Accurate chemical measurements

[ParkerSmithPhoto]

I recently made a 2% solution of phenidone in glycol (2g phenidone in 100mL) to make it easier to measure small amounts for Pyrocat-HD. I assume that I could do the same thing with ascorbic acid without any undue effects. Is this correct?

 

[AgX]

In case you want to make a stock solution: Ascorbic Acid solution is not very stable, first it gets oxidized, later polymerisatiion takes place.

 

[Photo Engineer]

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

 

[Raghu Kuvempunagar]

That is not a 2% solution!

PE, if OP's objective is, say, to be able to use 10ml of concentrate as the equivalent of 0.2g of phenidone, then he should dissolve 2g of Phenidone in 50ml of hot glycol, wait till it has cooled down to room temperature and bring up the volume to 100ml by adding more glycol at room temperature. Wouldn't this work well? Or is it always best to make 2% wt/wt solution and weigh out 10g of the concentrate?

 

[AgX]

Ron, the problem here is that the definitions/designations are utmost confusing....

see here:

We thus got 3 different kinds of concentrations.

Then there is the term "Mass-Concentration". Guess to what equation the average person would relate that term term to? Likely equation (i).
However it is equation (iii) ...


To make it even worse: I just got a german textbook in hand "Calculations in Chemistry". It got 9 pages on concntrations alone, but does not even contain equation (i) ! But lists only (ii) and (iii).

 

[ParkerSmithPhoto]

PE, if OP's objective is, say, to be able to use 10ml of concentrate as the equivalent of 0.2g of phenidone...

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

 

[wyofilm]

In nearly every molecular biologybiochemistry lab or even chemistry lab I worked in or directed, wt/vol solutions were by far the most common scenario. However, as PE correctly points out volume changes can be unpredictable, especially when the solute amount is high relative to the solvent amount.

Stock solutions are great for convenience, but also when weighing out small amounts is impracticable or likely to lead to unacceptably large errors. In the latter case, one might make a stock solution even if the shelf life is short. This will lead to waste of reagent, but sometimes it is necessary.

 

[Vaughn]

I am assuming that the extra water of the stock solution is compensated for when "bringing the total volume up to...xxx" of the final developer solution.

 

[wyofilm]

I am assuming that the extra water of the stock solution is compensated for when "bringing the total volume up to...xxx" of the final developer solution.

Yes.

 

[Mainecoonmaniac]

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

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".

 

[AgX]

"Add Water to make 100ml" , "mix 1/5" or "mix 1+4" volume parts is easy to understand.
But already at "make an X% solution" the ambiguity starts.

And "mix x/y" seems easy to understan, but is ambiguous nonetheless, and best avoided.

 

[Mainecoonmaniac]

"Add Water to make 100ml" , "mix 1/5" or "mix 1+4" volume parts is easy to understand.
But already at "make an X% solution" the ambiguity starts.

Yep. I ran into this many times. If there are expensive compounds, it could get expensive fast.

 

[Vaughn]

...If there are expensive compounds, it could get expensive fast.

Platinum and palladium salts get measured by the drop! EDTA and Citric acid for the clearing baths get measured in heaping tablespoons per liter.

 

[Raghu Kuvempunagar]

I assume that I could do the same thing with ascorbic acid without any undue effects.

Patrick Gainer was a big advocate of Vitamin C developers and this is what he had to say on this subject :

A 10% solution of ascorbic acid in propylene or ethylene glycol can be made and has quite a long life until water is added.

 

[Rudeofus]

Since precise scales are universally available, I recommend weight/weight concentrates over weight/volume concentrates. While it is quite simple to measure 2g with high accuracy, the 100ml volume will be inaccurate to begin with. If you then measure out 5ml to get your 0.1 g Phenidone, these 5ml will be inaccurate again. In the end you may be more inaccurate than if you would have measured the 0.1g of Phenidone directly. And you would be much more accurate with 2g Phenidone and 98 g (yes, grams, not milliliters) solvent. When you then need 0.1 g Phenidone, you'd put a suitable beaker onto your scale, and fill it with precisely 5.0g of solution.

 

[AgX]

Since precise scales are universally available,...

However one should not be fooled into believing a precision that there actually may not be, in general or at that very metering.
https://www.photrio.com/forum/threads/the-inaccuracy-of-digital-meters.166690/

 

[wyofilm]

Since precise scales are universally available, I recommend weight/weight concentrates over weight/volume concentrates. While it is quite simple to measure 2g with high accuracy, the 100ml volume will be inaccurate to begin with. If you then measure out 5ml to get your 0.1 g Phenidone, these 5ml will be inaccurate again. In the end you may be more inaccurate than if you would have measured the 0.1g of Phenidone directly. And you would be much more accurate with 2g Phenidone and 98 g (yes, grams, not milliliters) solvent. When you then need 0.1 g Phenidone, you'd put a suitable beaker onto your scale, and fill it with precisely 5.0g of solution.

