| Author | Message | ||
     David E Honigs (dhonigs) Junior Member Username: dhonigs Post Number: 7 Registered: 4-2012 |
Dear Howard, I wanted to thank you for your answers. I started out not really understanding my questions and your comments have helped me in many ways. I gave the thread its title because I understood that something would inherently be wrong in my question/correction but I had no clue as to what. I find it amazing how fast things became clearer. Sincerely, David Honigs | ||
| Donald J Dahm (djdahm) Senior Member Username: djdahm Post Number: 75 Registered: 2-2007 |
Well, I'm sure many of us have a story about how we learned about volume contraction on mixing. Mine was "shelled" and "ear" corn. It turns out that sometime there's even a volume expansion, but I don't have the reference handy. I think chemistry books share part of the blame for some of the misunderstanding surrounding volume fraction. The definition of volume fraction is: "the volume of a constituent divided by the volume of all constituents of the mixture prior to mixing". Using the density and all that is taught fine. The trouble is with what we are willing to call a concentration. A "true" concentration is expressed in "amount per volume". (The amount is usually most properly a weight.) If we mix two liquids and have a contraction we cannot calculate a "true" concentration without knowing the final volume. In my classes, I call "volume fraction" a "recipe", rather than a concentration, beause it tells how to make up the solution. A spectroscopist can use any concentration (amount per volume) they want, but if they want the concentrations to add to "one" they have to use "adjusted" volume fractions. In Howard's paper, it shows that non-adjusted volume traction is far better than the other measures they tried. The mystery to me is why folks think any other measure ought to add to one. After all, its volume in the denominator. | ||
| Howard Mark (hlmark) Senior Member Username: hlmark Post Number: 487 Registered: 9-2001 |
Well, Dave, it depends on the liquids. Sure, if you mix, say, water and alcohol, then it's well-known that the volume shrinks by about 5% and the spectra are distorted because of interactions. If you've been following my columns lately, then you'll have seen in the May, 2010 issue of Spectroscopy, that we originally tried mixtures of water, alcohol and acetic acid, and they don't work very well for a CLS calibration, exactly because of the effects of the interactions. In this case, the reconstruction (or "prediction") of the mixture spectrum was terrible. In such a case, you wouldn't want to use the values for analysis, and we didn't try. In the case of the hydrocarbons published in the Appl. Spect. article, the interactions were much reduced, and the reconstruction of the mixture spectra much better. If you read that article carefully, you'll note that toward the end there's a caveat about just that point - that you shouldn't try to get analytical information unless the spectra are well-behaved in the first place. I daresay, Dave, that you're being just a bit presumptuous to think you're the only person who knows any physical chemistry or spectroscopy, just because some real-world results don't jibe with your preconceived notions - not a good thing for a scientist. (I can be arrogant with the best, too!) \o/ /_\ | ||
| David E Honigs (dhonigs) Junior Member Username: dhonigs Post Number: 6 Registered: 4-2012 |
Well Howard, in Kansas they had a requirement that one must take Chemistry in 8th grade. One of the experiments we did was to demonstrate that volume is not conserved when you mix liquids. I saw this same thing when I was a graduate student and mixing up NIR samples for several of the papers we published such as the ones on predicting physical properties. To me, it is a glaring thing to see taking a density at a single temperature, mixing the samples, and thinking that nothing has changed. It ignores all of the interaction effects. By answering the question I guess you did answer my real question. I was wondering how most people see these things. I would guess that you and the authors do a good job of representing most people and I am an outlier. Thanks for the help. David | ||
| Howard Mark (hlmark) Senior Member Username: hlmark Post Number: 486 Registered: 9-2001 |
Dave - I saw your posting come through, but it didn't mean too much to me until I got my copy of NIR news this afternoon. It would seem that the "Mythbusters" picked up on the Applied Spectroscopy article we published in 2010 (Appl. Spect., 64(9), p.995-1006 (2010)) and ran a bit further with it than we did. I gotta admit I'm tickled pink. Usually when you publish something you never know if anybody even noticed it, much less read it. That said, it seems that the Mythbusters pretty much followed what we did before, and showed, on a larger sample set than we had, much the same results. I can't speak for the Mythbusters, but since their results are so similar to ours, if I apply your comments to our own work, I don't see what your problem is with it. Calculating density from weights and volumes is pretty basic physical chemistry; it's the definition of density, after all. That hardly qualifies as an experiment, much less a gedankenexperiment. Measuring the spectra of mixtures is an experiment, but it's a REAL experiment, also not a gedankenexperiment. If you want to wait we can talk about this more in Chambersburg, but if you want to, we can also discuss it further publicly here on this forum. \o/ /_\ | ||
| David E Honigs (dhonigs) New member Username: dhonigs Post Number: 5 Registered: 4-2012 |
Did anyone read the most recent myth busters about concentration? Did anyone see how they calculated volume fraction? Anyone care to comment on what actually happens with mixed liquids in the real world outside of gedanken experiments? |