B A R R E T T RESEARCH REPORTS O N :

A New Standard of Purity

1 Unusual photomicrographs show the formation of crystal phthalic ο anhydride from the liquid state—key to Barrett purity measure­

ments. Above, first crystals appear as liquid is cooled to freezing point. 2 Crystals grow rapidly from "root" of first solid formation.

ο Temperature variations are carefully plotted during this phase change to determine freezing point of sample.

3 Crystals have now engulfed 75% of area. When determined, ο the freezing point is compared with that of 100 mole % phthal­

ic anhydride—permitting accurate calculation of over-all purity. 4 Completely crystallized, phthalic anhydride looks like this, mag-

o nified 25 times. By the method of freezing point determination, Barrett commercial phthalic measures 99.7 mole % pure.

7 8 A INDUSTRIAL AND ENGINEERING CHEMISTRY

for Phthalic Anhydride S U M M A R Y : The Barrett laboratories have established the purity of Barrett Phthalic Anhydride

{commercial specifications) at 99.7 mole per cent. To achieve this accuracy, they used a method known

to physical chemists as the colorimetric freezing point determination. It,involves the indirect measure­

ment of the freezing point of 100 mole per cent phthalic anhydride. When determined, this freezing point

becomes the standard of purity against which phthalic samples are measured. Barrett researchers now

have set this standard, permitting new accuracy in the purity measurement of phthalic anhydride.

When you're measuring purity, that extra place beyond the decimal point often seems to be just beyond the reach of your analysis. You turn to mass spectrometry, infra-red spectro­photometry, or chromatography for the analysis of specific impurities. But when it comes to finding the sum of all impurities, you may have to abandon electronic instrumentation and return to the white-coated methods of physi­cal chemistry.

First, find the freezing point

The calorimetric freezing point deter­mination, a method accepted by the U.S. Bureau of Standards, is a real boon to the decimal point chaser. The crux of this method is the extrapola­tion of the freezing points to 100 mole per cent material. Once you can estab­lish this value for a particular com­pound, you are well on the way to new accuracy in measuring its purity.

After determining that the method was theoretically applicable to phthalic anhydride, Barrett chemists went to work on the freezing point problem. Though phthalic anhydride (or any­thing else) of absolute purity is non­existent, its properties can be arrived

at indirectly. Barrett research estab­lished the freezing point of 100 mole per cent phthalic anhydride to be 131.11 ± 0.01°C.

With this absolute value now avail­able, the purity of phthalic anhydride can be measured with a new accuracy. The second law of thermodynamics describes the relationship between the mole fraction of a substance and the freezing point depression. Working from there, Barrett researchers have measured the mole fraction of Bar­rett Phthalic Anhydride (commercial specifications) as 0.997.

Pitfalls of titration

This is a particularly impressive de­gree of assured purity when you com­pare it with the vagaries of phthalic purity measured by titration. Say, for instance, a manufacturer quotes a purity of 99 .5% by titration. This may sound precise but actually it gives you no firm limit on total impurities.

Phthalic acid, maleic anhydride, ma-leic acid and benzoic acid are some common impurities which go unde­tected by titration. These masquer-aders are all included in the purity

figure determined by titration.

On the other hand, the purity of Bar­rett Phthalic Anhydride can now be stated positively at 99.7 mole per cent. This gives customers the comforting assurance that Barrett Phthalic An­hydride will react smoothly and con­tribute its full share to uniformity of end products—be they alkyd resins, polyesters, plasticizers or chemical intermediates.

Microscopic devi ls

While keeping careful tabs on over­all purity, Barrett also pays close at­tention to special trouble-brewing im­purities. Maleic anhydride and other impurities can cause trouble in phthalic anhydride at percentage levels you wouldn't notice in your drinking water. Another report in this series will carry the full story on these elu­sive devils and how they are exorcised.

If you would like the deep-down de­tails of the Barrett lab's work on phthalic purity, you're welcome to a reprint of the published article, Calorimetric Determination of the Freezing Point of Phthalic Anhydride. Drop us a postcard now, while you're thinking of it.

B A R R E T T D I V I S I O N 4 0 R e c t o r S t r e e t , N e w Y o r k 6 , N e w Y o r k In Canada: The Barrett Co. Ltd., 5 5 5 1 St. Hubert St., Montreal, P. Q.

VOL. 50, NO. 5 · MAY 1958 7 9 A

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