Activity versus Concentration

Since the pH glass electrode is sensitive to hydrogen ion activity 2h+, the factors which influence activity and its definition are of primary importance. The activity of the hydrogen ion can be defined by its relation to concentration (Ch+, molality) and the activity coefficient /h+  

If the activity coefficient is unity, then activity is equal to concentration. This is nearly the case in dilute solutions, where the ionic 

Strength is low. Since the objective of most pH measurements is to find a stable and reproducible reading which can be correlated to the results of some process, it becomes important to know what influences the activity coefficient and therefore the pH measurement. 

The factors that affect the activity coefficient are the temperature r, the ionic strength w, the dielectric constant e, the ion charge Zf, the size of the ion in angstroms and the density of the solvent d. All of these factors are characteristics of the solution which relate the activity to the concentration by two main effects. The first is the salt effect designated as/ H+ It can be approximated for the hydrogen ion by the expression 

The more exact definition of this salt effect is found by using the Debye-Huckel equation described in the Glossary. The other factors mentioned are used in defining this equation, thus showing their effect on the measurement. 

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Activity Versus Concentration In describing metallic and membrane indicator electrodes, the Nernst equation relates the measured cell potential to the concentration of analyte. In writing the Nernst equation, we often ignore an important detail—the... 

Definitions - Activity versus Concentration Thermodynamic Scales... 

Because of its prominent appearance in the whole cell Michaelis-Menten equation, Kt is frequently mistaken as a measure of the substrate affinity. However, from equations (2) and (4), it becomes obvious that the activity versus concentration relationship is characterised by the two independent parameters, 2max, as a descriptor of the zero-order part at high substrate concentration, and a°A, as a descriptor of the slope of the first-order part of the curve. In his much-cited review paper, Button [9] has listed the specific affinities of various organisms for a range of carbon sources and other elements. Reported variations for the same substrates extend over up to four orders of magnitude. Table 1 updates... 

Equations (2.16) and (2.17) describe equilibria in terms of concentrations rather than activities. The concept of activity versus concentration of a species is discussed in Section 2.5. 

Know the concept of activity versus concentration and how activity is defined for gases, solutes, pure solids, and liquids. 

Figure 11. Activity versus concentration curve for Na in sodium amalgam at 25°C-...

Activity versus Concentration Electrode response is related to activity rather than to analyte concentration. Wo are usually interested in concentration, and the determination of this quantity from a potentiometric measurement requires activity coefficient data. Often, activity coefficients arc unavailable because (he ionic strength of the solution is eitiier unknown or so high that the Debye-Huckel equation is not applicable. Unfortunately, (he assumption (hat activitN and concentration are identical may lead to serious errors, particularly w hen the analyte is polyvalent-... 

The net proton surface excess amount (Ah h/oh, moles per gram) is defined as the difference of H ( "+) and OH ( oh ) surface excess amounts related to umt mass of solid, = n + — n jj. The surface excess amount of any solute, like H and OH here, can be determined directly from the initial and equilibrium concentration of solute for adsorption from dilute solution [23]. The values of and /Iqh calculated at each point of the titration from tne electrode output using the actual activity coefficient from the slope of H /OH" activity versus concentration straight lines for background electrolyte titration. 

To avoid the problem of activity versus concentration, a direct calibration of the electrode is carried out in known electrolytes. Typically one prepares known solutions by dissolving known amounts of a given salt in... 

In most quantitative analyses we are interested in determining the concentration, not the activity, of the analyte. As noted earlier, however, the electrode s response is a function of the analyte s activity. In the absence of interferents, a calibration curve of potential versus activity is a straight line. A plot of potential versus concentration, however, may be curved at higher concentrations of analyte due to changes in the analyte s activity coefficient. A curved calibration curve may still be used to determine the analyte s concentration if the standard s matrix matches that of the sample. When the exact composition of the sample matrix is unknown, which often is the case, matrix matching becomes impossible. 

Figure 5 Madon-Boudart test for mass transfer limitation over Pd/C catalysts -a plot of In (activity) versus In (Pd surface concentration).

The results of this study show a definite quenching of the 418 nm phosphorescence emission of DMT. One would expect that the quenching effect, in a rigid glass, would fit the Perrin model (73). A plot in In 4>0/4> versus concentration of 4,4 -BPDC yielded a straight line, the slope of which was identified with NV. The radius, R, of the active volume of quenching sphere was calculated by the following equation ... 

Figwe 8. The effect of centrifugal force in the BPA treatment protocol on the clarification parameters. Results are expressed as a percentage of those for the untreated sample spun at 16,000 g. All treatments involved 2 min centrifugation. The KCl, BPA-1000, and BPA-1050 concentrations are the same as in Figure 7. A turbidity versus relative centrifugal force (RCF) B protein versus RCF C RNA versus RCF D DNA versus RCF E GUS activity versus RCF. Legend = untreated extract = BPA-1000 treated extract = BPA-1050 treated extract... 

In most kinetic investigations, one assumes the enzyme remains stable over the course of the measurement. When this is the case, corrective measures must be taken to obtain valid kinetic data. A useful test for any enzyme system is to plot enzyme activity versus time. This is readily accomplished by using a standardized assay (usually at optimal or saturating substrate concentrations) to measure the enzyme s specific activity periodically during the course of some experiment. This approach may fail to detect a reduction in activity characterized by lower affinity for substrate however, use of a subsaturating substrate concentration in a time-course study will reveal this behavior. 

Figure 4. Michaelis-Menten type plot of percent of contractile activity versus the histamine substrate concentration and the toxin concentration in the saline solution.

FIG. 7.16 Schematic plots of the Langmuir equation showing the significance of the initial slope and the saturation value of the ordinate (a) the fraction covered versus solute activity and (b) the number of moles of solute adsorbed per unit weight of adsorbent versus concentration. 

Effect of Molar Ratio of Reactants Several mixtures containing different molar ratios of arginine to xylose were refluxed in 25 ml of nonbuffered water for 20 h. The total concentration in all cases was 3.0 M. Figure 7 is a plot of the final activity versus the molar ratio of arginine to xylose, and clearly indicates that the 1 1 ratio was the superior combination. In addition, the bell-shaped... 

The choice of the concentration of the drug to be tested should consider therapeutic levels that could be attained with clinically employed doses. In the case of a compound under pre-clinical evaluation for a potential antitumor activity, a concentration limit of 100 pg/ml is recommended. Pharmacokinetic data are available for various anti-neoplastic clinically used drugs, with information about their maximum plasma concentration, concentration versus time, and pharmaceutical half-life in plasma. When these data are not available, an approximation of plasma levels could be obtained by calculating the theoretical concentration obtained when the administered dose is uniformly distributed throughout the body fluid. 

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