Making solutions from powder - how do you measure weight against volume?

[jeztastic]

A simple question and I'm sure the answer will be simple. I often see advice to make a solution up at a percentage or ratio of weight of solid to volume of liquid. Does one milligram equate to one millilitre? So say a 5% solution of potassium ferricyanide. Would that be 50 milligrams of chemical in 1 litre of water?

Thanks,

Jez

 

[AgX]

It would be 50 grams filled up with water to make 1kg.

Though many recipes have enough tolerance (or are worded wrongly from the start) to put in this case 50grams in 1Liter of water.

 

[jeztastic]

Thanks

It would be 50 grams filled up with water to make 1kg.

Though many recipes have enough tolerance (or are worded wrongly from the start) to put in this case 50grams in 1Liter of water.

Thanks.

 

[pdeeh]

the normal advice is to start with c. two-thirds of the final volume of water, dissolve the full weight of chemical required in that, then top up to the final volume.

in the case of a 5% ferricyanide solution, it wouldn't matter too much if you just started with a litre and added 50g.(well it might easily matter in a lab, but probably not in your darkroom)


but with other chemicals and stronger solutions, it would definitely matter because of the "water of crystallisation" those chemicals contain (e.g. sodium carbonate decahydrate) for instance

 

[Gerald C Koch]

People get into trouble when they try to make these calculations in their head and/or do not include the units in their calculation. Remember that units are important. If the units of the answer do not agree with the units in the equation this is a tipoff that things are not right. For a 5% solution

(total volume) X (percentage expressed as a decimal fraction) = (amount to add)

1 l = 1000 g X 0.05 = 50 g

The above example is pretty simple but other chemical calculations are not.

 

[darkroommike]

Percentage solutions can either be made by weight/weight or by weight/volume.

weight per weight are true percentage solutions weight/volume dilutions are close enough for all practical (read that as photographic) purposes when the solvent is water and the solute is a small weight.

 

[jeztastic]

Got it, I think. I seem to remember from GCSE chemistry that a mililitre of water weighs a gram, which bears out what you are saying here.

 

[darkroommike]

Got it, I think. I seem to remember from GCSE chemistry that a mililitre of water weighs a gram, which bears out what you are saying here.

And it displaces one cubic centimeter.

 

[Photo Engineer]

But, as you add materials to water, it expands or contracts. This is confusing to many.

You can make g/g or what is called weight / weight solutions to make up something or you can use g/v or weight / volume. The former says weight out 5 g of x and add to water to make 100 g of total solution and this is w/w. OTOH, g/v suggests using 5 g of x and adding water to make 100 ml of solution. This is a very subtle difference that becomes greater as percentage goes up or if the reaction generates or absorbs heat.

Then there is the school that uses g/g in this sense, 5 g of x and 100 g of water. This uses a method called "molal" which varies from compound to compound and really is not good.

If you measure viscous liquids such as HC110, you should use g/g, another method. In this you take 5 g of HC110 and add water up to 100 g as one example.

And 1 cc of water is 1 gram of water only at 20C.

PE

 

[mshchem]

But, as you add materials to water, it expands or contracts. This is confusing to many.

You can make g/g or what is called weight / weight solutions to make up something or you can use g/v or weight / volume. The former says weight out 5 g of x and add to water to make 100 g of total solution and this is w/w. OTOH, g/v suggests using 5 g of x and adding water to make 100 ml of solution. This is a very subtle difference that becomes greater as percentage goes up or if the reaction generates or absorbs heat.

Then there is the school that uses g/g in this sense, 5 g of x and 100 g of water. This uses a method called "molal" which varies from compound to compound and really is not good.

If you measure viscous liquids such as HC110, you should use v/v, another method. In this you take 5 ml of HC110 and add water up to 100 ml as one example.

And 1 cc of water is 1 gram of water only at 20C.

PE

The great example of add 50ml of water to 50ml of alcohol you end up with 95ml. I will leave it to PE to explain the whole hydrogen bonding , shielding thing .

My advice is, you know all the millions of dataguides and technical bulletins Kodak printed . FOLLOW THE RECIPE .
I love my dataguides the really choice stuff was printed in the 40's and 50's , plus the commercial photography that was used to illustrate these books is stunning
Best Regards Mike

 

[jeztastic]

But, as you add materials to water, it expands or contracts. This is confusing to many.

