Mercury standard, calibration

Calibration standards are made from a soluble mercury salt, such as, mercuric chloride. The standard solutions are analyzed first prior to the sample, following acid digestion, oxidation, and reduction, as described above. A standard calibration curve is constructed by plotting absorbance vs. concentrations of Hg (or mg Hg). The concentration of Hg in the sample is then determined by comparing the absorbance with that in the calibration curve. 

Experimental. A Parr model 1221 oxygen bomb calorimeter was modified for isothermal operation and to ensure solution of nitrogen oxides (2). The space between the water jacket and the case was filled with vermiculite (exploded mica) to improve insulation. A flexible 1000-watt heater (Cenco No. 16565-3) was bent in the form of a circle to fit just within the jacket about 1 cm. above the bottom. Heater ends were soldered through the orifices left by removing the hot and cold water valves. A copper-constantan thermocouple and a precision platinum resistance thermometer (Minco model S37-2) were calibrated by comparison with a National Bureau of Standards-calibrated Leeds and Northrup model 8164 platinum resistance thermometer. The thermometer was used to sense the temperature within the calorimeter bucket the thermocouple sensed the jacket temperature. A mercury-in-glass thermoregulator (Philadelphia Scientific Glass model CE-712) was used to control the jacket temperature. 

The vacuum line used in this synthesis is a standard, calibrated, high vacuum system equipped with a two stage mechanical forepump and a dual stage mercury diffusion pump. The operation pressure in this system is 10 torr or lower. ... 

All samples are analysed for Cd, Pb, Cr against standard calibration curves prepared from 0.0, 0.5, 2.5, 5.0 and lO.Oppm of each metal in 0.25 M HNO3. The ultrasonic nebuliser is used for the determination of Cd, Pb and Cr while the continuous cold vapour trap method is used for the determination of Hg. The recovery of each metal is determined for each metal. The Hg forms the vapour ion of the metal in solution after reduction with SnCl2(Sn2+ + Hg2+ > Sn4+ + Hg°) and the metallic mercury is swept to the plasma torch by the argon gas. This method is sensitive for Hg and has the advantage that it removes the analyte from the main solution and has very low limits of detection. 

Figure 5.13. Determination of chloride by the mercury thiocyanate method with the hydrodynamic injection manifold in Fig. 5.12a, where C was 100 cm long and the volume of conduit L was 25 xL. The pumping rate x = z was 1.1 mL/min and the aspiration rate y was 3.0 mL/min, operated for 12 s. The detector was tuned at 490 nm. (a) Standard calibration run of samples in the range 10-50 ppm Cl (b) stopped-flow experiment with the 40 ppm Cl standard recorded at high paper speed to demonstrate the fast rate of reaction and (c) monitoring of the content of chloride in a solution of NaCl in which the analyte concentration was changed intermittently and measured at fixed time intervals by the system.

A calibration factor (f) is obtained by repeated analyses of a pre-purged ordinary seawater sample which has been spiked with increasing amounts of mercury standard solution. The concentrations of the spiked seawater, c.g., 0.2, 0.5 and l.Ong/L of Hg, should cover the concentration range expected in the field. Between runs with spiked samples, the system blanks should be established, t.e., by applying the analytical procedure without any addition of mercury. 

GC analysis of samples is accomplished by measuring peak heights from lOyl sample injections and obtaining the concentration from standard calibration curves prepared daily. Relevant equations illustrating the use of 203Hg recovery data for calculating the amount of mercury in a sample are ... 

Examples of Newtonian fluids include water, mercury, treacle, tar, mineral oils, glycerol, sucrose solutions, standard calibration oils (e.g. octane), milk, fruit juices, and honey. Fluids that do not fall into this classification are known as non-Newtonian fluids. 

From the ventilation point of view, the fixed points -38.83 °C (triple-point of mercury), 0.010 °C (triple-point of water), 29.76 °C (melting point of gallium), and 156.60 °C (freezing point of indium) are of relevance. The triple-point of water is relatively simple to achieve and maintain with a triple-point apparatus. Some freezing point cells are covered in standards. In practical temperature calibration of measuring instruments, the lTS-90 fixed points are not used directly. 

Bond et al. [791 ] studied strategies for trace metal determination in seawater by ASV using a computerised multi-time domain measurement method. A microcomputer-based system allowed the reliability of the determination of trace amounts of metals to be estimated. Peak height, width, and potential were measured as a function of time and concentration to construct the database. Measurements were made with a potentiostat polarographic analyser connected to the microcomputer and a hanging drop mercury electrode. The presence of surfactants, which presented a matrix problem, was detected via time domain dependent results and nonlinearity of the calibration. A decision to pretreat the samples could then be made. In the presence of surfactants, neither a direct calibration mode nor a linear standard addition method yielded precise data. Alternative ways to eliminate the interferences based either on theoretical considerations or destruction of the matrix needed to be considered. 

Graphite furnace atomic absorption spectrophotometry has been used for the determination down to 5 ng/1 inorganic and organic mercury in seawater [61]. The method used a preliminary preconcentration of mercury using the ammonium pyrrolidine dithiocarbamate-chloroform system. A recovery of 85 - 86% of mercury was reproducibly obtained in the first chloroform extract and consequently it was possible to calibrate the method on this basis. A standard deviation of 2.6% was obtained on a seawater sample containing 529 ng/1 mercury. 

To calibrate an instrument such as a thermometer, we correlate a physico-chemical property (such as the length / of the mercury) using the temperature-dependent properties of a known standard. 

A mercury-based maximum-minimum thermometer may be used to monitor temperatures during periods when the laboratory is not staffed. Large-volume refrigerators/freezers can be monitored with an upper- and lower-limit alarm system or a recording thermometer. Ah thermometers are to be calibrated with an appropriate National Inshtute of Standards and Technology (NIST) thermometer when put in service and every 6 mo thereafter. 

The interface provides efficient transfer of samples into the Merlin, and, most importantly, a rapid flush-out there is no hold up of mercury (which is a feature of the commonly used atomic absorption techniques). To aid the transfer of mercury vapour, the tin(II) chloride regime is used, together with a gas/liquid separator designed for this task. Mercury is sparged from the reaction vessel into the Merlin Detector. Full automation is provided by using a simple standard DIO card fitted into an IBM compatible computer system with the PSA Touchstone software. This is an easy-to-use menu-driven system which controls the modules used in the instrumentation, calibrates the system, collects, collates and reprints the results, and which finks to host computer systems. 

Several types of electrodes have been successfully used with electrochemical detectors including carbon-paste, glassy carbon, platinum, mercury, and gold. Most widely used is the carbon-paste electrode, which is inexpensive and has low residual current. All analytes drat oxidize or reduce at the selected electrode potential can be detected. However, tliis electrode must be frequently standardized to maintain precise calibration because of changes that occur in its surface (128). 

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