Dissolved carbon dioxide measurements

Different enzymes exhibit different specific activities and turnover numbers. The specific activity is a measure of enzyme purity and is defined as the number of enzyme units per milligram of protein. During the purification of an enzyme, the specific activity increases, and it reaches its maximum when the enzyme is in the pure state. The turnover number of an enzyme is the maximal number of moles of substrate hydrolyzed per mole of enzyme per unit time [63], For example, carbonic anhydrase, found in red blood cells, is a very active enzyme with a turnover number of 36 X 106/min per enzyme molecule. It catalyzes a very important reaction of reversible hydration of dissolved carbon dioxide in blood to form carbonic acid [57, p. 220],... 

Clinical chemistry, particularly the determination of the biologically relevant electrolytes in physiological fluids, remains the key area of ISEs application [15], as billions of routine measurements with ISEs are performed each year all over the world [16], The concentration ranges for the most important physiological ions detectable in blood fluids with polymeric ISEs are shown in Table 4.1. Sensors for pH and for ionized calcium, potassium and sodium are approved by the International Federation of Clinical Chemistry (IFCC) and implemented into commercially available clinical analyzers [17], Moreover, magnesium, lithium, and chloride ions are also widely detected by corresponding ISEs in blood liquids, urine, hemodialysis solutions, and elsewhere. Sensors for the determination of physiologically relevant polyions (heparin and protamine), dissolved carbon dioxide, phosphates, and other blood analytes, intensively studied over the years, are on their way to replace less reliable and/or awkward analytical procedures for blood analysis (see below). 

Lehaitre et al. [23] have described a fibre-optic spectrophotometric method for the in situ measurement of biological and chemical species. This instrument can measure phytoplanktonic species, and the potential for chemical measurement such as dissolved carbon dioxide is analysed. 

The difference in the conductivity of the calibration buffers and sample can cause a very large error on the sample measurement, due to junction potentials in different environments. Solid samples should be dissolved in purified water. It is necessary that the water be carbon dioxide-free. The presence of dissolved carbon dioxide will cause significant bias in the measurement of samples with low buffering capacity. For pH measurements with an accuracy of 0.01 to 0.1 pH unit, the limiting factor is often the electrochemical system (i.e., the characteristics of the electrodes and the solution in which they are immersed). 

Standard addition. A particular COz compound electrode like the one in Figure 15-22 obeys the equation E = constant — [p/ T (In 10)/2F log[C02], where R is the gas constant, T is temperature (303.15 K), F is the Faraday constant, and P = 0.933 (measured from a calibration curve). [C02] represents all forms of dissolved carbon dioxide at the pH of the experiment, which was 5.0. Standard additions of volume Vs containing a standard concentration cs = 0.020 0 M NaHC03 were made to an unknown solution whose initial volume was V0 = 55.0 mL. 

Figure 10.11 shows the results of measuring the pH of a selection of liquids and beverages. Fresh lemon juice has a pH of 2.5, corresponding to an H30+ molarity of 3 X 10-3 mol-L 1. Natural rain, with an acidity due largely to dissolved carbon dioxide, has a pH of about 5.7. 

Pattison RN, Swamy J, Mendenhall B, Hwang C, Frohlich BT (2000), Measurement and control of dissolved carbon dioxide in mammalian cell culture processes using an in situ fiber optic chemical sensor, Biotechnol. Prog. 16 69-74. 

When preparing the solutions the conductance of which is to be measured, water of the maximum possible purity is used. Absolutely pure water has a specific conductance of about 5 x 10-8 mho. As measurement then has to be undertaken in the absence of air, such water is used for the most accurate work only. So called conductance water meets the requirements of general practice it is prepared by a redistillation of normal distilled water with a small addition of permanganate and sodium hydroxide in a hard glass apparatus. Its higher conductance of some 0,3 to 3,0 x 10 mho is mainly due to dissolved carbon dioxide. 

Dissolving 3.00 g of an impure sample of calcium carbonate in hydrochloric acid produced 0.656 L of carbon dioxide (measured at 20.0°C and 792 mmHg). Calculate the percent by mass of calcium carbonate in the sample. 

