Standard buffers temperature dependence

Since every practical pH electrode can be regarded only as a somewhat imperfect tool that functions more or less unevenly over the whole pH range, and every practical pH reading involves a (possibly variable) liquid-junction potential, the NBS has adopted a series of six primary standard pH buffer solutions (Table 2.2). The pH of the standards is temperature dependent, primarily because of the variation of the Kg, of the buffer system with temperature. 

Before the pH electrode is used, it should be calibrated using two (or more) buffers of known pH. Many standard buffers are commercially available, with an accuracy of 0.01 pH unit. Calibration must be performed at the same temperature at which the measurement will be made care must be taken to match the temperature of samples and standards. The exact procedure depends on the model of pH meter used. Modem pH meters, such as the one shown in Figure 5-8, are microcomputer controlled, and allow double-point calibration, slope calculation, temperature adjustment, and accuracy to 0.001 pH unit, all with few basic steps. The electrode must... 

The pH values derived by the standard buffers are temperature dependent. The pH values between 0 and 60 °C are given in Table 7.5, which also indicates the change of pH at 1 1 dilution (ApHi i). 

Temperature Effects The pH of a buffer solution is influenced by temperature. This effect is due to a temperature-dependent change of the dissociation constant (pK ) of ions in solution. The pH of the commonly used buffer Tris is greatly affected by temperature changes, with a ApKa/C° of —0.031. This means that a pH 7.0 Tris buffer made up at 4°C would have a pH of 5.95 at 37°C. The best way to avoid this problem is to prepare the buffer solution at the temperature at which it will be used and to standardize the electrode with buffers at the same temperature as the solution you wish to measure. 

G-H. Dependence of the Rate on Temperature. In order to determine the temperature dependence of the enzyme-catalyzed rate, the reaction is carried out at several different temperatures, which are held constant to within 0.3°C or better. Choose from mns E a set of initial concentrations that gives an absorbance of —0.5 at room temperature after the standard assay. Convenient temperatures for these runs, denoted as runs G, are 0°C (ice bath), 12, 25, 35, and 45°C. In each case, prepare the enzyme-water-buffer mixture in the assay tube, put about 2 mL of the sucrose stock solution in another test tube, and immerse both tubes in the thermostat bath for a few minutes to achieve temperature equilibrium. Next, pipette 1.0 mL of the equilibrated sucrose solution into the assay tube and read the bath temperature. Leave the assay tube in the constant-temperature bath until, 5.0 min after the addition of the sucrose solution, the reaction is terminated by the addition of 2.0 mL of the dinitrosalicylate reagent. Then complete the assay as usual. 

Been varies only as a function of the test solution pH if the temperature is constant. It is significant to point out that the potential of modern glass electrodes is a linear function of pH (equation 39). By using a test solution of known pH it is possible to relate the cell potential to hydrogen ion activity of a test solution. This standardization must be done each time a pH meter is used because of subtle changes in the various potentials owing to aging of the electrode. Therefore, the accuracy of a pH determination depends on the accuracy of the standard buffer. Table 1-2... 

Pacifici and Viani, commenting on comparisons among results from different methods applied to protein binding, warn that methods for drug binding need to be standardized. Currently, results depend greatly on technique and interpretation of results thus, experimental details such as anticoagulant used, type of dialysis or filtration membrane, buffer characteristics, duration of experiment, and temperature should be taken into account. 

The compositions and pH of NIST standard buffers are given in Table 13.2. Although the absolute value of the pH accuracy is no better than 0.01 unit, the buffers have been measured relative to one another to 0.001 pH. The potentials used in calculating the pH can be measured reproducibly this closely, and the discrimination of differences of thousandths of pH units is sometimes important (i.e., an electrode may have to be calibrated to a thousandth of a pH unit). The pH of the buffers is temperature dependent because of the dependence of the ionization constants of the parent acids or bases on temperature. 

Figure 1 displays the dependence of electromotive force on temperature for the hematite suspension as well as for two buffers having pH approximately 4 and 7. The pH values of the suspension were calculated by using the tabulated values for two standard buffers [4]. These pH values were taken as points of zero charge [7]. The temperature dependence of p.z.c. of hematite is presented in Fig. 2. Linear regression according to Eq. (8) resulted in...

Also, for proper technique, the effect that temperature has on hydrogen ion activity should be considered. The label on a bottle of buffer lists the standardization values at specific temperatures for that buffer (see Table 4.1). The operator should always observe the buffer temperature and corresponding standardization value before calibrating the meter. For the most accurate results, the buffer and the sample should have as nearly equal a pH as possible and be brought to the same temperature. This minimizes any errors arising from differences between the ideal temperature-dependent slope factor and the actual electrode response due to temperature changes. 