You can make g/g or what is called weight / weight solutions to make up something or you can use g/v or weight / volume. The former says weight out 5 g of x and add to water to make 100 g of total solution and this is w/w. OTOH, g/v suggests using 5 g of x and adding water to make 100 ml of solution. This is a very subtle difference that becomes greater as percentage goes up or if the reaction generates or absorbs heat.

Then there is the school that uses g/g in this sense, 5 g of x and 100 g of water. This uses a method called "molal" which varies from compound to compound and really is not good.

If you measure viscous liquids such as HC110, you should use v/v, another method. In this you take 5 ml of HC110 and add water up to 100 ml as one example.

And 1 cc of water is 1 gram of water only at 20C.

PE

Very interesting. Thanks to all who have replied. So this is why development times always assume 20C.

And mixing up the Ilford powdered developers (I've only ever used Ilford, living in the UK), the instructions on the pack are to add water up to 1 litre of volume at 40C. Am I correct in thinking this is the g/v method, and that the higher temperature is to speed up the dissolving? Presumably also the g/g method would not be optimal since the temperature change would affect the weight, although presumably also the difference would be negligible...

 

[darkroommike]

Very interesting. Thanks to all who have replied. So this is why development times always assume 20C.

And mixing up the Ilford powdered developers (I've only ever used Ilford, living in the UK), the instructions on the pack are to add water up to 1 litre of volume at 40C. Am I correct in thinking this is the g/v method, and that the higher temperature is to speed up the dissolving? Presumably also the g/g method would not be optimal since the temperature change would affect the weight, although presumably also the difference would be negligible...

Follow the instructions packaged with the developer or whatever.
20C is pretty close to the average room temperature, although I have seen some old formulas using 65F (about 18C) back in the early days of photography before central heat. Processing at room temp simplifies temperature control.

 

[Photo Engineer]

I have edited my egregious error in that last post. For viscous liquids use wt/wt due to the fact that a viscous liquid cannot pour easily! You cannot get it all out of the measuring container.

My apologies for this terrible error. It must have been the hour. IDK.

PE

 

[jeztastic]

I have edited my egregious error in that last post. For viscous liquids use wt/wt due to the fact that a viscous liquid cannot pour easily! You cannot get it all out of the measuring container.

My apologies for this terrible error. It must have been the hour. IDK.

PE

Nearly lost all respect for you there PE! I take it wt/wt means weight/weight? Would you really advocate weighing out hc110 rather than using a syringe and rinsing it out for example?

 

[Photo Engineer]

A syringe which can be evacuated totally, due to its construction, is fine as long as there is no residue left in the tip of any significant amount. Many syringes are made to compensate for the tip residue, but it often does not consider viscous or dense materials. I know we weight emulsions out and we weighed out practically all viscous solutions.

Wt/wt is almost exactly the same as wt/vol as long as water is being used and the temperature is 20C.

PE

 

[RalphLambrecht]

A simple question and I'm sure the answer will be simple. I often see advice to make a solution up at a percentage or ratio of weight of solid to volume of liquid. Does one milligram equate to one millilitre? So say a 5% solution of potassium ferricyanide. Would that be 50 milligrams of chemical in 1 litre of water?

Thanks,

Jez

that's how I use it 1mg =1 ml

 

[AgX]

A syringe which can be evacuated totally, due to its construction, is fine as long as there is no residue left in the tip of any significant amount. Many syringes are made to compensate for the tip residue, but it often does not consider viscous or dense materials.

Syringes are made for medical use and that typically is water-based solutions of high surface tension. Thus a syringe tip will be filled completely.
The same with a viscous solution. Difference may only be on the form of stem-tip.

I do not see a problem with measuring viscous solution in a syringe. Did I overlook something? I did no test on this matter.

 

[Photo Engineer]

There are veterinary syringes such as Monoject, which I use, and which have no marks but merely indicate total capacity. The tip is a permanent part and retains quite a bit of solution. You have to be careful.

PE

 

[Craig]

For viscous liquids use wt/wt due to the fact that a viscous liquid cannot pour easily! You cannot get it all out of the measuring container.

What I have done for measuring small amounts of a viscous concentrate, such as Ilfotec HC or T Max developer is to use something like a graduated cylinder to measure what I need, then rinse it out with the make up water until it's clean. I'll usually dump the concentrate and rinse water into a beaker to mix it up to final volume. Although a positive displacement device like a syringe is much easier.

 

[Photo Engineer]

Craig, good idea, but try doing that in the dark, in an emergency.

PE