With the exceptions of analytical labs working to high levels of precision for specific reasons measuring out 100 in the example above with the appropriate graduated cylinder is the way to go. Here is an extreme example where if I were drunk I might measure out 5 ml too much or 5 ml too little with a graduated cylinder.

Dissolve 2.0g (carefully weighed out before drinking) phenidone in 100ml, 95ml and 105ml of water to yield solutions of 2.0%, 2.1%, and 1.9% phenidone, respectively. Now when sober with no hangover, carefully weigh out 5ml of the above solutions to get 0.10g, 0.11g, and 0.10g of phenidone, respectively. A negligible error in whatever development you are going to do. Of course, this is with a gross error of +/- 5ml water for illustrative purposes. With the appropriate graduated cylinder you can easily and quickly deliver the correct amount of phenidone to less than 10mg.

Since I don't drink, I feel comfortable with this method. Carefully weighing out the phenidone is important. Errors here will matter.

Now this how I would do the example above, but everyone needs to find his or her own comfort level.

 

[AgX]

Maybe there is not enough emphasis in general on acquiring the right lab stuff when making up a darkroom. Thus not only a few big beakers but also one higher-precision thin, tall graduate.

 

[wyofilm]

Maybe there is not enough emphasis in general on acquiring the right lab stuff when making up a darkroom. Thus not only a few big beakers but also one higher-precision thin, tall graduate.

You might be right, I don't know how others set up their darkrooms. I would not recommend using beakers to measure out critical or even semi-critical volumes. A set of graduated cylinders that covers a range of volumes is useful. In the example above one would want to use a 100ml graduated cylinder and not a 500 ml cylinder, for example.

 

[ParkerSmithPhoto]

Thanks for all the help. I do have three glass graduated cylinders. Pyrocat has definitely got me thinking with precision!

 

[Mainecoonmaniac]

Platinum and palladium salts get measured by the drop! EDTA and Citric acid for the clearing baths get measured in heaping tablespoons per liter.

I've got platinum taste on a citric acid budget. The cheaper cousin Palladium chloride is $50/gr.

 

[Rudeofus]

That graduated glass cylinder is your friend as long as the ingredient dissolves very quickly in the solvent. This is not the case with Phenidone and Propylene Glycol, Diethylene Glycol or Glycerol. Little me put a brown glass bottle onto a scale, weighed 98 g Propylene Glycol and 2.0 g Phenidone, then tightly capped the bottle . That bottle then sat in a cabinet, and the Phenidone was completely dissolved after a week. Little impatient me was also able to recruit a heat blower to get the Phenidone into solution within an hour, but neither method would have been feasible with an open container like the suggested tall glass cylinder.

 

[Sirius Glass]

I have a 10ml graduated cylinder that I use for measuring 5ml Pyrocat HC in Glycol. After each solution I rinse out the cylinder with water and put that water in the water measuring cylinder with the measured solution.

I bought it from Photographers' Formulary.

 

[wyofilm]

That graduated glass cylinder is your friend as long as the ingredient dissolves very quickly in the solvent. This is not the case with Phenidone and Propylene Glycol, Diethylene Glycol or Glycerol. Little me put a brown glass bottle onto a scale, weighed 98 g Propylene Glycol and 2.0 g Phenidone, then tightly capped the bottle . That bottle then sat in a cabinet, and the Phenidone was completely dissolved after a week. Little impatient me was also able to recruit a heat blower to get the Phenidone into solution within an hour, but neither method would have been feasible with an open container like the suggested tall glass cylinder.

Who said I was dissolving the phenidone in the grad cylinder? I used it to measure out the the volume of solvent. That small portion (2.0g) of phenidone will not change the volume by any measurable amount. Now if I were back in my academic lab I would have taken a square milk dilution bottle with a single 99ml graduation, dumped in the phenidone and brought the vol. up to 99 ml. Like I said, everyone needs to find their comfort zone. I simply illustrated another way doing things, and a rough analysis of the possible error involved.

Now that I have finished my spiel, I will say that since you are using viscous liquids there is a stronger case to be made for weighing the liquid.

 

[Vaughn]

I've got platinum taste on a citric acid budget. The cheaper cousin Palladium chloride is $50/gr.

My mix is two parts palladium to 1 part platinum -- so technically I suppose I should call them palladium/platinum prints rather than the other around...

Occasionally I make Palladium prints with Na2 to increase contrast. The Na2 is as costly as regular pt, but I use very little per print...a drop.