Reference and buffer solutions

For assessing the viabihty and accmacy of high-temperature potentiometric measurements, suitable reference systems should be used. 

It is highly desirable to establish a set of pH buffer solutions which can be used at temperatures above 100 °C. Thus far, little has been done to develop the necessary sets of the high-temperature buffer systems as primary standards. Only a 0.05 mol-kg potassium hydrogen phthalate solution has been adopted by lUPAC as an appropriate primary buffer system to be used at temperatures up to about 225 °C. However, the acid dissociation constants of many organic and inorganic buffers have been measured with the hydrogen-electrode concentration cell (see discussion below) and these results are currently available for developing the secondary pH standards to 250 °C. 

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The ultraviolet chemical assay for erythromycin remains largely unchanged from that described by Kuzel et al.9 in 1954. This procedure is essentially as follows. The reference standard, alkali reagent, and buffer solutions are prepared prior to the assay. 

TABLE 8.5 National Institute of Standards and Technology (formerly National Bureau of Standards U.S). Reference PH Buffer Solutions. 

The buffer value for the National Institute of Standards and technology (U.S.) reference pH buffer solutions is given below ... 

In all cases, the storage temperature for this study was 38° C. (100° F.) and the pH value was maintained at 3.0 0.03. All juice and buffer solutions stored at 38° C. were saturated with thymol for prevention of fermentation and mold growth. Thymol has been found to be clearly inert as far as the color loss is concerned. Buffer solution in all cases refers to Sorensen s citrate buffer, adjusted to pH 3.0 with hydrochloric acid, and saturated with thymol. The importance of buffer concentration is shown below. 

In Section 8, the material on solubility constants has been doubled to 550 entries. Sections on proton transfer reactions, including some at various temperatures, formation constants of metal complexes with organic and inorganic ligands, buffer solutions of all types, reference electrodes, indicators, and electrode potentials are retained with some revisions. The material on conductances has been revised and expanded, particularly in the table on limiting equivalent ionic conductances. 

Because the ionic product of water = [H ] [OH ] = 1.04 x 10" at 25°C, it follows that pH = 14 - pOH. Thus, a neutral solution (e.g., pure water at 25°C) in which [H j = [OH ] has a pH = pOH = 7. Acids show a lower pH and bases a higher pH than this neutral value of 7. The hydrogen ion concentrations can cover a wide range, from -1 g-ion/liter or more in acidic solutions to -lO" " g-ion/liter or less in alkaline solutions [53, p. 545]. Buffer action refers to the property of a solution in resisting change of pH upon addition of an acid or a base. Buffer solutions usually consist of a mixture of a weak acid and its salt (conjugate base) or of a weak base and its salt (conjugate acid). 

In some of the details which follow, reference is made to the addition of a buffer solution, and in all such cases, to ensure that the requisite buffering action is in fact achieved, it is necessary to make certain that the original solution has first been made almost neutral by the cautious addition of sodium hydroxide or ammonium hydroxide, or of dilute acid, before adding the buffer solution. When an acid solution containing a metallic ion is neutralised by the addition of alkali care must be taken to ensure that the metal hydroxide is not precipitated. 

Details for the preparation of the solutions referred to in the table are as follows (note that concentrations are expressed in molalities) all reagents must be of the highest purity. Freshly distilled water protected from carbon dioxide during cooling, having a pH of 6.7-7.3, should be used, and is essential for basic standards. De-ionised water is also suitable. Standard buffer solutions may be stored in well-closed Pyrex or polythene bottles. If the formation of mould or sediment is visible the solution must be discarded. 

It has become fairly common to adopt the manufacture of combinations of internal reference electrode and its inner electrolyte such that the (inner) potential at the glass electrode lead matches the (outer) potential at the external reference electrode if the glass electrode has been placed in an aqueous solution of pH 7. In fact, each pH glass electrode (single or combined) has its own iso-pH value or isotherm intersection point ideally it equals 0 mV at pH 7 0.5 according to a DIN standard, as is shown in Fig. 2.11 the asymmetry potential can be easily eliminated by calibration with a pH 7.00 0.02 (at 25° C) buffer solution. 