Magnesium oxide has very limited solubility in water. However, an accurate determination is complicated by trace quantities of dissolved carbon dioxide in the water and the purity of the sample, where the presence of lime can introduce error. Another complicating factor is the source of the periclase and what heat treatment it had received. All these factors influence the rate at which equilibrium is reached in solution and hence solubility. Many solubility measurements have been made that have produced a wide variation in results. The most accurate determination to date has produced a result of 8.6 mg/L at 30°C. Caustic-calcined magnesia readily hydrates to the hydroxide on exposure to moisture. 

The case is obvionsly similar for dissolved carbon dioxide whose concentration is essentially determined by the equilibrium of the aquatic carbonate phases. As for the alkalinity, no remarkable variations were found in measured values, even at high concentrations, when the measurements were performed successively on adjacent parts of the same sediment core (cf. alkalinity profile shown in Fig. 3.1). 

A sample of the mineral was dissolved in excess hydrochloric acid and the solution made up to 100cm with water. During the process 48 cm of carbon dioxide, measured at 23 C and 1 atmosphere pressure, were evolved. 

During the fermentations pH, dissolved oxygen and dissolved carbon dioxide were measured continuously with electrodes which were in the fermenter. The first sample for calcium activity and electrical conductivity measurement was taken just after inoculumn. Thereafter samples for calcium activity and electrical conductivity measurements were collected at hourly intervals. 

Chemical Sorption Methods. Chemical sorption methods to determine ky a are based on a chemical reaction between the absorbed gas and a chemical that is added to the liquid phase. Four of these methods will be presented here, although many others exist. The sulfite oxidation, hydrazine, and peroxide methods are applicable to systems studying oxygen transport, while the carbon dioxide absorption method, as its name implies, is for measuring dissolved carbon dioxide. 

Eleetric conductivity is the reeiproeal of speeifie resistanee. The units typically used are either ohm m or, because the eonduetivities of solvents are very small picosiemens per meter which is equivalent tol0 ohm m . The eleetrie eonduetivity of solvents is very low (typically between 10 - 10 ohm m ). The presenee of aeids, bases, salts, and dissolved carbon dioxide might contribute to increased eonduetivity. Free ions are solely responsible for the electric conductivity of solution. This can be eonveniently determined by measuring the conductivity of the solvent or the conductivity of the water extract of solvent impurities. The electronic industry and aviation industry have the major interest in these determinations. 

The consumption of base in a natural water is essentially caused by dissolved carbon dioxide (CO2). Humic acids or other weak organic acids can also play a part. The pH of a water can in such cases lie in the range 4.3 to 4. If mineral acids are present, the pH will fall below 4, so the pH measured in a water already gives an indication of the measured values to be expected. 

Taking as a basis the distribution factor of the CO2 between the gas phase and water at the measured temperature, the CO2 content of the pure CO2 solution can now be determined, without external influence. The result can then be converted to yield the content of free, dissolved carbon dioxide in the water sample taken. Hydrogen carbonate ions and alkaline earth ions in the natural water are certain to be excluded from the distilled water. 

The free dissolved carbon dioxide level is measured in the field (sampling location) by means of back titration (Chapter 1) (b). 

The oldest clinical appKcations of po-tentiometry involved blood gas analysis, and this method is still used to quantitate dissolved carbon dioxide a glass pH electrode is housed behind a membrane permeable to CO2, and a thin solution layer of hydrogen carbonate separates the pH electrode from the membrane. This device, called the Stow-Severinghaus electrode, remains unchanged in principle from its original version reported in 1958 [18]. The measurement is based on the hydrolysis of CO2 to HC03 andH+ in the thin solution... 

Measuring the pH with a pH meter is only possible if the conductivity is increased by the addition of a little potassium chloride. Distilled water has a low pH, which quite often is caused by dissolved carbon dioxide. This is removed during the distillation process but it can re-enter by diffusion. This is a strong acid with a limited solubility. It is partly the cause of distilled water s aggressiveness towards base metals (beware with aluminium). Dissolved gases may also be found in demineralised water since the process is not designed to remove them, neither is the demineralisation process designed to remove heavy metals, so also these can be found. 

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