Mixing solids with liquids to make a percentage solution.

Megrez · May 26, 2011

I apologies for asking a tedious question, but I would really appreciate help with this.

How do I make a 10% solution of sodium thiosulphate crystals with 1 litre of distilled water, i.e. mixing solids with liquids to make a percentage solution.

Many thanks in anticipation,

Gary

Marek Warunkiewicz · May 26, 2011

Measure 100 grams of any chemical in 1,000 ml gives you ten percent. For a 12.5% solution in 1,000 ml (one litre) you would use 125 grams, for a 1.25% solution you would use 12.5 grams and so on. One gram in 100 ml is a 1% solution, 10 grams in 1,000 ml is also a 1% solution.

Marek

Tony Egan · May 26, 2011

Not such a tedious question as I have read of three different ways:
1. As above: 100g +1000ml (1ml weighs 1g)
2. Add 100g to 900ml of water only
3. Dissolve 100g in less than 1000ml and when dissolved top up to = 1000ml
Not sure there is much difference between 1 and 3 but 2 might give different results if a high level of precision is required. I guess the question is does the mass of chemical make any difference to the volume of liquid once dissolved in solution. Chemists will weigh in I'm sure. I invariably use method 1.

Doremus Scudder · May 26, 2011

Tony has a valid point, which was not originally addressed.

When making a solution of dry chemicals in water, the chemicals usually (not always) take up some space of their own, increasing the total volume. In order to be accurate, one needs to start with less water than the total desired volume, add the dry chemical and then, after it has dissolved, top up the solution to the desired volume with water (method 3 above).

Example: Making a 10% solution of sodium thiosulfate:
Start with 750ml or so of water.
Add 100g of dry chemical while stirring and dissolve completely
(add your sodium bicarbonate here: 2g)
Add water to the solution to make 1 liter

This is the most accurate. For less critical solutions or when just adding a small amount of chemical to water, it is often the practice to simply add the dry (or liquid) chemical to the total amount of water, thus increasing the volume by a small amount. I mix wash aid this way, i.e., 1 liter water, 1 Tablespoon of sodium sulfite and a pinch of sodium metabisulfite. This makes a tad bit more than one liter, but the proportions are not all that critical, and this saves a bit of time. For making stock solutions of known content, I always use the first method.

Best,

Doremus Scudder
www.DoremusScudder.com

jochen · May 26, 2011

Hello,
the method Doremus wrote is the most convenient to work with percentage solutions because you can measure parts of an amount of substance simply with a graduated cylinder without a balance.
But in some cases you find weight percent, than you have to take 100 g of substance and fill up to a total of 1000 g with water to have a 10 % (m/m) solution. (m/m) means mass/mass. For example the commercial available 60 % Ammoniumthiosulfate solution is m/m. For measuring you need a balance or you have to know the specific gravity.

Megrez · May 26, 2011

Thank you everyone for you replies. I really do appreciate the time you took and, of course, your answers. Please accept my apologies for not being clearer.

I am trying to make sense of a recipe for making fixer for salt printing:

"After the initial rinse salt prints must be thoroughly fixed. Be sure to use fresh fixer. I use a 10% solution of sodium thiosulfate (hypo) adding 2 grams of sodium bicarbonate to each liter of fixer."

Does this mean I weigh out 100g of sodium thiosulfate and add 900 ml of water, and then add 2g of sodium bicarbonate to the mixed 1 litre solution?

Doremus Scudder · May 26, 2011

Jochen,

I think you misunderstood what I was saying. My instructions are for dry chemicals.

Mergan,

Mix the 10% solution as per my instructions above, but add the sodium bicarbonate before topping the water up to 1 liter. It's really pretty basic... Don't add water to the chemicals, but add the dry chemicals to the water slowly while stirring.

Best,

Doremus Scudder
www.DoremusScudder.com

holmburgers · May 26, 2011

1mL = 1cc = 1 gram, for all intents and purposes.

Mix the dry chemicals into a volume that is somewhat less than your final desired volume. Fill to the desired volume.

Nicholas Lindan · May 26, 2011

This dilution method is properly "weight/volume" - it isn't really a percentage, but so many grams/liter of solution.

But the common parlance is to refer to it as a percentage - "10% thiosulfate" - when it is really 100gm thiosulfate/liter of solution and there isn't any "10%" of anything to anything to be found anywhere.

In US units it isn't uncommon to find 10% w/v as 1 oz avdp per 10 oz fluid = 28.35gm in 295.7cc which would be 96 gm/l (9.6%) in metric speak.

Then one really needs to specify which crystaline form of s. thiosulfate and s. bicarb.. S. Thiosulfate is commonly supplied as the pentahydrate with 5 water molecules per thiosulfate molecule.

Then if you want to make more of a hash of things one needs to specify the isotopic make-up of the chemical added to the water. Carbon can be up to 25% C13 which is 10% heavier than the standard C12.

To mitigate all this gratuitous pedantic confusion solutions made up by chemists are specified by molar concentration (molarity) which is the number of molecules of a substance per volume of solution. The number of molecules is specified in 'moles' where one 'mole' is 6.022 x 10^23, the average number of the small rodents found in Avogadro's back garden.

