Six commercial gum arabic samples and authenticated samples of gum from Acacia Senegal and Acacia seyal have been characterized using a range of chemical and physicochemical techniques including determination of specific rotation, sugar composition, nitrogen and amino acid content, and molecular mass distribution. Although some of the gums have slightly different chemical and physicochemical characteristics, gel permeation chromatography shows that each consists of essentially three molecular mass fractions classified as an arabinogalactan, an arabinogalactan-protein complex, and a glycoprotein.The proportions of each varied for the individual samples. The gums were tested using a specifically developed enzyme-linked immunosorbent assay which can identify gum from A. Senegal and chemically related species. Their interaction with the antibody could be correlated with differences observed in their molecular compositions. Such evidence will assist in their chemotaxonomic classification.
sixteen of the alloys investigated a transformation point was noted (in addition to the three freezing points) at a fixed temperature, approximately 124~. In every instance observed the transformation took place below the temperature a t which binary eutectic had begun freezing. The phenomenon was not noticed early in the investigation, and the data at hand are not sufficient to enable me to fix the limits of composition within which the change takes place. If the halt is caused by the formation in the partly-solid alloy of a binary or ternary compound, the extent to which this compound may form mix-crystals with the three pure metals will decide whether it is necessary to introduce another surfaceroughly triangular in outline-at the point now assigned to the ternary eutectic. I intend to resume the investigation of the transformation a t an early date, and ask for a reasonable time reservation for this purpose.
to correct for the difference in atomic weight of atmospheric oxygen and aqueous oxygen, the density of D-free water containing normal oxygen is 15.4 p. p. m. lighter than normal water. From this the ratio of deuterium atoms to hydrogen atoms in Lake Michigan water is calculated to be 1 : 6900. Our density value of -15.4 y agrees within 1.1 p. p. m. with that of Hall and Jones, -16.5 y, and confirms their belief that the commonly accepted value for the D/H ratio is too high and should be revised downward. Our result indicates, however, that the downward revision should be somewhat more than that recommended by Hall and Jones.Isotopic exchange equilibrium constants involving Dz016 and Dz0'8 have been calculated and tabulated.Published measurements of solubility of salts in deuterium water' hitherto have been limited to a few determinations near room temperature. The present work is one of a series of studies over a wide temperature range undertaken in the hope of gaining insight into portions of the general subject of aqueous solubility.Method and Materials.-For all our experiments with deuterium water, and for most of those with ordinary water, the method used was that described by Meozies.2 A few experiments requiring much ordinary water in relation to the quantity of solute were carried out by the older synthetic method, where each experiment demands a separate sealed tube. Our method of securing and measuring steady temperatures has already been described;* and it may be added that about one-half of the measurements were carried out in the jacketed air-oven referred to in the earlier publication.The deuterium water was similar to that described elsewhere,' with a content of 98.2% DzO. The relative density of this water compared to ordinary water a t 25'ewas taken as 1.1059 for the purposes of conversion of measured volumes to masses. In order to apply the small correction for the weight of water vapor present in the gas phase, values of the vapor pressure of saturated solutions of potassium chloride were derived, by extrapolation when necessary, from the work of Leopold and Johnston;' and these pressure values could, with adequate approximation, be adapted to suit the case of deuterium water solutions from a knowledge of solubility and of the vapor pressures of deuterium water relative to that of ordinary water.8 Potassium chloride is easily obtained of highest purity, and we estimate the impurity of our sample, other than (1) Cf. Waterwater, as less than five parts per 10,000. Water was removed by slow heating to incipient fusion.6Manner of Stating Solubility.-The solubility of potassium chloride in common water is reported in moles of solute per 1000 g. of water (55.51 moles HoO), conformably to the practice of "International Critical Tables." To secure a comparable statement of solubility in deuterium water, we state the number of moles of solute per 1111.7 g. of solvent, which is 55.51 moles of D20. To arrive at this value from experiments made with 98.2% DzO, we first computed, from the weights of solvent...
The method here described is a comparative one because the aqueous pressure over an aqueous sulfuric acid solution, whose concentration can later be found, becomes equal at the same temperature to the aqueous pressure of the system under investigation. Since the variation of aqueous pressure with temperature of the two systems thus under comparison is, fortunately, very similar, refined temperature control is not a necessity. Measurements may therefore be made with ease up to temperatures well over 100°C., at which close control of temperature over lengthy periods has proved difficult to most workers. It is in part due to this difficulty that records of measurements of dissociation pressures of salt hydrate systems are scanty as well as notoriously discordant for temperatures much above room temperature, and we believe that this situation can be remedied by use of the method here described. It is essential to realize, however, that in measurements of dissociation pressures, there enters at the lower temperatures another little understood source of error which must be avoided, especially in dealing with systems of lower hydrates which of necessity exliibit very large surface areas when prepared by efflorescence from higher hydrates. The errors in question have been attributed to adsorbed water (23) and are here further discussed in the light of experimental results which we believe are little vitiated by the better recognized sources of error.We shall show below the application of our procedure to the measurement of the aqueous pressures of saturated solutions and to the dissociation pressures of salt hydrate systems. COMMENTS UPON CERTAIN OTHER METHODSThe first three methods mentioned below, because absolute rather than relative or comparative, require accurate temperature control, which is the more difficult the higher the temperature.1 From a thesis presented by Edward M. Collins to the Faculty of Princeton University in partial fulfillment of the requirements for the degree of Doctor of Philosophy, January, 1933.
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