ANALYST'S COLUMN not only behave as acid-base color indicators but also exhibit marked complexforming properties. T h e color change induced in these indicators on complex formation lies within the limits of the acid-base color change, thus indicating a fundamental relation between protonation or dissociation on one hand and complex formation on the other. The mechanism of action of metallochrome indicators thus appears to b e : The color changes of acid-base indicators are due to changes in the electronic structure of the dye system. If the auxochrome responsible for such color changes also forms part of a chelating group, complex formation will lead to the same type of change in the electronic structure and hence also to the same kind of color change. This analysis indicates the points which should be borne in mind in searching for, or designing, new metallochromic indicators. Consideration along these lines led to the conclusion that a dye system eminently suitable as the basis for synthesis of metallochromic indicators was that of the sulfonphthalein and phthalein type; some azo dyes have also been examined in this respect. A suitable complex-forming group seemed to be the iminodiacetic acid grouping. Even the introduction of the aminoacetic acid grouping (glycine) has been found to confer marked metallochromic properties on acid-base indicators. Some of the new indicators Pribil mentioned include xylenol orange, methylthymol blue, xylenol purple, and methylxylenol blue. New indicators of the phthalein type include thymolphthalein Complexone and fluorexone, a fluorescein derivative. By using certain reagents which are capable of forming very stable complexes, it is possible to screen a number of cations and thus make titrations more selective. There are several problems involved, however, including requirements that the complexes be much more stable than those formed with E D T A , be colorless and water-soluble, form quantitatively at stoichiometric ratios, and proceed without side reactions. Most of those used to date have been effective in the alkaline p H region. With the new metallochromic indicators it is possible to work outside the alkaline p H region. This then raises the possibilities of new screening agents. In complex solutions, preliminary separations can often be speeded up if the requirements and possibilities of eomplexometry are considered. I t is often unnecessary, for example, to achieve complete separations. This is particularly true with respect to ion exchange methods which have not yet been applied extensively in complexometry.
reproducibility and stability
All other means of studying induced radioactivity levels are considered passé by those who have encountered our Detectolab® DZ46 Automatic Gamma R a y Spectrum Recorder. Grip tightly though he may, friend Sherman's method lacks provision for operational reliability, precision and stability for counting and recording of induced radioactivity levels. When deeply involved in activation analysis, shielding studies, flux patterns, isotope isolation or short half-life decay research, proceed with assurance . . .with a DZ46. While Sherm travails exceeding distance to obtain spectrometry results, leading universities, industrial labs and research establishments continue to count on our versatile instrument. Automatic single or repetitive scan, front panel selection of five scanning speeds and of scanning limits and automatic repetitive charting with no mirror image are distinguishing DZ46 features. It will operate reliably down to 8 Kev. At a 20 Kev window width, 1200 cpm may be attained from a 0.01 μο C s 1 ! 7 source in a Model D P 4 well-type detector. A stand ard 2 inch N a l ( T ) well crystal is used with background limited to approximately 40 cpm. If you would like full information on how to scan the energy band from top to bottom with the DZ46, write for technical bulletin 58-117. Long, but interesting.
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For further Information, circle number 57 A on Readers' Service Card, page 83 A
VOL. 30, NO. 3, MARCH 1958
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57 A
