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VARIABLE G R AD ΙΕ ΝT AMINO ACID ANALYZER MODEL VG-6000 LATEST IMPROVED COMMERCIAL VERSION OF THE APPARATUS DESCRIBED BY KARL A. PIEZ AND LOUISE MORRIS IN ANALYTICAL BIOCHEMISTRY, VOL. 1, 1960 For complete details write Dept. V G for Bulletin 6000
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ANALYTICAL CHEMISTRY
spect to impurities that are present as separate solid or liquid phases. Actual sensitivity will depend, of course, not only on the size but on the pattern of distribution of the foreign phases. One more word about these inclusions. They disappeared completely, so far as could be seen, even with the aid of a microscope, when a small concentration of nickel (250 p.p.m.) was added to the bath. The effect of the nickel was to retard the rate of growth of the crystal perpendicular to the pyramid faces, relative to the rate of growth on the prism faces. The nickel ion does this by a mechanism that is not understood but that does not involve more than a very slight contamination of the crystal with nickel. The concentration of nickel at the prism faces was 0.2 p.p.m., and below the sensitivity of the method of analysis at the pyramid faces. This change of relative growth rates appears to allow more time for a neat and perfect construction of the corner formed by two prism faces and the two adjacent pyramid faces. Orientation of the inclusions shows that this is where they occur. Perfecting this corner is a mechanical achievement that appears to be somewhat difficult for ammonium dihydrogen phosphate. Ammonium ions, hydrogen ions, and phosphate ions must arrange themselves in an orderly fashion in four intersecting planes if the corner formed by the four growing faces is not to be defective. All of this has to happen while the particles that are forming the crystal pattern are being jostled about by a vast number of water molecules moving in the disorderly fashion characteristic of the liquid state. Molecules of water do not feel an urge to find places in the crystal array, but nevertheless they are close onlookers that are constantly getting in the way and having to be pushed aside. Although no selfrespecting crystallographer would suggest this kind of picture, it may serve as an aid to understanding why growing good crystals from a solution is in general much more difficult than growing them from a melt. By experience it has been learned that good ADP crystals