A pH electrode is normally standardized using two buffers one near a pH of 7 and one that is more acidic or basic depending on the sample s expected pH. The pH electrode is immersed in the first buffer, and the standardize or calibrate control is adjusted until the meter reads the correct pH. The electrode is placed in the second buffer, and the slope or temperature control is adjusted to the-buffer s pH. Some pH meters are equipped with a temperature compensation feature, allowing the pH meter to correct the measured pH for any change in temperature. In this case a thermistor is placed in the sample and connected to the pH meter. The temperature control is set to the solution s temperature, and the pH meter is calibrated using the calibrate and slope controls. If a change in the sample s temperature is indicated by the thermistor, the pH meter adjusts the slope of the calibration based on an assumed Nerstian response of 2.303RT/F. 

From the data in Fig. 4.8b, estimate the shift factors required to displace the data at 0 = 0.5 (consider only this point) so that all runs superimpose on the experiment conducted at 128 C at 0 = 0.5. Either a ruler or proportional dividers can be used to measure displacements. Criticize or defend the following proposition Whether a buffered aqueous solution of H2O2 and 1. containing small amounts of S2O3 and starch, appears blue or colorless depends on both the time and the temperature. This standard general chemistry experiment could be used to demonstrate the equivalency of time and temperature. The pertinent reactions for the iodine clock are... 

Dissolution indicates the rate-limiting step for compound absorption when drugs are administered orally. The solubility of a pharmaceutical compound represents its maximum concentration in an aqueous buffer. Additional compound will not dissolve above this concentration. The solubility value is often heavily dependent upon pH and temperature and is typically measured at physiologically important pH levels and body temperature. The standards for dissolution testing are determined by the United States Pharmacopoeia (USP). Testing typically requires sampling of a solution at 15, 30, 45, and 60 min for immediate-release products. /./Pl.C is ideally suited for use in conjunction with USP apparatus types I or II and can rapidly analyze multiple time points or replicate samples. 

Viscosity differences-. Different sample vial temperatures create different viscosities, and thus different amounts injected. To reduce the effect, use the instrument temperature control (see Section IV). However, often the sample and buffer vials reside outside the temperature-controlled area. The effect of this might vary depending on the climate system in your lab and how the lab temperature varies over the year. Besides temperature control, it is important to match samples and standards in terms of viscosity and conductivity. 

The total free chlorine in wastewaters as measured by colorimetric techniques constitutes both the dissolved molecular chlorine, hypochlorite ion, OCl, and hypochlorous acid. An equilibrium exists between these species, the concentrations of which depend on the temperature and pH of the waste-water. Concentration of the hypochlorous acid may be estimated from the K value or from the ratio (33% of the measured concentration of free chlorine). The free chlorine may be measured by amperometric titration after the addition of a phosphate buffer solution to produce a pH between 6.5 and 7.5. The sample is titrated against a standard solution of phenylarsine oxide. Alternatively, the syringaldazine (3,5-dimethoxy-4-hydroxybenzaldazine) colorimetric test may be performed. This color-forming reagent in 2-propanol yields a colored product with free chlorine, the absorbance of which may be... 

An important factor in the reaction for CdSe deposition via the hydroxide mechanism was an observed gradual increase in pH, ca. 0.8 pH units over the course of the deposition. It is not clear what the cause of this increase is. However, it means that while Cd(OH)2 may not be formed at the start of the deposition, it may form during the deposition. This can then explain the induction time where no apparent reaction takes place initially. For the preceding experiments, the standard conditions were to set the solution pH to 10. Coloration of the solution (indicating formation of CdSe) occurred when the pH reached ca. 10.3 (depending, of course, on the temperature and complex Cd ratio). If the pH was adjusted to 10.3 at the beginning of the reaction, coloration began almost inunediately rather than after a more typical time of several minutes. This increase in pH did not occur for the ion-by-ion mechanism, probably because the excess NT A necessary in this case also acts as a buffer. In any case, an increase in pH is not required for this mechanism. 

A plot of the optical absorbance at 260 nm (the wavelength of maximum light absorption by nucleic acids) versus temperature is known as a melting curve (Fig. 5-45). The absorbance is lower, by up to 40%, for native than for denatured nucleic acids. This hypochromic effect (Chapter 23) is a result of the interaction between the closely stacked bases in the helices of the native molecules. The melting temperature Tm is taken as the midpoint of the increase in absorbance (Fig. 5-45). As the percentage of G + C increases, the nucleic acid becomes more stable toward denaturation because of the three hydrogen bonds in each GC pair. Tm increases almost linearly with increases in the G + C content. In the "standard" citrate buffer (0.15 M NaCl + 0.015 M sodium citrate, pH 7.0) Eq. 5-22 holds. The exact numerical relationship depends strongly upon the ionic composition and pH of the medium.37 72 552 553... 

A pH meter is standardized with buffer solutions of known pH before a measurement of an unknown solution is taken. It should be noted from Equation 2.2 that the voltage depends on temperature. Hence, pH meters must have some means for temperature correction. Older instruments usually have a knob labeled temperature control, which is adjusted by the user to the temperature of the measured solution. Newer pH meters automatically display a temperature-corrected pH value. 

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