I am a bit confused, here. Theoretically, PE is correct, but, in the case of water, mL = g. Thus 2g of a powder chemical HAS to make up a total of 100mL of solution. What is the alternative? What am I missing? - David Lyga

 

[Vaughn]

You might have missed PE's statement

Volumes change when mixing chemicals and sometimes the change can be very large and either positive or negative.

Adding 20 grams of a chemical to 100ml of water might yield 110 ml of solution. A different scenerio than adding 20 grams to 75 ml and after mixing, adding water to make 100 ml. The former is 20grs in 110ml of solution and the latter, 20 grs in 100 ml of solution.

For making tissue for carbon printing, I add 90 grams of gelatin to 750 ml of water and end up with about 875 ml of gelatin solution (60 grams of white sugar and 5 grams of pigment is also added). For simplicity's sake, I have called this a 12% gelatin solution (90/750 = 12%), though I suppose with the final volume of 875ml, it could be called 10% (90/875). As long as the people one is communicating with understand, its cool.

 

[Sirius Glass]

What we are talking about is good chemistry pouring and mixing practices. It occurs to me that I have never seen an article or book describing how to measure, pour, mix, ... any chemicals and photochemicals in particular. I have had to learn it on the street or by watching others. Any recommendations or pointers? This would make a good thread with a pin to hold it in view.

 

[Photo Engineer]

To add insult to injury, 1 ml of water is only 1 g at 20C, not at any other temperature, and this is only true of distilled water.

As for such a book on mixing, it is probably a futile effort. People will do what they want. This is not the first time this has come up here.

PE

 

[Sirius Glass]

I do not care about people who do what they want. It is about time I reviewed and or learned good chemistry practices.

 

[Rudeofus]

I am a bit confused, here. Theoretically, PE is correct, but, in the case of water, mL = g. Thus 2g of a powder chemical HAS to make up a total of 100mL of solution. What is the alternative? What am I missing? - David Lyga

Concentration of an ingredient in a liquid can be given in different percentages. There is weight per volume, where 10% NaOH means that 100ml solution contain 10 g NaOH. Then there is weight per weight, where 60% Ammonium Thiosulfate mean 60g Ammonium Thiosulfate and 40g water are mixed together to give a total of 100g of liquid. Since the ingredient dissolved in water changes its density, and since some compounds contract the solvent (meaning you add compound to 100ml water and get less than 100ml solution) whereas others expand it (meaning you add compound to 100ml water and get more than 100ml solution), that "water weighs 1g/ml" number won't help much.

@wyofilm : The reason I brought up Phenidone in Propylene Glycol is because that's what the original question revolved about. It's a prime example where weight/weight concentrates make matters a whole lot easier.

 

[Mainecoonmaniac]

What helps with precision is mixing in bulk. The percentage of error is much less as you mix a larger amount. Mixing 100ml with precious metals is a lot of pressure for me. I calculate the amount and read the recipe at least twice.

 

[wyofilm]

To add insult to injury, 1 ml of water is only 1 g at 20C, not at any other temperature, and this is only true of distilled water.

As for such a book on mixing, it is probably a futile effort. People will do what they want. This is not the first time this has come up here.

PE

It is fine to bring up the fact that 1ml of water is only 1.00g at 20C, but it is a bit of a disservice to suggest that the change is meaningful to the work here. For example, at 100C the weight of water per ml drops to 0.982, a loss of 1.8%. A largely meaningless error at a ridiculous extreme. When I worked in an analytical chemistry lab we even applied a buoyancy correction when weighing compounds out. You are doing this aren't you?

People will do what they want, but some realism as to where error matters is important. There is always error. If I were to beat the drum about wt/wt vs. wt/vol without basis of laboratory sense, someone out there might start weighing out water to make a batch of clearing agent. A useless endeavor for sure. Scoops from a kitchen tablespoon into a bucket would be close enough.

For sure other times, a greater degree of care is needed. Discussions like this thread happened with each new student that joined my laboratory. If the student understood when extreme care was required vs. when close enough was good enough it meant that he or she probably understood the experiment.

I'm not trying to pick a fight here, but it is helpful to understand error, control, and troubleshooting. If someone fails to see any film development it wasn't because they were 5, 10 or 20% off in the amount of phenidone present. The problem was more fundamental. Worrying about the density of water at STP wouldn't help the troubleshooting process. Understanding error means understanding the experiment.

 

[wyofilm]

@wyofilm : The reason I brought up Phenidone in Propylene Glycol is because that's what the original question revolved about. It's a prime example where weight/weight concentrates make matters a whole lot easier.

You are absolutely right in this example. However, the thread was more trending more about wt/wt vs wt/vol in general. It was to this comparison I was focussed on. Even the first sentence of your first thread takes the position of always wt/wt. It is this point of view that disagree with. I take the position of pragmatism. Sometimes one way is preferred to the other. At what volume do you stop water out solvent (especially water) and use volume instead? I'm pretty sure you aren't doing everything wt/wt.

 

[Photo Engineer]

It is fine to bring up the fact that 1ml of water is only 1.00g at 20C, but it is a bit of a disservice to suggest that the change is meaningful to the work here. For example, at 100C the weight of water per ml drops to 0.982, a loss of 1.8%. A largely meaningless error at a ridiculous extreme. When I worked in an analytical chemistry lab we even applied a buoyancy correction when weighing compounds out. You are doing this aren't you?