Figure 2.81 (a) Schematic of the system for in situ X-ray reflectivity measurements. Syn = synchrotron source M = monochromator S = slit /0, /R = incident and reflected X-rays beams, respectively 9 = angle of incidence W = teflon windows WE = working electrode RE = reference electrode CF = counter electrode D = scintillation detector, (h) Cyclic voltammogram of Cu-on-Si electrode in borate buffer solution (pH 8.4), scan rate = lOmVs-1. From Melendres... 

To construct a standard curve of various biotin concentrations, first zero a spectrophotometer at an absorbance setting of 500 nm with sample and reference cuvettes filled with 0.05M sodium phosphate, 0.15M NaCl, pH 6.0. Remove the buffer solution from the sample cuvette and add 3 ml of the (strept)avidin solution plus 75 pi of the HABA-dye solution. Mix well and measure the absorbance of the solution at 500nm. Next add 2 pi aliquots of the biotin solution to this (strept)avidin-HABA solution, mix well after each addition, and measure and record the resultant absorbance change at 500 nm. With each addition of biotin, the absorbance of the (strept)avidin-HABA complex at 500 nm decreases. The absorbance readings are plotted against the amount of biotin added to construct the standard curve. 

The wireless pH capsule (Medtronic Inc.) is oblong in shape and contains an antimony pH electrode, a reference electrode at its distal tip, a battery, and a RF transmitter. The whole device is encapsulated in epoxy. The capsule is introduced into the esophagus on a catheter through the nose or mouth and is attached to the lining of the esophagus with a clip. The probe monitors the pH in the esophagus and transmits the information via RF telemetry at a rate of 6 per second (0.17 Hz) to a pager-sized receiver that is worn by the patient on a belt. Prior to implantation, the capsule is calibrated with its receiver in pH buffer solutions of pH 1.07 and pH 7.01 [168],... 

Smooth polycrystalline Au, Pt and Ir thin-layer electrodes were utilized (10-11). Electrodes were cleaned between trials by sequential electrochemical oxidation above 1.2 V [Ag/AgCl (1 M Cl-) reference] and reduction below -0.2 V in 1 M H2SO4 surface cleanliness was verified with the aid of cyclic voltammetry in the same molar sulfuric acid solution. Experiments were carried out in 1 M H2SO4, 1 M NaC104 buffered at pH 7 and 10, and in 1 M NaOH solutions were prepared with pyrolytically triply distilled water (12). Surface reagents employed were iodide, hydroquinone (HQ), 2,5-dihydroxythiophenol [DHT (13)1. and 3,6-dihydroxypyridazine (DHPz). 

For example, to prepare a pH = 9 buffer solution, one would prepare a solution of ammonium chloride (refer to Table 5.1), and then add a solution of sodium hydroxide while stirring and monitoring the pH with a pH meter. The preparation is complete when the pH reaches 9. The required conjugate acid-base pair would be NH3 - NHj. Recipes for standard buffer solutions can be useful. Table 5.2 gives specific directions for preparing some popular buffer solutions. 

The pH meter is standardized (calibrated) with the use of buffer solutions. Usually, two buffer solutions are used for maximum accuracy. The pH values for these solutions should bracket the pH value expected for the sample. For example, if the pH of a sample to be measured is expected to be 9.0, buffers of pH = 7.0 and pH = 10.0 should be used. Buffers with pH values of 4.0,7.0, and 10.0 are available commercially specifically for pH meter standardization. Alternatively, of course, homemade buffer solutions (see Chapter 5) may be used. In either case, when the pH electrode and reference electrode are immersed in the buffer solution being measured and the electrode leads are connected to the pH meter, the meter reading is electronically adjusted (refer to manufacturer s literature for specifics) to read the pH of this soluiton. The electrodes can then be immersed into the solution being tested and the pH directly determined. 

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