Vapor-pressure osmometer

Aero Hydrolysis. A solution of kasugamycin hydrochloride (1.5 grams, 3.46 mmoles) dissolved in 15 ml. of 6N hydrochloric acid was heated at 105°C. for five hours in a sealed tube. The solution was condensed to 5 ml. under a reduced pressure and the addition of 50 ml. of ethyl alcohol afforded a crude solid overnight. It was recrystallized from aqueous ethyl alcohol, showing m.p. 246°-247°C. (dec.). It showed no depression in the mixed-melting point and completely identical infrared spectrum with d-inositol which was supplied by L. Anderson of the University of Wisconsin. The yield was 81% (503 mg., 2.79 mmoles). Anal Calcd. for CgH12Og C, 40.00 H, 6.71 O, 53.29 mol. wt., 180.16. Found C, 40.11 H, 6.67 O, 53.33 mol. wt., 180 (vapor pressure osmometer). 

Total plasma protein concentration was determined by the buiret method (16) after correction of plasma hemoglobin which was determined by the cyanmethemoglobin method (15). Plasma osmolality was measured by a Wescor model 5100 vapor pressure osmometer. 

Number-average molecular weight was measured in 1,2-di-chloroethane at 40°C using a Wescan vapor pressure osmometer. 

Number-average molar masses were determined using a vapor pressure osmometer (VPO) (Hitachi 117 Molecular Weight Apparatus) at 54.8 0.1°C in toluene (Fisher Scientific, certified A.C.S.) which was distilled from freshly crushed CaH2. The VPO apparatus was calibrated with pentaerythritol tetrastearate (Pressure Chemical). Gel permeation chromatographic (GPC) analyses were performed in tetrahydrofuran by HPLC (Perkin-Elmer 601 HPLC) using six y-Styragel columns (106, 105, 10l, 103, 500, and 100 A) after calibration with standard polystyrene samples. 

Vapor Pressure Osmometry. Number-average molecular weights were evaluated with a vapor pressure osmometer (Knauer) following a previously described method (18). 

Number average molecular weight was determined in benzene solution by Hitachi 117 vapor pressure osmometer(VPO) at 42°C. 

There are many measurement techniques for activity coefficients. These include measuring the colligative property (osmotic coefficients) relationship, the junction potentials, the freezing point depression, or deviations from ideal solution theory of only one electrolyte. The osmotic coefficient method presented here can be used to determine activity coefficients of a 1 1 electrolyte in water. A vapor pressure osmometer (i.e., dew point osmometer) measures vapor pressure depression. 

The number average molecular weight was measured in benzene solution by Mechrolab membrane osmometer 502 and also by Hitachi vapor pressure osmometer 115. 

Apparatus Use a suitable vapor pressure osmometer, such as the Hewlett-Packard Model 302A, or equivalent, equipped with dual thermistor beads. 

Several vapor pressure osmometers are now commercially available. Although they are mainly used for determining number-average molecular weights in aqueous and organic solvents, they can also be employed to evaluate the total osmolality of biological solutions or dissociation and activity coefficients. Each model has its own technical characteristics. However, all are comparable in terms of general measurement procedure and sensitivity. 

Fig. 8.3.1. Simplified representation of the Knauer Model 11.00 Vapor pressure osmometer / Thermostat 2 measurement cell 3 lid 4 porous wicks 5 thermostated heating block 6 hypodermic syringes 7 bead thermistors 8 thermistor probe...

Brzezinski J, Glowala H, Kornas-Calka A (1973) Note on the molecular weight dependence of the calibration constant in vapor pressure osmometry Eur Polym J 9 1251-1253 Burge DE (1979) Calibration of vapor pressure osmometers for molecular weights measurements J Appl Polym Sci 24 293-299... 

Thioacetophenone polymerizes spontaneously to a white solid. Conversion to polymer is increased by irradiation with ultraviolet light Highest conversion reported is 74%. Polymerization is also brought about by such anionic initiators as AlEt2Cl, AlEtj, BF3 - OEt, and SnC. Molecular weights measured on tetra-hydrofuran or brazene solutions using a vapor-pressure osmometer were in the 1000-2000 range. These are minimum values, since the polymer decomposes readily. 

Number average molecular weights were determined using a Knauer Vapor Pressure Osmometer. 

Another type of osmometer is the vapor pressure osmometer. In reality, osmolality measurement in these instruments is not related directly to a change in vapor pressure (in millimeters of mercury), but to the decrease in the dew point temperature of tlie pure solvent (water) caused by the decrease in vapor pressure of the solvent by the solutes. In this instrument, temperature is measured by means of a thermocouple, which is a device consisting of two dissimilar metals joined so that a voltage difference generated between the points of contact (junctions) is a measure of the temperature difference between the points. 

In clinical laboratories, the vapor pressure osmolality technique has been reported to be less precise than the freezing point depression method. For serum samples, the coefficients of variation obtained for the vapor pressure osmometer are about twice those obtained for the freezing point depression osmometer. The lesser degree of precision is related to the lower slope of dew point decrease compared with freezing point decrease (i.e., 0.303 °C versus 1.86 °C per osmol/kg H2O). 

An important clinical difference between the vapor pressure technique and the freezing point depression osmometer is the failure of the former to include in its measurement of total osmolality any volatile solutes present in the serum. Substances such as ethanol, methanol, and iso-propanol are volatile, and thus escape from the solution and increase the vapor pressure instead of lowering the vapor pressure of the solvent (water). This makes use of vapor pressure osmometers impractical for identifying osmolal gaps in acid-base disturbances (see Chapter 46). Thus use of this type of osmometer cannot be recommended for most clinical laboratories. 

For samples with higher molecular weight (up to 3000 or more) and unusual composition or for polymers, another method (ASTM D-2503) is recommended. This method uses a vapor pressure osmometer to determine molecular weight. Low-boiling samples may not be suitable if their vapor pressure interferes with the method. 

Molecular weight measurements on the polymers were made using a Knauer vapor pressure osmometer (Utopia Instrument Co.) with pyridine as the solvent at 60°C. Additional measurements with a similar osmometer were made by Galbraith Laboratories, Inc. using chloroform as the solvent at 45°C, the osmometer calibration factor having been obtained with phenacetln. 

In the vapor-pressure osmometer, there is no membrane. Two matched thermistors are located in a thermostated chamber that is saturated with solvent vapor (Fig. 4.8). With a hypodermic syringe a drop of solution is placed on one thermistor and a drop of solvent on the other thermistor. The solution has a lower vapor pressure than the solvent at the same temperature, and so the solvent vapor condenses on the solution droplet. The solution droplet, therefore, starts getting diluted as well as heated up by the latent heat of condensation of solvent condensing on it. Owing to the temperature rise and increased dilution, the vapor pressure of the solution droplet increases steadily. The process of condensation and the resultant temperature rise continues till the vapor pressure of the solution droplet at the new elevated temperature becomes equal to that of the pure solvent at the original temperature. In a steady state, the total rise in temperature AT can be related by an analog of Eq. (4.67) ... 

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