Carbon dioxide, automatic determination

During the combustion the rate of flow of bubbles in the counter often increases rather appreciably because the rapid absorption in the sodium hydroxide-asbestos tube of the carbon dioxide produced causes a slight decrease in the pressure. If this happens reduce the rate of dropping of water from the aspirator or even stop it altogether temporarily. No harm is done by this decrease in pressure. Should there be any small leaks at the constricted parts of the tube there would, it is true, be a danger that air might be sucked in. In view of the short duration of the reduction in pressure, however, the errors caused would have no appreciable effect on the determination. Obviously no adjustment of the flow from the aspirator must be made while the reduced pressure prevails. The former rate of dropping is automatically restored. 

It is not-usually necessary to determine the carbon dioxide in beer, since the external characters of the latter generally indicate if the gas is present in sufficient quantity. When, however, the determination is necessary, loss of gas during the extraction of the beer must be avoided. To this end, the vessels may be well cooled before they are opened, or special automatic extraction apparatus may be employed by means of which the beer is transferred directly, without loss of carbon dioxide, from the cask or bottle to the flask used in the determination. 

A number of instrument manufacturers offer automatic coulometric titrators, most of which employ a potentiometric end point. Some of these instruments are multipurpose and can be used for the determination of a variety of species. Others are designed for a single type of analysis. Examples of the latter are chloride titrators, in which silver ion is generated coulometrically sulfur dioxide monitors, where anodically generated bromine oxidizes the analyte to sulfate ions carbon dioxide monitors, in which the gas, absorbed in monoethanolamine, is titrated with coulometrically generated base and water titrators, in which Karl Fischer reagent (see Section 20C-5) is generated electrolytically. 

Carbon and hydrogen are commonly determined by combustion analysis in which the sample is burned in an oxygen stream where carbon is converted to carbon dioxide and hydrogen to water. These compounds are absorbed, and the composition is determined automatically from mass increase (ASTM D-5291). Nitrogen may be determined simultaneously. 

Figure 33-20 is a schematic of a commercial automatic instrument for the determination of carbon, hydrogen, and nitrogen. In this instrument, samples are oxidized at 9(K) C under static conditions in a pure oxygen environment that produces a gaseous mixture of carbon dioxide, carbon monoxide, water, clemenlal... 

In principle, any metabolic process that terminates in the production of ammonia or carbon dioxide can be similarly quantified in a continuous, automatic, incubation-quenching technique. Already some progress has been made with creatinine, another nitrogenous terminal metabolite, using an ammonia gas electrode [376], while glutaminase activity (arising from isoenzymes present in rat tissues and tumours) may be similarly determined [378]. Urea and tyrosine can be assayed [367] by using the appropriate decarboxylase and a carbon dioxide gas electrode. 

A specific-ion meter is capable of direct potentiometric determinations of electroactive species using pH, redox, or various ion-selective electrodes (i.e., chlorine, calcium, nitrate, ammonia/carbon dioxide, copper, and halides). Microprocessor control allows for instrument calculation of analyte levels by known additions, standard additions, or activity. An automatic temperature-compensation feature helps to reduce analytical errors. 

The PE 2400 CHN Analyzer (Fig. 4) can be modified to perform automatic determination of oxygen. The combustion tube is filled with platinized carbon reagent. The samples are pyrolyzed in an inert atmosphere of argon or helium. The reaction product CO is separated by frontal chromatography and measured by thermal conductivity. In the Leco CHN Analyzer, carbon monoxide is converted to carbon dioxide to be measured by infrared absorption. Neither CHN method is suitable for the analysis of organic substances which contain fluorine, phosphorus, silicon, or most metallic elements. 

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