Calibration conductivity cells

In the dynamic method the powder is flushed with an inert gas during degassing, nitrogen is then adsorbed on the powder in a carrier of helium gas at Known relative pressure while the powder is in a container surrounded by liquid nitrogen. The changing concentration of nitrogen is measured by a calibrated conductivity cell so that the amount adsorbed can be determined. 

Standard KCl Solutions for Calibrating Conductivity Cells Molar Conductivity of Aqueous HF, HCl, HBr, and HI Equivalent Conductivity of Electrolytes in Aqueous Solution Ionic Conductivity and Diffusion at Infinite Dilution Activity Coefficients of Acids, Bases, and Salts... 

In fact, one can construct conductance cells with accurately known values of l and A in which the conductivity of standard electrolytes can be calibrated however, in analytical practice cells with less restricted shape requirements are applied as their cell constants can be adequately established by measurements on a standard electrolyte (generally KC1) of known conductivity (see Fig. 2.3). Suppliers of commercial cells usually give the cell constants. 

For an evaluation of measurements the conductivity cell is calibrated with a solution of a known conductivity. Both the electrolyte and the solvent must be carefully purified. 

Samples and standards are injected into the IC in 100-yL aliquots. The ions are separated by their varying affinites for the ion exchange resin in the anion separator column, the oppositely charged ions are stripped away by the suppressor column leaving only the separated anions and water to be detected by the conductivity cell. Samples are quantitated by comparison with a calibration curve. 

Reagent grade anhydrous lithium bromide powder (Matheson, Coleman and Bell) was used after drying in a vacuum oven at 100°-120°C. Potassium chloride (Fisher certified ACS) was dried at 110°C and used to calibrate the conductance cell. Deionized water from the laboratory supply was piped directly into a Pyrex glass still (Corning model AG-la) and distilled into a polyethylene vessel, where it was kept until needed. The specific conductance of the water was 1 X 10 6 12 1 cm-1. 

Flow controllers utilizing conductivity cells can address the problem of variable work flows. Water flows only when needed, and only to a set value of contamination (cleanliness) thus reducing the volume of water requiring management or treatment. Calibration to this set value is a critical activity, and can be confounded by rinsing more than one type of process solution in the same rinse system. The equipment also requires diligent maintenance, since the probes can become coated, and must be cleaned regularly. Installation should account for this eventuality.[19]... 

Conductivity Meter. A second system utilizes a simple ohmmeter circuit, shown in Fig. 3. A meter, transformer secondary winding, and conductivity cell are connected in series so that the current is a function of the cell conductance. The meter may be calibrated in resistivity or conductivity units. 

The catalyst compositions were determined by chemical analysis at the Central Service of Chemical Analysis of the CNRS (Lyon) except for the samples Aj and A2 which were analyzed by Ugicarb Morgon. The analytical method for carbon has been described previously.8 The total amount of carbon was obtained from the combustion of the sample with oxygen. Carbon dioxide was then quantitatively detected by a calibrated thermal conductivity cell. For the polymeric carbon content, the sample was attacked by a hot mixture of nitric and hydrofluoric acids which dissolved every component except for the non-combined carbon. Then, this remaining carbon was transformed into C02 which was analyzed with an electrochemical titration cell. 

The timescale over which the conductance of the medium changes is a fundamental issue standard conductivity cells are designed for use with alternating current (AC), but the period of this current imposes a limit on rates of reactions that can be followed. To investigate reactions faster than the AC conductivity cell can handle, it is necessary to build and calibrate appropriate direct current (DC) conductivity cells, which is not a routine business. Conductivity meters that record continuously are uncommon. Nowadays, however, it is easy to interface a simple apparatus to a computer and collect the data with ad hoc software. 

Most conductance cells do not have uniform cross-sectional area, which requires that the cell constant be determined by calibration with solutions of known conductivity. The cell constant, HA, may be thought of as the ratio of effective length and cross-sectional area of the conducting path. Some values of the conductivity standards are tabulated in Table 6.6. 

SEC is measured with a conductivity meter, which normally consists of an AC bridge and a conductivity cell or electrodes. The conductance is measured between two electrodes. Two solutions of known conductivity should be used, one to calibrate the metre and the other to check the slope. It is important to correct all data for water temperature, either by calculation or by automatically using the metre s auto-temperature correction mode, since SEC is highly dependent on temperature. SEC increases by about 2% per degree centigrade rise in temperature due principally to an increase in water viscosity. 

The TPR experiments were performed in 5% H Ar gaseous mixture at 20 cm3/min. Samples were heated at 5 K/min within the temperature range of 298-1073 K. Prior to TPR experiments, samples were treated in situ with air at 723 K for 2 h, The TPD runs were carried out in the same apparatus. Catalysts were reduced in H2 at different temperatures for 1 h. After cooling the reactor down to room temperature, samples were heated under Ar at 5 K/min from 298 up to 873 K The amounts of H2 desorbed were calculated from the integrated peaks areas measured by a heat conductivity cell, which was calibrated by injection of known volumes of H2 into the carrier gas. 

Conductivity. The conductivity bridge has been described (6). A 0.01000M standard KC1 solution was used to calibrate the Beckman conductivity cell. A value of 0.488 cm"1 was obtained for the cell constant. Measurements were made at 30°C on 50 ml of 0.1-0.5M acid solutions in sulfolane with successive additions of water or ether from a 2-ml Teflon glass syringe. All acid solutions were processed in a glove box. 

In the exchange reactions between HCI-D2 or DCI-H2, the amount of D in the hydrogen produced was followed with calibrated thermal conductivity cells of both the linear and coil type (this permits distinction between H2-D2 and H2-HD-D2 mixtures). These experiments showed that during the initial stages of the reaction only HD was formed. Analysis of these results indicated that for the reactions... 

As suggested in Example 29-4, urea can be determined by measuring the initial rate of production of the products of this reaction. The high selectivity of the enzyme permits the use of nonselective detection methods, such as electrical conductance, for initial rate measurements. There are commercial instruments that operate on this principle. The sample is mixed with a small amount of an enzyme-buffer solution in a conductivity cell. The maximum rate of increase in conductance is measured within 10 s of mixing, and the concentration of urea is determined from a calibration curve consisting of a plot of maximum initial rate as a function of urea concentration. The precision of the instrument is on the order of 2% to 5% for concentrations in the physiological range of 2 to 10 mM. 

In general, the measurement of electrical conductivity is not an absolute method and the conductance cell used has to be calibrated using the molten salt with a known conductivity. When the calibration of the cell is done at ambient temperatures, the accurate resistance measurement requires correction for the change in the resistance cell capacity with temperature. The total accuracy of measurement is then affected by knowing the coefficient of thermal dilatation of the material used from which the cell was made. 

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