A universal pH scale

Problems involved in the establishment of a universal pH scale applicable to all solvent systems have been discussed by Bates (1964). The essential difficulty is in the independent determination of the activities of individual ions and of liquid junction potentials. More recently, de Ligny Alfenaar 

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It may be noted finally that if a precise scale of medium effects on the proton were established, we should then have a universal pH scale because of the relation... 

Determinations of pH are relevant only in water-based products/solutions. A universal pH paper is usually satisfactory for clinical work. A few drops of the solution/emulsion to be investigated are applied on the pH paper, and the resulting color is compared with the color scale on the package of the pH paper. Moistened with water, pH paper can be applied to solid subjects to demonstrate residual acid or alkaline... 

In 1909, Sorenson described the development of the pH scale based on the work of Arrhenius and the characteristics of water. Experiments and the resulting calculations show that water dissociates into hydrogen ions (H+) and hydroxide ions (HO-) and that the product of their concentrations equals close to 10"14 ions in aqueous solution. From this, a pH scale from 0 to 14 was developed and the scale describing this relationship using the abbreviation pH was developed (in the older literature [13], one may encounter both the p and the h capitalized, i.e., as PH). Today, it is universally designated as pH [13,22]. 

This indicator is a mixture of many other indicators. The colour shown by this indicator can be matched against a pH scale. The pH scale was developed by a Scandinavian chemist called Soren Sorenson. The pH scale runs from below 0 to 14. A substance with a pH of less than 7 is an acid. One with a pH of greater than 7 is alkaline. One with a pH of 7 is said to be neither acid nor alkaline, that is neutral. Water is the most common example of a neutral substance. Figure 8.4 shows the universal indicator colour range along with everyday substances with their particular pH values. 

Universal indicator papers permit pH determinations over the total pH range from 0 to 14. The determination of the pH is made by comparing the color of the paper with a reference color scale. The accuracy is ca. 1 pH unit. 

The influence of solvents on the ionization equilibrium is related to their electrostatic and their solvation properties. The value of the ionization constant of an analyte is closely determined, in practice, by the pH scale in the particular solvent. It is clear that it is most desirable to have a universal scale which is able to describe acidity (and basicity) in a way that is generally valid for all solvents. It is, in principle, not the definition of an acidity scale in theory which complicates the problem it is the difficulty of approximating the measured values in practice to the specifications of the definition. The pH scale, as is common in water, is applicable only to some organic solvents (i.e., mainly those for which the solvated proton activity is compatible with the Brpnsted theory of acidity). The applicability of an analog to the pH scale in water decreases with decreasing relative permittivity of the solvents and with their increasing aprotic character. 

The paH (which can also be termed the pa or the pfi) is a practical unit consistent both with the experimental methods for pH measurements and with the thermodynamic equations for acid-base equilibria in the solvents concerned, but it should be noted that paH is a succession of scales rather than a single, universal scale of acidity. Consequently, two solutions in different solvent media may have the same paH but behave in totally different ways in acid-base reactions. 

Until procedures are available by which standard universal pcH scales can be established, presumably with a very sophisticated procedure, it will be necessary to check whenever possible the hydrogen ion activity by measuring its effect on a known pH-dependent reaction. 

PH Value. An operational use of the pH term should occure naturally after students have tested various household solutions with universal indicator paper with pH scale - they don t have to deal with any logarithm (see Fig. 7.9). It is more difficult to teach the quantitative meaning of pH value one has to work with concentrations, the logarithm and the mol term. In this case, it is advantageous to relate the meaning of 1 mol to a specific amount of small particles and to decide the type of particles, which are to be counted 18 g water do not contain 1 mol of water , but rather 1 mol of H20 molecules . A liter of 1 M hydrochloric acid contains 1 mol of H30 + (aq) ions and 1 mol of Cl (aq) ions the concentration is equal to 1 mol/1 for both kinds of ions. Dilution in the volume ratio 1 10 results in a solution with the H + (aq) ion concentration of 0.1 mol/1, the dilution 1 100 leads to the 0.01 M or 10 2 M solution. 

Problem The scale of pH values of adds is unfortunately structured in such a way that along with an increase in concentration of hydronium ions, the pH value decreases that for 1M hydrochloric add or nitric add the pH value 0 results and all diluted solutions show pH values higher than 0. This unusual scale can be studied using universal indicator paper and the pH scale with different colors. This paper can be dipped into solutions from the kitchen, bathroom and laboratory pH values are numerically listed in a tabular form (see Fig. 7.9). 

FIGURE 8.4 Sorption isotherms—w in g per g dry starch versus water activity uw— of native potato starch, obtained when decreasing water content (desorption) and when increasing it (absorption), (a) Linear scales, (b) Part of the same data on a log/ log scale, (c) Part of the same data, also showing intermediate curves (broken). (After results by C. van den Berg, Ph.D. thesis, Wageningen University, 1981.)... 

