Reference buffers

The temperature compensator on a pH meter varies the instrument definition of a pH unit from 54.20 mV at 0°C to perhaps 66.10 mV at 60°C. This permits one to measure the pH of the sample (and reference buffer standard) at its actual temperature and thus avoid error due to dissociation equilibria and to junction potentials which have significant temperature coefficients. 

Reference buffer solutions, 14 25-26 Reference data, standard, 15 747 Reference dose (RfD), 25 238, 239 Reference electrodes, 9 571-574 ... 

Accuracy and Interpretation of Measured pH Values. To define the pH scale and pertnil the calibration of pH measurement systems, a scries of reference buffer solutions have been certified hy the U.S. National Institute of Standards and Technology iNIST). The acidity function which is the experimental basis for the assignment of pH. is reproducible within about O.IKl.I pH unit from It) to 40T. However, errors in the standard potential of the cell, in the composition of the buffer materials, and in the preparation of the solutions may raise the uncertainty to 0 005 pH unit. The accuracy of ihe practical scale may he furthei reduced to (I.Ot)X-(l.(ll pH unit as a result of variations in the liquid-junction potential. 

The original work of Swift and Connick [82] paved the way for modern applications of this NMR method in paramagnetic contrast agent research. The Lausanne group of Merbach and co-workers have developed elegant applications of the method [83]. Most frequently, the difference between the transverse relaxation rates of 170 in reference buffer (1/T2A) and in solutions containing the BPCA (1/T2) are measured as a function of temperature. The analysis fits the temperature dependence of the reduced transverse relaxation rate, 1/T2R, defined as ... 

The various steps for the assignment of pH(PS) to the primary pH reference buffer are summarised in Fig. 1. In this figure also the main sources of uncertainty for the primary method for pH are mentioned. 

As depicted in Fig. 18.2, the first step in BE-AES is sequential buffer exchanges using ultrafiltration spin columns. Buffer exchanges are necessary to get the analyte into a known reference buffer. The last round of buffer exchange is used to concentrate the sample. 

This relationship assumes that all anions behave in the same way. To have confidence that the cacodylate concentrations are reflective of the actual anions of interest, the two controls outlined in Fig. 18.5 need to be completed. As cacodylate has a pFCa of 6.3, it is necessary to make sure that the anion is the primary species present. As shown in Fig. 18.5A, this is done by verifying that the number of excluded anions does not vary as a function of the pH of the reference buffer. Additionally, as the size of the anion could affect how it is excluded, it is prudent to verify that the number of excluded anions does not change with the fractional abundance of the cacodylate. Figure 18.5B demonstrates that this assumption does hold for duplex DNA. 

S. Rodinini and A. Nese, Standard pH values for potassium hydrogen phthalate reference buffer solutions in acetonitrile-water mixtures up to 70 wt.% at various temperatures, Electrochim. Acta 32 (1987), 1499-1505. 

Table 1-2. pH Values of National Bureau of Standards Primary Reference Buffer Solutions... 

Fortunately, glass electrodes have wide applicability in solvents other than water. Even in a solvent as different from water as acetonitrile, glass electrodes respond reversibly to changes in hydrogen ion activity, in agreement with the Nernst equation. In setting up pH values for reference buffers, it is not always possible to use internal reference electrodes without liquid junction. For example, Ag-AgX electrodes are unstable in acetonitrile owing to slow formation of species of the type Ag X +j. When external reference electrodes must be substituted, the reliability of the measurements is reduced because uncertainties in the liquid junction are always present. 

Here the pH is the value for acidity referred to the standard state defined from the limiting reference behavior for the hydrogen ions in the solvent in question, and PH is the pH value of the aqueous reference buffer corrected for liquid junction and transfer activity coefficient as follows ... 

Acidity scales are used commonly to assess the chemical and biological state of seawater. The international operational scale of pH fulfills the primary, requirement of repro ducibility and leads to useful equilbrium data. Nevertheless, these pH numbers do not have a well defined meaning in respect to all marine processes. Seawater of 35%o salinity behaves as a constant ionic medium, effectively stabilizing both the activity coefficients and the liquid junction potential. It may be possible, therefore, to determine hydrogen ion concentrations in seawater experimentally. One method is based on cells without a liquid junction and is used to establish standard values of hydrogen ion concentration (expressed as moles of H /kg of seawater) for reference buffer solutions. 

Kviss. Ale. I is the dissociation constant of the indicator in alcohol. The factor [basic form]/[acid form] is the experimentally determined ratio of the quantities of both forms of the indicator present in the buffer solution, while [A]/[B] is the ratio of the concentration of the acid and its sodium salt in a reference buffer. solution. Kuiaa. Aic. Acid is the dissociation constant of the buffer acid in alcohol (cf. accompanying table). 

Longhi, P, Mussini, T., and Rondinini, S., Predicting standard pH values for reference buffer solutions in solvent mixtures with water, Anal. Chem., 58, 2290, 1986. 

Typical values of the pH(PS) of the seven solutions from the six accepted primary standard reference buffers, which meet the conditions stated above, are listed in Table 2. Batch-to-batch variations in purity can result in changes in the pH value of samples of at most... 

E = the electromotive force of a cell containing the unknown solution = the electromotive force of a cell containing a standard reference buffer solution of known or defined pH, that is, pH,... 

Nonionic surfactant vesicles have been reported successfully, as ocular vehicles for cyclopen-tolate. In the in vivo study, niosomes, independent of their pH, significantly improved the ocular bioavailability of cyclopentolate, with respect to reference buffer solution, indicating that it can be used as an efficient vehicle for ocular drug delivery. ... 

Cell Loading. Sample and reference buffer are loaded into the cells using the same procedure described above for DSC to eliminate air bubbles. Erratic performance usually indicates either bubbles or leaking cells. The cells are overfilled outside the calorimeter to about 2 mm above the tip of the loading capillary and then inserted stepwise into the calorimeter to gradually equilibrate the cells and minimize perturbation of the calorimeter. The syringe is put into place at the end of the equilibration to minimize the effects of leakage from the tip of the injection needle. Total equilibration requires approximately 2 hr. 

Figure 4.35 DSC curve of DMPC vesicles of various sizes (511, Sample, DMPC vesicle suspension liquid sample mass, lipid concentration 2 mmol L heating rate, 0.75"C min atmosphere, Nj sample vessel, tantalum-niobium alloy, 1.2 ml, scaled type reference, buffer solution vesicle size I,. 300-1000 nm, muti-layers II, average 200 nm, 3-4 layers HI, 40 nm, single layer...

Dialyze the delipidated CRABP against the buffer to be used in the fluorescence studies Save a portion of the buffer unused as this will serve as the reference buffer This will be called buffer blank. 

Potassium dihydrogen citrate 0.05 molal solution (pH=3.776 at 25 °C) is used for calibration purposes because it exhibits better stability than primary pH reference buffer solutions of tartrate or phthalate [44, 45]. The saline sodium citrate buffer (SSC) prepared from tri-sodium citrate and sodium chloride (pH=7.0) is applied in biochemistry. Citric buffers with different HjCit Na3Cit ratios are clinically effective, for example in reducing gastric acidity [46-48]. Compositions of buffers and corresponding pH values are presented in Table 3.7. 

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