I'm not a chemist - help Mr. Wizard.

[Dan Dozer]

Please help me with my dumb question. When mixing chemicals and you are supposed to mix a certain percent solution, how much powder chemicals do you mix with how much water to get the percentage you want. In other words, to get a 1% solution, how much powder by weight do you mix per liter (or per gallon) of water to get that?

 

[Dana Sullivan]

1 gram of anything in 100 ml of liquid makes a 1% solution. As you guessed, you'd use 5 grams in 100 ml to makes a 5% solution.

-Dana

 

[Dan Dozer]

Thanks Dana - I knew that there was a real simple answer to this.

 

[Akki14]

The real chemists have a minor fit about this because it's not figuring out the solution based on mole but for our purposes in photography it's good enough and it's a standard (kind of) across photography chemistry.

 

[RobertP]

Ok so if you want to mix 50 ml of a 27% solution of ferric oxalate and you just add 13.5 grams of FO to 50 ml of H2O(+ 1 gram or so of edta) you end up with more than 50 ml of solution. So would you add 13.5 grams to say 35ml of H2O and then add H2O to bring it to 50ml? The reason I ask is I've done it both ways without any adverse effects that I could notice. Also there is the matter of measuring water at 68 degrees and then remeasuring it after it has been heated to 180 degrees. Doesn't water expand when heated and also expand when frozen? I know this is splitting hairs for what we do but I am no chemist also. These are just a few things I notice when mixing solutions.

 

[Jordan]

Actually, I'm a "real chemist" and we use percentage solutions all the time when exact stoichiometry (getting a certain known number of molecules) is not necessary.

The best way to make 50ml of a 27% FO solution would be to add the solid first to about 35ml, dissolve it, then make it up to the 50ml. The density changes of water within the temperature range needed can probably be ignored -- they're no more than a few percent.

 

[Photo Engineer]

I'll have to go with what Jordan says. All of the others are just approximations, and only that. They work at very low concentrations of 1% or less on average. They become increasingly in error as percentage increases.

A weight of solid placed into a liquid is known as wt/vol. If you weighed the water as Jordan alludes to, then the solution would have percentage expressed as wt/wt or weight for weight. This corrects for density.

PE

 

[keithwms]

Indeed, per the original definition of the mL, the density of water is 1 g/mL.

 

[RobertP]

Thanks for clearing that up guys. So the bottom line is when all the materials are combined you actually end up using a little less than 50ml of water to make 50 ml of a 27% FO solution. As I said I have done it both ways, so I assume that the 27% mixed either way is a solution with enough saturation that regardless of the extra vol. of water there is suffient FO to reduce the platinum/palladium metal salts in Pt/Pd print. I usually mix it as you have said, by mixing in 35ml then topping it to 50 ml.The time I mixed it the other way I made a mistake and started with 50 ml of water but it didn't seem to effect the print.

 

[Akki14]

Maybe I'm missing something here but when something goes into solution, you don't get "more" or "less" water/solution. As long as it's dissolved fully it should still be 50ml. In both cases you should be ending up with 50ml... right? The two stage dissolving process is merely to help you dissolve the chemical then when you top up with water, you are sort of washing down the sides of the beaker/graduated cylinder/container. Basically, so you don't slop stuff out, yes?

 

[RobertP]

Heather you may not be missing anything. I may be the one missing something here. I started with distilled water room temp 50ml. I heated the water to 180 degrees. I added 13.5 grams of FO and 2 grams of edta. After mixing I ended up with over 50ml of vol. It was closer to 60ml. This is why I asked about the proper way to mix. I don't know if the increase of the vol. to around 60 ml was due to the addition of the FO and edta or the temp of the water or both. Like I said I'm no chemist. So if it is not possible to gain vol. by how I described then the only thing that could have gone wrong is my initial measuring of the H2O. Robert

 

[Photo Engineer]

Robert;

Probably both! What you had was about a 22.5% solution due to the error in starting with 50 ml of water and ending with 60 ml. When you add a solid to a liquid to make a solution, volume can increase or decrease.

PE

 

[RobertP]

Thanks PE, I understand now. That's what I suspected. Robert

 

[Ray Rogers]

When you add a solid to a liquid to make a solution, volume can increase or decrease.

PE

I understand that the volume of [liquid A + solid B] may be less than the sum of the uncombined volumes... but how can the volume of [liquid A + solid B] become less than the volume of liquid A, in the absence of a reaction between A and B that reduces intra ? and or inter-molecular distances?

It is intuitive that the combination of the two volumes might be additive.

It is even understandable that on occasion, the volume may remain unchanged
(if for example "empty" space were somehow present)

But,

Can someone explain how the combination of 2 volumes can result in a lower TOTAL volume?

A common example would be helpful too.

Thanks

 

[Photo Engineer]

Addition of Silver Nitrate to water can decrease volume, thereby increasing density.

Addition of Ammonia gas to water can increase volume thereby decreasing density.

So, 100 ml of 1 N AgNO3 is higher in weight than 100 grams, but 100 ml of 28% Ammonium Hydroxide is lower in weight than 100 grams. Just two for starters.

PE

 

[Ray Rogers]

Addition of Silver Nitrate to water can decrease volume, thereby increasing density.
QUOTE]

Yes.

