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

alanrockwood · Jan 22, 2009

I am glad someone brought partial molar volumes into the conversation. In older terminology "partial molal" was sometimes used for the same quantity, e.g. in Chemical Thermodynamics, revised edition by Klotz.

The underlying reason for negative partial molar volumes of some solutes, especially some ionic solutes, probably has to do with the coulombic interaction. ("Coulombic interaction" means the interaction between charges. "Interaction" is chemical and physical talk for "energy.") Through the coulombic interaction ions are very strongly attracted to molecules with large dipole moments, such as water. (A "dipole moment" means that one end of the molecule carries a positive or partial positive charge and the other end of the molecule caries an equal but opposite amount of charge.)

This strong interaction between the ion and the dipole moments of the solvent tends to overwhelm the attraction that the solvent molecules have for each other. Consequently, the solvent molecules tend to orient themselves around and closely associate with the ions rather than with other solvent molecules. This is a kind of re-packing operation, and it tends to be strong in the first molecular layer around the ion and less strong as you get further from the ion.

The repacked solvent molecules may be packed so as to take up less space than they would if they were distributed in the bulk solvent. This space savings can, in some cases, more than make up for the fact that the ion itself takes up some space. The net effect is that adding solute to the solvent can actually decrease the volume of the solution.

Jordan · Jan 23, 2009

I think Alan has it -- solvation is an organizing process and may pack solvent molecules more tightly than in bulk solvent.

In practice, if you want a 10% w/v solution in water, the correct procedure is to dissolve the solid first in somewhat less than your final desired volume, then make it up to the final volume. No need to worry about contraction / expansion in this case.

Photo Engineer · Jan 23, 2009

I can agree with the above, but in practice I have seen that the actual density equations are up to 8th order with both positive and negative terms, which indicates to me more than just "packing" is taking place. It is a complex reorganization of all of the molecules that lead to charts such as shown in the reference I gave above where you have both increases and decreases in volume wrt concentration. In that reference, they say that the ordering can be broken up as well as increased.

This, to me, implies that many many things go into determining the actual change in volume at any given concentration and with any given salt.

We were able to derive density equations for each and every chemical used in emulsion making and could predict single and multi salt solution densities over a temperature range from 20 - 40 deg C with 1% or better accuracy. These equations were extremely large, but derivable empirically, and the curves looked much like those shown in the reference.

So, I don't disagree that solvation is an organizing process, but it can also be a disorganizing process or packing could not be reversed causing increases in volume, and this requires more than one force.

PE

Ray Rogers · Jan 23, 2009

Photo Engineer said:
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.
...
The PMV of silver nitrate depends on concentration, as it varies with molarity.

Yes.

I found that ref. earlier and thought it comparativly lucid too!

I thought the PMV as given was, by definition, based on the volume change induced per mole of the solute.
The actual volume change will differ depending upon the amount of solute introduced but there must be a given value for silver nitrate's PMV somewhere.

I noticed the phrase "a large volume of water" was used (see link) rather than an exact volume.

Ray

Ian Grant · Jan 23, 2009

Not at all, Anchell has it written correctly

Photo Engineer said:
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

Ron, you really should read first before you write, you've been mis-informed and your source is very unreliable. This is quite false here is the relevant section, I quote from the 3rd Edition:

Percentage Solutions

For convenience, and when the amount of a chemical may be too small to be weighed accurately,the amount is often given as a percentage solution. This can be simply stated as how many grams of a chemical are dissolved in 100.0 ml of water. For example, a 10% solution has 10.0 grams of a chemical dissolved in 100.0 ml of water.

Regardless of the amount used in the formula, the percentage is always the same. That means that every 10.0 ml of a 10% solution contains 1.0 gram of the chemical. If a formula requires 2.0 grams of chemical, use 20.0 ml of the percentage solution.

The following is an example of the use of percentage solutions. Suppose the formula calls for:

Potassium ferricyanide, 5.0 g
Potassium bromide, 1.5 g
Water to make 1.0 liter (1.0 liter - 1000 ml)

If we start with stock solutions of 10% potassium ferricyanide and 10% Potassium bromide, we can quickly make our solution by multiplying the dry amount by 10 and taking:

Potassium ferricyanide, 10% solution, 50.0 ml
Potassium bromide, 10% solution, 15.0 ml
Water to make 1.0 liter

It is easy to see the advantage of this method, especially for chemicals that are often used in small amounts (e.g., Phenidone, potassium bromide, benzotriazole).

When mixing percentage solutions start with less than the total volume of water. After the chemical is fully dissolved, add the remaining water to make the required volume.