People will do what they want, but some realism as to where error matters is important. There is always error. If I were to beat the drum about wt/wt vs. wt/vol without basis of laboratory sense, someone out there might start weighing out water to make a batch of clearing agent. A useless endeavor for sure. Scoops from a kitchen tablespoon into a bucket would be close enough.

For sure other times, a greater degree of care is needed. Discussions like this thread happened with each new student that joined my laboratory. If the student understood when extreme care was required vs. when close enough was good enough it meant that he or she probably understood the experiment.

I'm not trying to pick a fight here, but it is helpful to understand error, control, and troubleshooting. If someone fails to see any film development it wasn't because they were 5, 10 or 20% off in the amount of phenidone present. The problem was more fundamental. Worrying about the density of water at STP wouldn't help the troubleshooting process. Understanding error means understanding the experiment.

Yes, you are right, but then this becomes a valuable lesson if you understand the problems associated with exothermix and endothermic additions. It was just an exemplary lesson. Nothing more.

PE

 

[Raghu Kuvempunagar]

Yes, that is exactly what I was trying to do.

While this thread has been very enlightening on the tricky issue of measurement of chemicals, the following observation by Sandy King might be helpful to OP:

The amount of phenidone in the Pyrocat-HD formula is not as critical as one might think. In my own tests I have seen very little difference with amounts ranging from 1.5 g per liter to 2.5 g per liter of Stock Solution A. The minimum amount appears to be about 1.0 g, and the maximum about 3.0. Below 1.0 g there is not as much synergism (= longer development times) and above 3.0 g there is an increase in B+F.

 

[RalphLambrecht]

So it's the final volume of 100mls that you're after. If that's the case, the recipe should say "Water to make 100 mls".

just to be picky, there is no such thing as 'mls'. it's simply 'ml'; there are no plurals in ISO dimensions.

 

[AgX]

What we are talking about is good chemistry pouring and mixing practices. It occurs to me that I have never seen an article or book describing how to measure, pour, mix, ... any chemicals and photochemicals in particular.

Good point. If I remember well, at school we only learned no to add water to a strong solid alkaly but doing it the other way...

I got the popular east-german textbook "Photographic Chemistry" at hand. I offers about 11 pages on general lab practice. However it does not hint at concentrations in the sense we are discussing here. It is not needed as either recipes are given or volume-mixing instructions of A+B. (Not the ambiguous A/B as typical for Kodak.)

 

[ParkerSmithPhoto]

While this thread has been very enlightening on the tricky issue of measurement of chemicals, the following observation by Sandy King might be helpful to OP:

Yes that was very helpful. I was convinced an error in measuring phenidone would be very critical but am glad to see that it is not.

FWIW, I mixed Pyrocat-HDC and got scorching hot negs with the times I used for HD. I thought the phenidone was the problem, and that I had somehow gotten a significant error when measuring. I did mix the % solutions as discussed here and made a new batch of HDC, and...got scorching hot negs, even with a 25% weaker dilution. (insert sound of towel being tossed into the ring)

So, HD it is! Or, that metol version looks nice...maybe I should follow that rabbit down the hole...

 

[john_s]

just to be picky, there is no such thing as 'mls'. it's simply 'ml'; there are no plurals in ISO dimensions.

And, to avoid the confusion in our alphabet between i and I and l and 1, I understand that the much clearer mL is permitted even though it contravenes a rule that says that capital letters can be used only for units named after individuals, such as P for Pascal, N for Newton and so forth.

 

[Raghu Kuvempunagar]

FWIW, I mixed Pyrocat-HDC and got scorching hot negs with the times I used for HD.

Times for HDC are not different from those for HD.

Before abandoning HDC in favour of another flavour of Pyrocat, I would add a gram of sodium bicarbonate to HDC working solution to reduce the pH a little and tame the contrast.

 

[ParkerSmithPhoto]

Times for HDC are not different from those for HD.

Before abandoning HDC in favour of another flavour of Pyrocat, I would add a gram of sodium bicarbonate to HDC working solution to reduce the pH a little and tame the contrast.

I will give that a try, thanks.

 

[GLS]

This whole discussion perfectly highlights why chemists use molarity when accurate concentrations of things are required. With % soutions the confusion between which "%" is being discussed (i.e. wt/wt, wt/vol or vol/vol, and often it wont be defined in written procedures) as well as how they are calculated can all too easily introduce errors when precision is called for.

 

[Photo Engineer]

Ah, what is the molarity of 10 g/l of gelatin?

PE

 

[GLS]

True, polymers are a special case. Of course, the resulting mixture in your example isn't technically a solution but a gel, but that's another discussion entirely

 

[AgX]

As I indicated the trouble already starts with something as a/b mixing instructions. We had several threads on this with members interpretating such differently.
(That is why a+b is the better way to put it.)

 

[Vaughn]

Ah, what is the molarity of 10 g/l of gelatin?

PE

Don't know about molarity, but it can be a mess.

 

[Photo Engineer]

Yep!

 

[AgX]

And there also is molality...

 

[GLS]

Molality (i.e. mass (moles) of a solute per mass of solvent) is not generally used outside of situations where extreme precision is necessary. In day to day chemistry, the typical dilutions and reagent stoichiometry used in reactions renders the slight variances in volume (due to user error or changes in temperature & pressure) insignificant. Molarity is therefore used as it is fit for purpose and operationally simpler to use than molality (especially in mixtures containing more than one solvent).