Harshaw Chemical Co

HARSHAW

Crystal Products

Performance tested crystals for use as gamma radiation detectors, IR windows and UV components.

The Harshaw Chemical Co.

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HARSHAW

Infrared Optical Materials

Harshaw infrared materials transmit to 70 microns. Available as blanks or polished to your finished specifications.


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HARSHAW

IR Pressed Pellet Powders

KBr, Csl, and TIBr, IR Pressed Pellet Powders are suitable over a wide spectral and refractive index range.


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Λ HARSHAW

Silver Chloride

This unique IR material is insoluble in water, transmits out to 22 microns, available as windows only .016"thick.


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HARSHAW

Performance Tested Crystals

Crystals for use as X-ray monochromators, X-ray detectors, reststrahl plates, and refracting components.


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Your inquiries for complete information are invited.

The Harshaw Chemical Co. 194S East 97th St · Cleveland, Ohio 44106 · Phone 216 721-8300 Utrecht. Netherlands —Harshaw-Van Der Hoorn N.V. Frankfurt. W. Germany— Harshaw Chemie GmbH Circle No. 41 on Readers' Service Card 36 A

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ANALYTICAL CHEMISTRY

REPORT FOR ANALYTICAL CHEMISTS T h u s it m a y be easy to separate closely boiling solutes containing different functional groups with a selective stationary phase. I t m a y be somewhat more difficult to sepa rate closely boiling solutes (iso mers) with the same functional group (e.g. TO-, p-cresol). I t is most difficult to separate closely boiling solutes with no functional group (e.g.TO-,p-xylene) : To illustrate these ideas, let us ex amine the separation of TO- and pcresol. [ T h e ortho isomer can be easily separated from the meta and p a r a isomers because of the steric hindrance of O H by t h e methyl group when this group is in the or tho position (the well-known ortho effect) (32).] T h e methyl group is known to exhibit a different elec tronic effect on the hydroxyl group, when the alkyl is in the meta and para positions. To t a k e advantage of this difference, one should search for solvents which interact strongly and specifically with the hydroxyl functional group. F o r example, a hydrogen bond accepting solvent would be able to effect separation from the fact t h a t the meta isomer is more acidic t h a n the p a r a isomer (33). If, however, such a solvent were also t o interact nonselectively with other p a r t s of the phenolic iso mer molecules, the separation would be diminished. Examination of E q u a t i o n 17a further indicates t h a t temperature affects yt", and we might thus expect a also to be temperature dependent from this term. I n general it is found t h a t a will improve as the temperature is decreased, if solutesolvent interactions are important to the separation, (we have already discussed how temperature can af fect a through the saturation vapor pressure r a t i o ) . Using E q u a t i o n 15 for two components, it is seen t h a t the temperature dependence of α arises from the \(Αββ") term. When the difference in excess par tial molar enthalpies of mixing is large, temperatures will affect a greatly. Unfortunately for closely related solutes, A(\Se°) is usually small, so t h a t temperature does not usually play t h a t prominent a role in chemical selectivity. T h e limits of this article do not provide us the opportunity to dis cuss in any greater detail the a c

tivity coefficient and the excess functions. W e should note, how ever, t h a t Pierotti et al. (34, 35) have developed an empirical a p proach for the prediction of activ ity coefficients in G L C . T h e y es sentially assign a t e r m for each t y p e of interaction between solute and solvent, and then sum up all the terms to obtain the total interac tion. Their work is certainly v a l uable, for it provides an estimate of γ " . One is able to predict, for ex ample, the linear relationship be tween In y" and carbon number in a homologous series (34) • How ever, for more accurate y=values, based on an understanding of the solution process, a fundamental a p proach m a y prove more valuable. Complexation Equilibria I n the previous section we noted t h a t for high solvent selectivity specific solute-solvent interactions should be exploited relative to non specific interactions. A logical ex tension of this principle is the use of reversible complexes between solute and solvent, or to go one step further, the use of an additive ca pable of complexing with the solute for selectivity. T h e first example of this latter approach was by B r a d ford et al. in 1955 (36). These workers added A g N 0 3 to an "inert" solvent and found olefins selectively retarded relative to saturates. T h e silver ion complexed with the u n s a t urated center, and this complex re sulted in the increased retention of the olefin in the stationary phase. Since the above paper, m a n y workers have used liquid phases impregnated with A g N 0 3 for the separation of olefins. As an exam ple of the selectivity possible with these types of columns, Phillips (36a) found t h e relative retention of benzene/n-hexane on liquid paraf fin t o be 1.6, and 40 on 3 0 % silver perchlorate in tetralin. Separations of isomeric olefins are also possible with Ag+ impregnated columns. C. S. G. Phillips and co-workers (37, 38) have continued this work with other cationic complexing agents as the solvent itself. M e n tion should also be made of the work of Rogers et al. (39, 40) in the use of heavy metal salts as adsorbents in gas-solid chromatography. This work was directly followed by Al-

Harshaw Chemical Co

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