So your source is completely un-trustworthy

Ian

Photo Engineer · Jan 23, 2009

Ian;

This part is unquestionably wrong!

"For convenience, and when the amount of a chemical may be too small to be weighed accurately,the amount is often given as a percentage solution. This can be simply stated as how many grams of a chemical are dissolved in 100.0 ml of water. For example, a 10% solution has 10.0 grams of a chemical dissolved in 100.0 ml of water."

Please look at the other posts to confirm this Ian. The example is correct, but the instructions to get there are wrong! And, if followed, the error increases with percentage.

PE

Ray Rogers · Jan 23, 2009

alanrockwood said:
I am glad someone brought partial molar volumes into the conversation.

I think that would be me

I appreciate your comments.

Do you consider this repacking, (which could involve more than one force) might occur somewhat slowly, over a period of minutes or possibly hours, before stabilizing??

Ray

Ray Rogers · Jan 23, 2009

Photo Engineer said:
Ian;

This part is unquestionably wrong!

"For convenience, and when the amount of a chemical may be too small to be weighed accurately,the amount is often given as a percentage solution. This can be simply stated as how many grams of a chemical are dissolved in 100.0 ml of water. For example, a 10% solution has 10.0 grams of a chemical dissolved in 100.0 ml of water."

Please look at the other posts to confirm this Ian. The example is correct, but the instructions to get there are wrong! And, if followed, the error increases with percentage.

PE

I have to agree.

I did not bother to read the example, but I think the wording is in error. I believe it should read something like:

"For convenience, and when the amount of a chemical may be too small to be weighed accurately,the amount is often given as a percentage solution. This can be simply stated as how many grams of a chemical are dissolved in 100.0 ml of solution. For example, a 10% solution has 10.0 grams of a chemical dissolved in 100.0 ml of total solution."

Ian Grant · Jan 23, 2009

Well being pedantic he should say dissolved in water to make a total of 100ml of solution. But to a layman a 10% solution is 10 in a 100, and he does say "simply stated".

Ian

Ray Rogers · Jan 23, 2009

Ian Grant said:
Well being pedantic he should say dissolved in water to make a total of 100ml of solution. But to a layman a 10% solution is 10 in a 100, and he does say "simply stated".

Ian

I understand what all three of you were thinking.
Unfortunately, some people would read those simply stated directions and proceed to combine 10 grams solute with 100 grams water...

Photo Engineer · Jan 23, 2009

OTOMH, if you mixed up a 68% AgNO3 solution using the method given by Anchell, (about 4 molar which is common in emulsion making), the error would be on the order of 50%!

In chemistry and photography, if we cannot be exact then each of us is talking about something different. This gets back to the WIKI thread. Who is to judge. Being pedantic in this case is the only "right" way as it is chemically correct at all concentrations.

The wrong way is in error and error is moving as concentration moves. It also can be positive or negative. That is my point!

PE

Ray Rogers · Jan 23, 2009

Photo Engineer said:
In chemistry and photography, if we cannot be exact then each of us is talking about something different... Being pedantic in this case is the only "right" way as it is chemically correct at all concentrations.

The wrong way is in error and error is moving as concentration moves. It also can be positive or negative.PE

Communication is not an easy task.

I have a feeling that the guys siting on ANSI or ISO or JIS or DIN frequently wish it were easier to say things simply, and unambigously. I gather that even after years of working on projects, they still can't always agree.

Photo Engineer · Jan 23, 2009

Actually, after doing some lengthy calculations, I find that my estimate is much too high. The error in mixing is a funtion of the Molal vs Molar values. This should give a rough approximation.

With my handy calculator a 4 Molar AgNO3 solution is 1.54563 and a 4 Molal (ain't no such beast but close) is 1.48432. (This is actually 4 moles of AgNO3 in 1 Liter of water as decribed) and so the approximate error is....

About 13% by volume, still nothing to sneeze at and something that cannot be tolerated in most cases.

The molal solution will make up 1.13 liters of solution with the above density which results in the approximate error above.

Sorry. That is what happens when you are in a hurry and working complex math in your head.

PE

Kirk Keyes · Jan 24, 2009

Photo Engineer said:
This, to me, implies that many many things go into determining the actual change in volume at any given concentration and with any given salt.

How did I miss a fun thread like this for so long!?

Packing certainly has a large effect, especially when organics are involved. Some molecules, like water, can fit in-between molecules like ethanol, acetone, or diethyl ether and give you a final volum that's noticable different from the sum of the two initial volumes. And there are certainly things going on with dipoles and charges, especially with ionic compounds.

