known. Barring the presence of impurities that form mixed crystals whose solubility differs but little from that of the desired pure crys­talline compound, and certain other relatively unusual situations, re-crystallization from solution is a powerful tool of purification with respect to almost all impurities with one exception—the solvent. In the usual process of crystallization, where crystal growth is rather rapid and the aim is to produce a granular product, the mother liquor cannot be completely removed, even by centrifuging. I t is held both on the surfaces of the crystals and in the interstices of masses of crystals which may be in close proximity or partially grown together. This hitter condition is illustrated by a photomicrograph of s tandard sam­ple sodium oxalate (Figure 2 ) . Normally, water retained on the surfaces of the crystals can be re­moved by drying. But solvent is also retained in interstices between crystals and as inclusions of mother liquor within crystals. In this form the water, or other solvent, is sel­dom removable by drying. I t can be removed only by heating the substances to a temperature at which the vapor pressure of the trapped liquid becomes sufficient to rupture the crystals. The amount of mother liquor entrapped by crys­tals can be controlled to some de­gree by varying the rate of crystal growth and by other variations of technique known to the skilled tech­nician, but the absence of readily detectable inclusions in crystals is very rare. This is illustrated by microphotographs of crystals of so­dium chloride and of NBS s tandard sample potassium biphthalate and potassium dichromate (Figures 3A, 3B, 3Cl . Inclusions of mother liq­uor are plainly visible in each of the photographs, some of them having a geometrical outline similar to the form of the crystal. In some of the inclusions the presence of a gas bubble within the liquid is also visi­ble. This may be a foreign gas or a water vapor phase resulting from the contraction of this liquid inclu­sion on cooling.

Water that cannot be removed by drying is not easy to determine. For accurate assaying of NBS pure

a e r o g r a p h

HY-FI A complete High Temperature Programming Gas Chromatograph with Hydrogen Flame Ionization Detector. The phenominal sensitivity and high operating temperature of the Hy-FI extends ana­lytical possibilities into a whole new area not possible using Thermal Conductivity Instruments.

High temperature oven variable to 35CPC with fan for air circulation and rapid cooling. Stainless steel injector with adjustable temperature to 400°C. Panel controlled stream splitter. Sensitive electrometer with coarse and fine zero control, accurate attenuator, and ignitor button. Stainless steel flame head with ignitor coil. Air pump, air filter, packed Vs" stainless steel column and other accessories. Aerograph linear 6-way gas sampling valve, optional.

PRICE COMPLETE $895.00

For special details on applications write for Aerograph Research Notes Spring 1961. If you have a difficult problem requiring great

sensitivity and high temperatures such as sex hormones, waxes, etc., please send a sample to our Research Department for free

analysis on the Hy-FI. The Aerograph Hy-FI is guaranteed against any defects and to

meet your most exacting requirements. Your order by phone or letter will receive prompt attention.

W I L K E N S I N S T R U M E N T & R E S E A R C H I N C . P . O . B o x 3 1 3 , W a l n u t C r e e k , Cal i f . · P h o n e A T l a n t i c 4 - 7 1 6 6

Circfe No. 109 on Readers' Service Card

VOL. 3 3 , N O . 4 , APRIL 1961 · 2 7 A

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gas chromatograph