Some textbooks state that the pH scale runs from 0 to 14, while others that it runs from 1 to 14. They both state that 7 is in the middle It seems that the pH scale can rvm from a practical value of around -1.6 to around 15.6. Typical wide-range indicator paper available in schools runs from 3 to 11, and the same is true for universal indicator solutions. The colour of pH 7 is green for some universal indicator solutions, but yellow for others. The recipe used for the mixture of indicators determines the actual colour seen. Yamada created and patented the first recipe (water, propan-l-ol, phenolphthalein sodium salt, sodium hydroxide, methyl red, bromothymol blue monosodium salt and thymol blue monosodium salt) in 1923. 

Sorensen, Soren Peter Lauriti (1868-1939) Danish chemist who in 1909, while describing the effect of hydrogen ion concentration on enzyme activity, proposed the use of a negative logarithm of this concentration as a measure of acidity and alkalinity. This became the standard pH scale now in universal use. He also studied amino acids, enzymes, and proteins. He and his wife Magrete were the first to crystallize the egg protein albumin. 

Characteristic width of the photon beat spectrum Itk of polystyrene solutions as a function of the wave vector AT. The width is normalized by the diffusion width />o A, and the wave vector is expressed through KR when R is the coil radius. The squares and the triangles correspond to two very different molecular weights, suggesting that there is indeed a universal scaling form. From M. Delsanti, Ph.D. Thesis, Orsay, 1978. 

O Universal pH paper has been soaked in a universal indicator, allowing a wide range of pH s to be measured easily by comparing the test strip to the color scale on the container. 

Sources of current measurements (4) S.J. Tans etal. Individual single-wall carbon nanotubes as quantum wires, Nature, 386, 474 77, 1997 (5) M.A. Reed, Electrical Properties of Molecular Devices, presented at 1997 DARPA ULTRA Program Review Conference, Santa Fe, NM, October, 1997 (6) M.A. Reed, Molecular-scale electronics, Proc. IEEE, 87, 652-658, 1999 (7) C. Thou, M.R. Deshpande, M.A. Reed, and J.M. Tour, Nanoscale metal/self-assembled monolayer/metal heterostructures, Appl. Phys. Lett., 71, 611-613, 1997 (8) C. Zhou, Atomic and Molecular Wires, Ph.D. dissertation, Yale University, 1999. With permission. 

The standard molar Gibbs energies of transfer of ions from water to nonaqueous solvents are dealt with in Section 4.3.2.1 and those for transfer into mixed aqueous-organic solvents in Section 6.1. Specifically, the standard molar Gibbs energies of transfer of hydrogen ions from water to solvents S, A G"(H, W S), are available in Table 4.2 for nonaqueous solvents, in Table 6.1 for equimolar mixtures of water with cosolvents, and in the compilations by Kalidas et al. [17] and by Marcus [18] for other compositions. The pH scale is a universal one, because it refers to the same standard state, infinite dilution of hydrogen ions in pure water, where its activity coefficient is unity. The acidity in other solvents, pH, is related to this universal one by Equation 8.8. 

We can determine an approximate value of the pH of an aqueous solution very quickly with a strip of universal indicator paper, which turns different colors at different pH values. More precise measurements are made with a pH meter (Fig. 10.11). This instrument consists of a voltmeter connected to two electrodes that dip into the solution. The difference in electrical potential between the electrodes is proportional to the hydronium ion activity (as will be explained in Section 12.10) so, once the scale on the meter has been calibrated, the pH can be read directly. 

Matthew V. Tirrell (Co-Chair) is Dean of the College of Engineering at the University of California at Santa Barbara. He was previously Professor and Head of the Department of Chemical Engineering and Materials Science at the University of Minnesota, where he served as Director of its Biomedical Engineering Institute. He received a B.S. from Northwestern University and a Ph.D. from University of Massachusetts. His interests are in transport and interfacial properties of polymers, with particular emphasis on molecular-scale mechanical measurements, bioadhesion, and new materials development. He is a member of the National Academy of Engineering. 

Fig. 3. EPR spectra of 100 pairs of Rhodnius salivary glands homogenized in 125 xl of phosphate-buffered saline at pH 7.2 (A) before argon equilibration (B) after equilibration in an argon atmosphere for 4 h (C) after equilibration of (B) with NO for 2 min. (D) difference spectrum, that is B — C. (E) homogenate as in (B) treated with dithionite (DT) to reduce Fe(III) to Fe(II), followed by equilibration with NO for 2 min. (The small signal at g = 2 in A-C is due to copper oxide in the liquid helium which had been condensed at the University of Arizona in a copper-plumbed helium liquiflcation apparatus ) All spectra are plotted on the same scale except (E), which is reduced in amplitude by a factor of 3. Reproduced with permission from Ref 24).

Dry crystalline fructose is reported to have a sweetness level of 180 on a scale in which sucrose is represented at 100 (Andres, 1977). In cool, weak solutions and at lower pH, sweetness value is reported to be 140-150. At neutral pH or higher temperatures, the sweetness level drops, and at 50cC (122°P) sweetness equals that of a corresponding sucrose solution. A synergistic sweetness effect is reported between sucrose and fructose. A 40-60% fructose/sucrose mixture in a 100% water solution is sweeter than either component under comparable conditions (Unpublished report, University of Helsinki, 1972). 

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