You gave 2 examples, but I only am really concerned with the first case.

It is interesting in fact that you mention the case of silver nitrate; I was introduced to this phenomenon years ago by a researcher who was showing me his emulsion lab... he posed the question to me if I knew the explanation... he was always curious to find that after a few hours he would return to the lab to find the volume of silver nitrate (prepared in a volumetric flask) was always lower than upon inital mixing... not having been there during mixing, I could only make a wild guess or two, but it was claimed to be a real difference presumably not related to evaporation nor temperature or pressure changes.

My question is- What is the explanation for this behaviour?

 

[Photo Engineer]

Among other things, ionization may increase or attractive forces may increase, decreasing molecular bond distance and so volume may decrease. There are a variety of reasons. Look up van der Walls force among others.

PE

 

[Ray Rogers]

"... how can the volume of [liquid A + solid B] become less than the volume of liquid A, in the absence of a reaction between A and B that reduces intra ? and or inter-molecular distances?"

Among other things, ionization may increase or attractive forces may increase, decreasing molecular bond distance and so volume may decrease. There are a variety of reasons. Look up van der Walls force among others.

PE

Thanks.

I imagine I won't be able to discover much by myself... but I will try.

I welcome contributions from others who have some insight on this
(to me at least) curious behaviour.
I am particularly interested in what is unique about AgNO3 in this respect.

TIA

Ray

 

[Photo Engineer]

Ray, the same thing is observed with most inorganic salts as well as Silver Nitrate. I used that as an example only as it is familiar to most of us.

PE

 

[Ray Rogers]

Ray, the same thing is observed with most inorganic salts as well as Silver Nitrate. I used that as an example only as it is familiar to most of us.

PE

OK.

But the question still remains as to how and why the molecular bond lengths / inter molecular distances change...
specifically, Why do they shrink in one case and grow in another?

(Perhaps it is a very simple explanation after all;I often overlook Van der Walls and Hydrogen bonding so it shouldn't be hard to imagine.)

I am curious, have you noticed this to be "instant", or can you confirm a time component is involved?

Ray

 

[Photo Engineer]

Read here: http://en.wikipedia.org/wiki/Van_der_Waals_force to see that it is attractive OR repulsive.

The only time effects on this ever observed was with Thiotimoline. This is explained as follows:

"...thiotimoline is notable for the fact that when it is mixed with water, the chemical actually begins to break down before it contacts the water. This is explained by the fact that in the thiotimoline molecule, there is at least one carbon atom such that, while two of the carbon's four chemical bonds lie in normal space and time, one of the bonds projects into the future and another into the past."

Thiotimoline was first reported in 1948.

In this case, all bets are off.

PE

 

[Photo Engineer]

A breaking update....

In the new version of Anchell, the instructions for making up solutions to a given percentage make the same error as noted earlier. He has (I am told) said that using 100 ml of water and 10 grams of chemical will prepare a 10% solution. This, as explained earlier, is incorrect.

Now, not having a copy of the new version of Anchell, my profound apologies to Steve if the information I have is incorrect. At this time, I am unable to verify it either way, as I don't have a copy of the book. Perhaps someone will chime in here!

PE

 

[Ray Rogers]

Well I was able to place the question (not about cooking shortcuts) under the category of compounds with negative partial molar volumes.

I would still like to know more... for example, what is the PMV for silver nitrate? Where can a list of PMVs (for various photographic chemicals) be found?

Is there a situation where this behaviour interfers with proper preparation of known concentrations?

 

[Photo Engineer]

Ray;

This reference is probably the most lucid: http://www.everyscience.com/Chemistry/Physical/Mixtures/a.1265.php

I was trying to simplify a bit above, because I have been recently accused of using technobabble to explain something complex. So, I simplified.

PMV is generally determined by empirical measurements and these are reduced to the proper equations and constants. The PMV of silver nitrate depends on concentration, as it varies with molarity. Some of these are quadratic equations with 8 terms or so, and some are additive. Most behave in a fixed manner with temperature, and to my knowledge there is no case where the behavior in question interferes with preparation of a solution as long as you do the following:

If you adhere to MOLARITY or Moles in 1 Liter of total solution then everything works including percentage strength, but if you use MOLALITY or Moles in 1 Liter of solvent as proposed by many, then this breaks down and your solution is in error.

Another caveat is that wt/wt and wt/vol are not always the same, but wt/wt should be used in the dark when making emulsions or when using viscous solutions due to the inherent difficulty of measuring volume accurately when working with viscous solutions, especially in the dark. It is also useful sometimes when working with developer syrups unless you use a syringe for accurate dispensing.

Always specify the method used to avoid confusion and remember that errors creep into crossing molarity and molality as concentration goes up. The PMV can become very very large at high concentrations.

PE

 

[Photo Engineer]

An afterthought.....

All of this can be simplified if one thinks in terms of density. If density of the solution is greater than the density of the original solvent, then one can speak of the volume having decreased, but if the density is less than the density of the original solvent, then one can speak of the volume having increased.

This is a large oversimplification, but in fact, it works with the quadratics mentioned above for solvent, solute, temperature and mixtures. And, using geared pumps, gravimetric flow can give a precise molar flow rate that volumetric flow cannot for this very reason.

PE