Ethylene glycol, temperature calibrations

Fig. 1 Polystyrene, PSt, and poly (ethylene glycol), PEG, calibration curves obtained on PL Gel column set (Mixed C, 10 A, 5 X 10 A, and 10 A). Eluent THE flow rate 1 mL/min temperature 40°C. Inset SEC elution profiles of poly(ethylene glycol)s (PEGs) at the same analysis conditions.

NMR Apparatus. NMR spectra were obtained on a Varian A-60 equipped with a Varian V-6040 variable temperature controller and probe. Scans were made at 7-second intervals. Care was taken to avoid radiofrequency saturation of the proton resonance. The temperatures at the beginning and end of the runs were determined in the standard manner from the chemical shift of the two ethylene glycol peaks. Calibration of the heating characteristics of the apparatus as a function of time, heater current, and temperature was carried out prior to the triglyceride spectra determinations with the ethylene glycol peaks. Knowing heater current and time elapsed from the onset of heating permitted temperatures to be determined to ztl°C. in 10 trial runs. 

Variable temperature measurements were carried out on a Varian A-60 spectrometer with a Varian-4060 probe heater attachment. Probe temperatures were calibrated with a standard ethylene glycol sample. Samples were prepared by dissolving up to 25 per cent by weight of polymer in tetrachloroethylene. Even with such concentrated solutions no viscosity problem was encountered above 60°. 

The Evans method gives excellent results provided adequate care is taken. A most important requirement is that the solution temperature is measured reliably. One effective means of accomplishing this for H NMR is to insert into the NMR tube a capillary or additional coaxial sample of an NMR temperature calibrant solvent, usually methanol (158) or ethylene glycol (88). In this way the temperature measurement is made simultaneously with the susceptibility measurement. A second important factor is the variation of the solvent density with temperature (126). Because the density difference between the solvent and solution depends linearly on the concentration of the solute, it is only... 

NMR spectrometer, with variable temperature capability if possible several 10- or 25-mL volumetric flasks NMR tubes melting-point capillaries syringe torch for sealing capillaries ethylene glycol and methanol in NMR tubes for temperature calibrations. 

For materials applications, the chemical shifts of methanol and ethylene glycol can be monitored in the liquid state to follow temperature [Hawl]. The most sensitive ehemical shift is the Co resonance of aqueous Co(CN)e with a sensitivity of 0.05 K at 7 T and 0.2 K at 2T [Dorl]. Furthermore, dibromomethane dissolved in a liquid crystal is a temperature sensitive NMR compound [Hed 1 ], and known phase-transition temperatures can be exploited to calibrate the temperature control unit [Hawl J. In temperature imaging of fluids, temperature can be determined from the temperature dependence of the selfdiffusion coefficient but convective motion may arise in temperature gradients [Hedl]. In the solid state, the longitudinal relaxation time of quadrupolar nuclei like Br is a temperature sensitive parameter [Suil, Sui2]. In elastomers, both T2 and Ti depend on temperature (Fig. 7.1.13). In filled SBR, T2 is the more sensitive parameter with a temperature coefficient of about 30 xs/K [Haul]. 

Table 2. Temperature calibrations for methanol and ethylene glycol...

Many spectrometers are equipped with facilities to monitor and regulate the temperature within a probe head. Usually the sensor takes the form of a thermocouple whose tip is placed close to the sample in the gas flow used to provide temperature regulation. However, the readings provided by these systems may not reflect the true temperature of the sample unless they have been subject to appropriate calibration. One approach to such calibration is to measure a specific NMR parameter that has a known temperature dependence to provide a more direct reading of sample temperature. Whilst numerous possibilities have been proposed as reference materials [41], two have become accepted as the standard temperature calibration samples for solution spectroscopy. These are methanol for the range 175-310 K and 1,2-ethanediol (ethylene glycol) for 300-400 K. 

Figure 3.65. Temperature calibration chart for neat Ethane-1,2-diol (ethylene glycol). The shift difference A8 (ppm) is measured between the CH2 and OH resonances.

Variable temperature was achieved using the standard spectrometer accessories. The temperature controller was calibrated using the proton shifts of ethylene glycol. The temperatures are considered accurate to 2. The samples were generally equilibrated in the probe at the desired temperature for 30-60 minutes prior to data accumulation. 

Fig. 4. Calibration curves for columns of PLaquagel-OH, designated 30 (A), 40 (A), 50 (O), 60 ( ), and Mixed ( ), for aqueous SEC at ambient temperature. Eluent water, flow rate 1 mL/min, calibrants poly(ethylene oxide/glycol) standards.

---