However, molarity is dependent on the temperature - as the volume of the solution changes with temperature.

The solution to this conundrum is molality - number of molecules per weight of solvent (water). However, as the solution is invariably measured out by volume the advantages (insert incredulous smiley face if needed) of using molality are not as great as they first appear (insert "Yeah, right! Tell me another one." smiley).

Photo Engineer · May 26, 2011

There are three ways to specify percentage of solution. They are w/v, w/w and v/v where w = weight and v = volume.

Assuming you are working at 20 deg C, then:

w/v is made by weighing out the solid, say 10 g, and then dissolving in 90 ml of solvent. Then this is made up to 100 ml to have a 10% w/v solution.

w/w is made by weighing out the solid, again 10 g, and dissolving it in 90 grams of solvent. This is made up to 100 grams to have a 10% w/w solution. This is the most accurate and useful for all conditions of mixing of solutions.

v/v is only important if both materials are liquids and the make up is obvious from the above.

All of these are used in chemistry and photography and all are important! They are especially important when the solvent is not water and/or is viscous. The method of mixing a solution using percentages should always be specified.

PE

holmburgers · May 26, 2011

Nicholas Lindan said:
This dilution method is properly "weight/volume" - it isn't really a percentage, but so many grams/liter of solution.

But the common parlance is to refer to it as a percentage - "10% thiosulfate" - when it is really 100gm thiosulfate/liter of solution and there isn't any "10%" of anything to anything to be found anywhere.

In US units it isn't uncommon to find 10% w/v as 1 oz avdp per 10 oz fluid = 28.35gm in 295.7cc which would be 96 gm/l (9.6%) in metric speak.

Then one really needs to specify which crystaline form of s. thiosulfate and s. bicarb.. S. Thiosulfate is commonly supplied as the pentahydrate with 5 water molecules per thiosulfate molecule.

Then if you want to make more of a hash of things one needs to specify the isotopic make-up of the chemical added to the water. Carbon can be up to 25% C13 which is 10% heavier than the standard C12.

To mitigate all this gratuitous pedantic confusion solutions made up by chemists are specified by molar concentration (molarity) which is the number of molecules of a substance per volume of solution. The number of molecules is specified in 'moles' where one 'mole' is 6.022 x 10^23, the average number of the small rodents found in Avogadro's back garden.

However, molarity is dependent on the temperature - as the volume of the solution changes with temperature.

The solution to this conundrum is molality - number of molecules per weight of solvent (water). However, as the solution is invariably measured out by volume the advantages (insert incredulous smiley face if needed) of using molality are not as great as they first appear (insert "Yeah, right! Tell me another one." smiley).

Sorry, but reading this makes me want to burn down my darkroom and go back to college.

Knowing that 1mL = 1 grams = 1cc is probably good enough for 98% of all circumstances.

Photo Engineer · May 26, 2011

The problem is that with dilute solutions and plain water 1 g = 1 ml (at 20 deg C) is a very good approximation, but when you go to 10% or 20% or higher concentrations, then 1 gram is NOT = 1 ml. With densities approaching 2.0, you can see that 1 g = 1/2 ml!!!!!!

And, it can go the other way, as 28% Ammonium Hydroxide has a density lower than 1.0. Organic solvents or mixtures of organic and inorganic chemicals give wildly different values of density so w/w often works best. Weight does not change with temperature or density and viscous solutions are easy to handle.

Working at 40 or 60 deg C, as we do in emulsion making, density is again way off due to the mix of heavy metals, gelatin and the other ingredients.

PE

Worker 11811 · May 26, 2011

Wouldn't a true 10% solution be based on the number of moles?

Water @ 18g/mole and thiosulphate @ 158.11g/mole would mean that 162ml of pure water would be used to dilute 158.1g of the chemical. That would mean that you would need 975g to make a liter of solution.

Or, would that mean that you need 15.8g to make a liter of 0.1M solution?

Photo Engineer · May 26, 2011

Randy;

Molarity would be used in your case as it is more convenient. In fact, Chemistry uses molarity as a "true" measure with percent being an arcane measure rarely but sometimes used. In photography, percent is used in amateur applications due to its supposed ease of use when compared to molarity. We never use percent in the labs except to express dilution of solutions that are hard to measure otherwise. We use g/l or moles / l in our work as it is more precise to chemists.

But, even though percent is thought to be easy to use, it often confuses people due to the ease of errors creeping in. And, you never know the "real" concentration in M/L which is often critical. There are some notable exceptions. A 4% solution of NaOH is 1 molar. It has 40 g/l of NaOH or 23 + 16 + 1 or 40 grams which is 1 mole.

PE

holmburgers · May 26, 2011

I just find it amusing that the OP asks how to make a peanut butter and jelly sandwich and all of a sudden we're telling him about George Washington Carver's childhood, the climate conditions necessary for raspberries and the finest conditions for yeast cultivation.

Not that there's anything wrong with that.... it's just funny.

It's an interesting discussion nonetheless

Worker 11811 · May 26, 2011

As some would say, we always use a helicopter when we just want to cross the street.

