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  1. #21
    Photo Engineer's Avatar
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    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

  2. #22
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    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

  3. #23

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    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?
    Last edited by Ray Rogers; 01-22-2009 at 01:49 PM. Click to view previous post history.

  4. #24
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    Ray;

    This reference is probably the most lucid: http://www.everyscience.com/Chemistr...res/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

  5. #25
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    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

  6. #26

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    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.

  7. #27

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    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.

  8. #28
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    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

  9. #29

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    Quote Originally Posted by Photo Engineer View Post
    Ray;

    This reference is probably the most lucid: http://www.everyscience.com/Chemistr...res/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

  10. #30
    Ian Grant's Avatar
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    Not at all, Anchell has it written correctly

    Quote Originally Posted by Photo Engineer View Post
    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
    Last edited by Ian Grant; 01-23-2009 at 03:21 PM. Click to view previous post history. Reason: typo

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