I always like to amaze the new chemists when I mix up a certain reagent that has 90% acetone with 10% water. I get the 2000 ml volumetric flask, add 200 ml water, and then carefully pour the acetone in so it does not mix much. I get it up to the 2000 ml line on the neck of the flask, and then dump in about 25 ml more acetone. I say something like "crap!", and then I say "watch this" and invert the flask a couple times to mix the acetone and water, and I've done it enough times, that I can get the mix to have a final volume that's pretty close to the line on the flask.

Kirk Keyes · Jan 24, 2009

PE - do you have an equation or a table for silver nitrate density vs. g/L? When I was planning my last emulsion, I found it really handy to have a table of KBr solution density vs composition. I knew U had so many moles of silver nitrate solution in X ml, and I needed to get the same munber of moles of KBr in the same volume X, so I could do a double run addition and have them both pump at the same flow rate and have the same time of addition. So I took my KBr solution info, and I knew the moles of silver, I needed the same moles of KBr, converted that to grams KBr, and then I figured out if I added so many ml of water, what volume would that give. It took a few iterations to get the answer so that I had the same volumes, and I knew how many grams of water I needed as I wanted to make the solution by wieghing out the water and not using a grad cylinder so I had a more accurate measurement.

So do you have a chart? I've looked in my 32nd Ed. CRC handbook and my 8th Ed Lange's Handbook, and a bit of internet searching and no luck for silver nitrate...

Kirk Keyes · Jan 24, 2009

Ian Grant said:
Well being pedantic he should say dissolved in water to make a total of 100ml of solution. But to a layman a 10% solution is 10 in a 100, and he does say "simply stated".

Being pedantic has little to do with it.

It's that one way is correct, and the way he states it is incorrect. It's simple, and it's disappointing that when it is "simply stated", it ends up being wrong.

Kirk Keyes · Jan 24, 2009

Ray Rogers said:
Communication is not an easy task.

I have a feeling that the guys siting on ANSI or ISO or JIS or DIN frequently wish it were easier to say things simply, and unambigously. I gather that even after years of working on projects, they still can't always agree.

Ray. I certainly agree. After spending the last few days writing our lab's SOPs for our procedures of EPA regulated analyses that we do, I coworker and I were lamenting how the EPA does not have consistency in their methods in how they state certain procedures. If only they could be consistent@!#$!

alanrockwood · Jan 24, 2009

Ray Rogers said:
I think that would be me

I appreciate your comments.

Do you consider this repacking, (which could involve more than one force) might occur somewhat slowly, over a period of minutes or possibly hours, before stabilizing??

Ray

Ray,

Once an ion is in solution (by this I mean freed from a crystal lattice) the process of re-organizing the solvent structure around the ion is probably extremely fast. I doubt it would take more than a microsecond, and it would probably take much less time than this. These numbers are guesses based on a physical chemist's intuition and they could be wrong.

One could probably think of the re-organization of water molecules as being roughly based on rotational motion of the water molecules - hindered rotation would probably be the best model. Rotational motion for a molecule like water is characterized by gigahertz frequencies. If we naively but conservatively assume a representative rotational frequency of 1 GHz (it is probably faster than this), and then assume that because of hindered motion the re-organization process would take about a thousand cycles we would end up with an estimated solvent re-organization time of about a microsecond. However, as I indicated above the process is probably much faster than this.

I am very sure that it would not take minutes or hours.

Alan

Ray Rogers · Jan 24, 2009

alanrockwood said:
Ray,

Once an ion is in solution (by this I mean freed from a crystal lattice) the process of re-organizing the solvent structure around the ion is probably extremely fast. I doubt it would take more than a microsecond, and it would probably take much less time than this. These numbers are guesses based on a physical chemist's intuition and they could be wrong.

One could probably think of the re-organization of water molecules as being roughly based on rotational motion of the water molecules - hindered rotation would probably be the best model. Rotational motion for a molecule like water is characterized by gigahertz frequencies. If we naively but conservatively assume a representative rotational frequency of 1 GHz (it is probably faster than this), and then assume that because of hindered motion the re-organization process would take about a thousand cycles we would end up with an estimated solvent re-organization time of about a microsecond. However, as I indicated above the process is probably much faster than this.

I am very sure that it would not take minutes or hours.

Alan

Thanks.

It is not that I would normally think so myself, either.

It is just that I was told of this observation (which I have not confirmed!) and that I am taking on face value; obviously, it could have been a mistaken observation.

Next time I make up a solution, I will try to look for it myself.
To be quite frank, I was skeptical when I first heard it, but after all these years I still can get his surprise out of my mind.