However, the OP's question had been answered. Why not ask questions to have a little fun? I might just accidentally learn something in the process!

Photo Engineer · May 26, 2011

Like GWCs childhood? Or all about peanuts in a nutshell?

PE

michaelbsc · May 27, 2011

I think that all the discussion results from the fact that "a 10% solution" isn't a very accurate specification. I know that's what the recipe said, so no offence to the original question. But I think a lot of the recipes we deal with are vaguely specified.

Perhaps, with the help of thousands of pairs of eyes bringing different cultural backgrounds to interpretation maybe we'll get better over time.

kwall · May 27, 2011

holmburgers said:
I just find it amusing that the OP asks how to make a peanut butter and jelly sandwich and all of a sudden we're telling him about George Washington Carver's childhood, the climate conditions necessary for raspberries and the finest conditions for yeast cultivation.

I was thinking the same thing (and having flashbacks to high school chemistry).

Q: "What time is it?"
A: "Well, you see, the earth goes around the sun..."

holmburgers · May 27, 2011

Hahaha.... it just goes to show you that nothing in photographic chemistry as easy as you want it to be. But if you're at least consistent in what you do with a % solution, you can adjust it depending your results.

I would assume the OP is making a simple fix solution (?) or a stock solution for some purpose, in which case it seems like he didn't know the that a gram of water will have a mL of volume, or a mL of water will weigh 1 gram. After I learned this "dumbed down" rule, I closed that chapter in my brian and put it back on the shelf.

Worker 11811 · May 27, 2011

Well, this brings up a question that I've had in my mind for a while.

Just how accurate do we need to be with our measurements?

According to the literature I read and, from what people like Photo Engineer say here, it would seem that we need to use scientific precision. It goes without saying since P.E. is a scientist. Even more so because his job is to make or design film and chemistry for commercial production. Scientific accuracy is essential.

However, many people I have known simply "eyeball" their measurements and say, "Close enough." A lot of those people produced work that is far better than I could.

So, it seems that there is a pretty wide margin between the published rules and practical, everyday procedure. How wide is that margin, really?

I took chemistry classes in high school and in college and I was taught to be as scientifically accurate as I could. To this day, I still try to be as accurate as possible with the tools that I have. When I work in the darkroom, I always try to measure right to the line. (Yes, I account for the meniscus.) Whatever measuring tool I use, be it a thermometer, a timer or a scale (balance), I usually try to keep my precision to within a half a graduation.

e.g. If my thermometer is graduated in 1º increments, I try to keep my temperature to ±0.5º for a 1º spread. If my graduate is marked off in 1ml increments, it's ±0.5ml. (etc., etc.)

Sometimes I think I'm a little O.C.D. about measurements. This topic makes me think that I'm often more precise than I need to be.

But, what the hell? I kind of like being as accurate, precise and repeatable as I can. It's part of what makes photography fun for me.

holmburgers · May 27, 2011

If that's what makes photography fun for someone, then I'm all for it.

On the other hand, I know that some people are intimidated by this type of precision. I can't personally say I find it fun; I'd rather see my negatives, or prints, or whatever. I see the chemistry as mostly a means to an end, and not an end in itself.

But of course, the more you get into it, the more you appreciate this side of it and do see some beauty in the process. So, I guess I'm learning.

But to your question, "How accurate do we need to be?", I think the average worker is better off being consistent than truly accurate.

Thats my 2¢ (± .005¢)

michaelbsc · May 27, 2011

In industrial settings the word is repeatable. So long as the instrument indicates the same thing for the same measurement it is usable even if it isn't "right" about what it is measuring. If it repeats you know whether to adjust up or down. But if it is erratic you are hosed trying to figure out what's wrong.

Photo Engineer · May 27, 2011

Michael has a good point. Related to this and coupled to what Randy says, I can add that accuracy depends on the ingredient. A 1% or even a 10% error in Sodium Carbonate will likely have little effect on prints unless pH changes. On film, sharpness may change. A similar error in Sodium Bromide will have a slight impact on the speed of the material, but the same error in KI can be catastrophic!

A similar error in Ammonium Thiosulfate in a fixer will change capacity and fix rate but will do nothing adverse to the results.

The list I could make up is long and tedious and would probably generate pages and pages of queries and rebuttals from "experts", but I have done the experiments on the chemicals above and found what I said to be true.

I would also add to further confound things that these effects in developers varied with film type or paper type. Some were hyper sensitive and some were virtually insensitive.

PE

michaelbsc · May 27, 2011

Probably the most important thing people can learn about repeatability revolves around temperature control. Especially if you have the round mechanical thermometers.

Get a cup of ice water, a cup of hot water (too hot for processes) and a temperature bath that you have brought to your process temperature.

Put the thermometer in the cold water, to force it below the desired measurement. Once it's significantly below the process bath temperature, move it to the process bath. Note the temperature when it is stabilized.

Now move the thermometer to the hot bath for a minute, and then bring it back to the process bath. Check the reading once it's stable.

Now you have measured the same temperature approaching from both directions, and if the instrument doesn't read the same both times you know you have a hysteresis problem.

Mixing solids with liquids to make a percentage solution.

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