NO CHARGE? no problem for Zetasizer II In studies of flocculation and colloid stability, the region close to the zero point of charge is the most critical. It is in precisely this region of low mobility that other electrophoresis systems are least accurate and most difficult to use.
With its high frequency optical modulator, the Malvern Zetasizer II solves this problem, allowing you to measure mobility and zeta potential with uncompromising accuracy right across the range. Measurements are simple, fast and operator-independent and Zetasizer II also gives you built-in submicron particle size capability.
Learn more about the potential of Zetasizer II in your laboratory.
MALVERN Malvern Instruments Inc 187 Oaks Road Framingham MA 01701, USA Telephone: (617) 626 0200 Telex: 311397
Ti 47.88
?
• It is exceedingly unlikely that any randomly selected shelf sample, mineral specimen, or industrial product will have an atomic weight outside the range indicated in the IUPAC table. (My personal interpretation of "exceedingly unlikely" is less than 1 in 100,000.) When we talk of accuracies, we refer to the difference between a measured and a "true" value. The concept of a "true" atomic weight becomes diffuse for elements with variable composition. You never know whether you have found specimens exhibiting the maximum as well as the minimum of an atomic weight range. You also wonder whether you should average the known extremes for the standard value or look for the most probable value relative to laboratory samples, or for the most widely distributed natural sources. The problems are real, for instance, for hydrogen. Hydrogen atoms on the average are lightest in the laboratory, heavier in river water, and still heavier in ocean water, which should not be allowed to weight the average. In other cases, you probably would wish to take an average for the entire Earth. Given that we cannot sample the core, the uncertainty for the overall average would become far greater than if we sought to average over all reasonably deliverable specimens. Such considerations spoil the notion of a "true" atomic weight for polynuclidic elements. Atomic-weight limitations from experimental uncertainties Most annoying is the estimated uncertainty in standard atomic weights that is not caused by natural variability but by experimental uncertainty so large that it equals or overshadows the uncertainty of good chemical analytical measurements. These cases, in
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which the atomic-weight data limit the analytical accuracy, are shown in Table IV. Standard reference materials cannot help, and it is surprising that so little has been done to improve our knowledge. The common belief is that the IUPAC values are better than they are said to be, and chemists appear to rely on them blindly. During the five years that I was secretary of the commission, I became aware of only one challenge of a standard atomic weight (that of tin), and the basis for that challenge was questionable and has since, in the light of much more reliable measurements, been found to have been erroneous. However, there is in my view plenty of opportunity to question CAWIA and for young experimenters to generate new or better data, especially for the elements in Table IV. In this discussion, the critical question for these 21 elements is this: Are the uncertainties of their atomic weights really as large as indicated in Table IV? Perhaps CAWIA underestimates, as Clarke did earlier, experimental errors or faces future evidence of larger natural variabilities in as yet undiscovered sources. Or perhaps CAWIA is playing it safe by overestimating the uncertainties and ranges to keep the reliability of tabulated data inviolate. My own impression is that CAWIA
Table IV. Elements whose standard atomic weights are estimated to be uncertain by 0.015% or more arising from experimental uncertainty Element
Ti Ni Zn Ge Se Ru Sb Te Xe Nd Sm Gd Dy Er Yb Hf W Os Ir Pt Hg
Uncertainty
0.063% 0.017% 0.031% 0.041% 0.038% 0.020% 0.025% 0.024% 0.023% 0.021% 0.020% 0.019% 0.018% 0.018% 0.017% 0.017% 0.016% 0.053% 0.016% 0.015% 0.015%