Ray

Photo Engineer · Jan 24, 2009

Some properties that we have not mentioned are Fugacity and Gibbs free energy, or "Fugacity is the phase which will be the most favorable, and will have the lowest Gibbs free energy" and "The Gibbs free energy is the maximum amount of non-expansion work that can be extracted from a closed system".

This combination is the lowest and most favored energy state of mixed phase systems. I believe that this is also an important player here. I thought of it while re-reading much of this thread.

Kirk has asked if I have some density values. I thought I posted a bunch of them here before. If not, let me know. I will have to re-calculate them as I no longer seem to have them here. Remember that these are multiorder and so you cannot draw a straight line through points to construct a table.

PE

RobertP · Jan 24, 2009

Although much of this conversation about solvent structure is over my head, thank you gentlemen for elaborating so thoroughly. I find it very interesting that by mixing solutions you can gain or lose volume. This topic even had me digging out the old college chemistry book. I will take notice the next time I mix up a 9% solution of silver nitrate for my wet plate collodion bath. Robert

Photo Engineer · Jan 24, 2009

Thanks Robert.

Something not really addressed here is the fact that temperature can go up or down as one chemical is added to another. We are familiar with the temperature increase when adding acid to water or base to water, but this is not always so. When adding ammonia to water (another base) temperature goes way down. The same is true when adding hypo to water.

Temperature must therefore be "normalized" to a constant (normally 20 deg C) before volume is measured.

PE

dwross · Jan 24, 2009

Good thread, guys. Great chemgeek stuff.

Having said that (and sincerely), I seem required by my nature to beat on my same, poor old dead horse.

I worry that a learning darkroom worker could be seriously put off by the seeming complexity of all this - perhaps never to attempt mixing a custom recipe. That would be a crying shame. If any newcomer to the darkroom is still reading this thread, please know this: It's not complicated. A certain amount of knowledge and precision is important, of course, but a darkroom is not the Ebola lab at the Center for Disease Control. There is an easily achieved place that is 'good enough'.

Even if you didn't take HS chemistry, a little thought and a bit of common sense will take you a very long way. It comes down to 'margin of error'. I spend a lot of time in the darkroom. I'm there for results, so efficiency counts for a lot. For less than 10% solutions, I add 10g to 100 ml water. For 10% and greater, I bring the solution up to 100 ml. I always use grocery store distilled water. I don't worry that it might not be 'deionized' by the time the jug is empty. I usually dissolve at 52C/125F, unless otherwise noted. I keep things clean, I wear eye protection, gloves, and I have a reasonable ventilation system. The only food I bring into the darkroom is the occasional glass of wine, and I've never mixed chemicals in a wine glass.

And, good grief, molarity, schmolarity.

If you are consistent with your own lab technique, you will have consistent results, year after year.

If you are following someone else's recipe, note what technique they use and follow that. You will get the results they did. The Darkroom Cookbook is an absolutely invaluable resource. Steve is very clear about his methods and I have never gone wrong using them.

For what it's worth,
Denise

Photo Engineer · Jan 24, 2009

Denise;

I would have to say you are both right and wrong at the same time.

If you are consistent, you will get the same results time after time. But, at the same time, your results may be different than what others get in speed, grain, contrast or other features of a given photographic material and this is what leads to us all having expressed different opinions here and having different opinions. Again, neither wrong nor right, good nor bad.

But, if it is true that a textbook, which in and of itself is good and useful, perpetuates or issues incorrect information, then it is wrong! Kirk said it best in his post above, #41.

We must be mindful of that. And, if we are to construct a useful WIKI page or Articles section for analog photography, we must deal with this type of problem regarding differences in technique.

PE

dwross · Jan 24, 2009

That there are differences in technique among photographers is, I think, a given fact. Here's an opinion (mine):

I seriously doubt the technique used to make a 10% solution has much effect on speed, grain, contrast, etc. More problems arise from the obsession with minutely quantifiable results than any variations from technique. Photography (the whole, not the individual components of chemistry, optics, and the rest) is far more qualitative than quantitative.

I love the phrase from digital photography, 'pixel peeping', and its connotation of vacuous obsession with quantification. I believe digital photography actually got the foothold it did, at the speed it did, precisely because there has always been a group of photographers obsessed with counting grains dancing on the head of a pin - and intent on proving loudly that their count is more accurate than anyone else's. Whereas this count is very difficult with analog photography (and is precisely why analog can be art and digital arguably isn't), digital seems custom made for the exercise. And it is we who have made it a Holy Grail.

If we continue to frame our dialogue around technical minutia, we are telegraphing to the rest of the photographic world (and like it or not, this means digital photography) that quantifiable trumps all other considerations. At that level of dialogue, electronics will win every time.

d

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

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