Instruments osmometers

For a typical experiment, a series of solutions with concentrations between 1 and 10 g/L is prepared. Solutions and pure solvent, which defines the baseline, are injected alternately into the solution chamber of the osmometer and enough time is given for the pressure to equilibrate. Equilibrium is reached after seconds to days, depending on the instrument, the membrane, and the sample. 

Membrane osmometry measurements were carried out with the capillary osmometer shown in Figure 3. Owing to the short equilibration time of the instrument and the low cut-off molar mass of the membrane, solute permeation through the membrane, which would show up as a drift of the baseline, did not cause problems even for the lowest molar mass fraction. M was obtained from... 

For example, the correction is often negligible in a Zimm-Myerson osmometer, but considerable in a Fuoss-Mead instrument. 

The collected urine is centrifuged to remove solid debris and analyzed by standard methods for sodium, potassium and chloride (Durst and Siggard-Andersen, 1999, Scott et al. 1999). Osmolality is also measured with an osmometer (the freezing point depression type of instrument is recommended (Scott et al. 1999). 

Procedure Following the manufacturer s instructions, balance the osmometer to zero with o-dichlorobenzene on both thermistor beads, and establish the calibration constant, K, at 100°, using the four Calibration Standards. When the temperature within the osmometer has re-equilibrated to 100°, place an aliquot of the most concentrated Sample Preparation on the sample thermistor bead. After 4.0 min, balance the instrument to zero with the potentiometer, and record the AR value. Repeat this procedure with the same Sample Preparation two or three times, and average the AR values for that concentration. In a similar manner, obtain the average AR values for each of the other three concentrations of the Sample Preparation. Plot the four average AR values for the Sample Preparations as a function of AR/concentration, and extrapolate the line to zero to obtain the constant, Kjj, for the sample. Divide K by Kv to obtain the molecular weight of the sample tested. 

Vapor phase osmometers differ in design details. The most reliable instruments appear to be those which incorporate platinum gauzes on the thermistors in order to ensure reproducible solvent and solution drop sizes. In any case, the highest purity solvents should be used with this technique, to ensure a reasonably fast approach to steady state conditions. 

An osmometer is an instrument which measures the osmolality of a solution, usually by determining the freezing point depression of the solution in relation to pure water, a technique known as cryoscopic osmometry. A small amount of sample is cooled rapidly and then brought to the freezing point (Fig. 6.1), which is measured by a temperature-sensitive thermistor probe calibrated in mosmol kg . An alternative method is used in vapour pressure osmometry, which measures the relative decrease in the vapour pressure produced in the gas phase when a small sample of the solution is equilibrated within a chamber. 

The instrument used is a freezing point depression osmome-ter but it is often referred to simply as an osmometer. The... 

Day-to-day precision of 2mOsmol/kg H2O should be obtainable by todays osmometers. More than 98% of the laboratories in the 2003 CAP surveys use freezing point depression osmometers, with the automated instrument from Advanced Instruments (Norwood, Mass.) being the most common. ... 

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. 

All chemistries were determined by the clinical laboratory at the University of Utah Medical Center. A detailed error analysis shows the range of errors for clearance value to be between 10% at the start of dialysis and 15% at the end of dialysis. Oncotic pressures were measured with a colloid osmometer (Model 186, Instrumentation Laboratories, Boston MA) to 0.2 mm Hg. 

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. 

Physical and Chemical Analysis. Elemental analyses were done by Elek Microanalytical Laboratories, Torrance, California. Molecular weights were determined on a Mechrolab Model 301A Vapor Pressure Osmometer using THF as the solvent at 40°C. IR spectra were recorded at a concentration of 25 mg/ml in CH Cl using 0.5 mm NaCl cells on a Beckman Acculab 6 instrument. Proton and carbon-13 NMR spectra were obtained on a Varian XL-100 spectrometer operating at 100.1 MHz for proton or 25.2 MHz for carbon. Proton NMR was measured in CD Cl (central residue peak at 5.3 ppm). Carbon-13 NMR was measured in CDCl (central peak at 77.1 ppm). A sample of asphaltene (0.5 g) was dissolved in 2.5 ml of CDCl with 35 mg of Cr(acac) added to it. To obtain reliable quantitative results, a delay time of 4 sec after each 35° pulse and 0.68 sec acquisition time was used in the gated decoupling sequence. All chemical shifts were reported in ppm downfield from TMS. 

Osmometric measurements were carried out Melabs Mod. CSM-2 membrane osmometer following procedures. Details of the instrumentation and... 

Osmometer An instrument for determining the osmotic pressure exerted by solvent molecules diffusing through a semipermeable membrane in contact with a solution or hydrophilic colloidal dispersion. See also Colloid Osmotic Pressure, Osmotic Pressure. 

Particle Counting and Identification Electrochemical Methods Ion-Specific Electrodes Radioactive Methods Coagulation Timers Osmometers Automation Trends in Laboratory Instrumentation Defining Terms References... 

Laboratory designed instruments were developed in the 40 s and 50 s, e.g. by Zimm or by Flory. Later on, high speed membrane osmometers are commercially available, e.g., from Knauer, Hewlett-Packard or Wescan Instruments. External pressures may be applied to balance the osmotic pressure if necessary, e.g., Vink. The principle scheme of a membrane osmometer together with the corresponding... 

Figure 2.4 Schematic diagram of a membrane osmometer. In a practical instrument, it is essential to ensure that the semi-permeable membrane is supported to prevent bulging under...

There are several drawbacks to this approach. Diffusion of solvent dilutes the solution and it can often take many hours to achieve equilibrium. While this is undesirable, it becomes more significant as the longer the time it takes to achieve equilibrium, the more likely it is that solute molecules will find their way across the membrane into the pure solvent. As will be seen later, even 1% of solute finding its way across the membrane in the wrong direction can introduce a substantial error in the measured value of the molecular weight. Modern osmometers aim to prevent the diffusion of any solvent across the membrane by applying an appropriate pressure to the solution compartment of the osmometer. The instruments depend on the rapid detection of solvent... 

In Figure 2.6, the movement of solvent is detected by the movement of a bubble in a very narrow tube attached to the solvent compartment of the osmometer. When the photocell detects movement of the bubble, an external head of solvent is established by the raising of the solvent reservoir. In Figure 2.7, a capacitance device is used to detect solvent movement across the membrane. The measured signal is used to generate an appropriate external solvent head equivalent to the osmotic pressure. Modern instrumentation, once set up properly, can reduce the measurement time to a matter of minutes thus reducing the errors which are found if the membranes are anything less than truly semi-permeable. 

Vapor-pressure osmometer n. An instrument for determining number-average molecular weight of a polymer utilizing the vapor pressure versus molecular weight relationship. ... 

Dynamic mechanical properties of the graft copolymer were studied by using a NB-1 Torsional Braid Instrument. Number average molecular weight of the copol3nner was determined by a Bruss Membrane Osmometer. 

Number-average molecular weights ("Rn) of several of the polysulfone oligomers were measured using chloroform solutions at 40 C with a Wescan model 233 vapor phase osmometer. Sucrose octaacetate was used to calibrate the instrument. 

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