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
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 LygaThat 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
Volumes change when mixing chemicals and sometimes the change can be very large and either positive or negative.
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.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
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
@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.
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, that is exactly what I was trying to do.
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.
just to be picky, there is no such thing as 'mls'. it's simply 'ml'; there are no plurals in ISO dimensions.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".
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.
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.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:
just to be picky, there is no such thing as 'mls'. it's simply 'ml'; there are no plurals in ISO dimensions.
FWIW, I mixed Pyrocat-HDC and got scorching hot negs with the times I used for HD.
I will give that a try, thanks.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.
Don't know about molarity, but it can be a mess.Ah, what is the molarity of 10 g/l of gelatin?
PE
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