Hydrogen ion concentration, measurement

Hydrogen-ion concentration measurements cannot distinguish between a bis-hydroxy species and a mono-oxy species because the equilibrium 2 OH — 02 + H20... 

Indicalors and test-papers. 2. Hydrogen-ion concentration—Measurement. I. Title. 

Thus, by the end of the nineteenth century the electromotive force from high resistance sources could be measured with a relatively high precision. The introduction of the glass membrane electrode for the hydrogen-ion concentration measurement presented a challenge and a test of instrument capabilities. The State of the Art (1905) for the measurement of membrane potentials had been described by Max Cremer in the paper entitled (14.) ... 

The pH scale from 0 to 14 covers all the hydrogen ion concentrations (Table 2.1) found in dilute aqueous solutions and biological systems. Pure water has a pH of 7 which is considered to be neutrality. When pH < 7, the solution is acidic and when pH > 7, the solution is basic or alkaline. Because of the logarithmic function a change of one pH unit represents a tenfold difference in hydrogen ion concentration. Measurements of pH can be easily performed using a pH-meter. 

A particular concentration measure of acidity of aqueous solutions is pH which usually is regarded as the common logarithm of the reciprocal of the hydrogen-ion concentration (see Hydrogen-ION activity). More precisely, the potential difference of the hydrogen electrode in normal acid and in normal alkah solution (—0.828 V at 25°C) is divided into 14 equal parts or pH units each pH unit is 0.0591 V. Operationally, pH is defined by pH = pH(soln) + E/K, where E is the emf of the cell ... 

Fig. 10-11. The pH scale is a measure of hydrogen ion concentration. The pH of common substances is shown with various values along the scale. The Adirondack Lakes are located in the state of New York and are considered to be receptors of acidic deposition. Source U.S. Environmental Protection Agency, Acid Rain—Research Summary," EPA-600/8-79-028, Cincinnati, 1979.

The now universally used measure of the hydrogen-ion concentration was introduced in 1909 by the Danish biochemist S. P. L. S0iensen during his work at the Carlsbetg Breweries Biochem. Z 21. 131. 1909) ... 

C 0.5012 mol 2 at 15°C). It is clearly unwise to associate a pH meter reading too closely with pH unless under very controlled conditions, and still less sensible to relate the reading to the actual hydrogen-ion concentration in solution. For further discussion of pH mea.surements, see Pure Appl. Chem. 57, 531-42 (1985) Definition of pH Scales, Standard Reference Values, Measurement of pH and Related Terminology. Also C E News, Oct. 20. 1997. p. 6. 

The oxidizing power of the halate ions in aqueous solution, as measured by their standard reduction potentials (p. 854), decreases in the sequence bromate > chlorate > iodate but the rates of reaction follow the sequence iodate > bromate > chlorate. In addition, both the thermodynamic oxidizing power and the rate of reaction depend markedly on the hydrogen-ion concentration of the solution, being substantially greater in acid than in alkaline conditions (p, 855). 

It is the rapid increase in rates of hydration with increasing hydrogen ion concentration that prevents measurement with existing apparatus of the -pKa values of anhydrous bases such as pteridine. For example, at pH 1, hydration of the anhydrous cation is half-complete in 0.01 sec at 20°. Conversely, it is the comparative slowness of the reactions in near-neutral solutions that makes it possible, by adding acid solutions to near-neutral buffers, using the stopped-flow technique, to determine the p STa values of the hydrated species. 

To apply the Equilibrium Law to acid solutions, a chemist must know the numerical value of the equilibrium constant, KA. Experiments which provide this information require the measurement of hydrogen ion concentration. Acid-sensitive dyes, such as litmus, offer the easiest estimate of [H+]. 

For some industrial operations an antimony electrode is used to measure hydrogen-ion concentrations. The electrode consists of a rod of antimony which invariably has a coating of oxide, and placed in an aqueous solution the equilibrium... 

To measure the hydrogen ion concentration of a solution the glass electrode must be combined with a reference electrode, for which purpose the saturated calomel electrode is most commonly used, thus giving the cell ... 

Coordination equilibria were investigated by pH and potentiometric titration in aqueous solution. The measured e.m.f. values (E) are converted into hydrogen-ion concentrations using the modified Nernst equation ... 

Among potentiometric methods of analysis that are important for ecological applications, the one most widely used is that of pH measurements with an indicator electrode whose potential is a function of the hydrogen ion concentration. More recently, ion-selective electrodes reversible to other cations such as those of heavy metals have become available. 

While the pH scale has made it convenient to describe the order of hydrogen ion concentrations and to give a measure of the acid strength or alkalinity of a solution, it suffers from a defect which is less obvious. A 4 10-5 N HC1 solution is clearly twice as acidic as a 2 10-5 N solution, but the pH values of these solutions, 4.40 and 4.70, provide no idea of the relative strengths of these solutions. 

Applications Potentiometry finds widespread use for direct and selective measurement of analyte concentrations, mainly in routine analyses, and for endpoint determinations of titrations. Direct potentiometric measurements provide a rapid and convenient method for determining the activity of a variety of cations and anions. The most frequently determined ion in water is the hydrogen ion (pH measurement). Ion chromatography combined with potentiometric detection techniques using ISEs allows the selective quantification of selected analytes, even in complex matrices. The sensitivity of the electrodes allows sub-ppm concentrations to be measured. 

The pH scale was invented in 1909 by a Danish biochemist named Soren Sorensen (1868-1939). The pH of a substance is a measure of its acidity. Because acids donate hydrogen ions, when they are added to a solution they increase its hydrogen ion concentration. The addition of a base decreases the hydrogen ion concentration in a substance because bases accept hydrogen ions. 

Classic Papers in Chemistry S. P. L. Sorenson, Enzyme Studies II. The Measurement and Meaning of Hydrogen Ion Concentration in Enzymatic Processes. Biochemische Zeitschrift 21, (1909) 131-200,... 

FIGURE 10.3 A schematic diagram of an ISFET pH measurement system. A potential responding to the hydrogen ions concentration in the solution is established on the pH sensitive gate-solution interface of the ISFET. It electrostatically influences the current (4-d) flow between the source and drain, therefore, the current 4 d change is directly related to the pH change in the solution. 

Implantable microelectronic devices for neural prosthesis require stimulation electrodes to have minimal electrochemical damage to tissue or nerve from chronic stimulation. Since most electrochemical reactions at the stimulation electrode surface alter the hydrogen ion concentration, one can expect a stimulus-induced pH shift [17]. When translated into a biological environment, these pH shifts could potentially have detrimental effects on the surrounding neural tissue and implant function. Measuring depth and spatial profiles of pH changes is important for the development of neural prostheses and safe stimulation protocols. 

The number of grams of hydrogen ions per litre of solution it is a measure of the acidity or alkalinity of a solution. The pH of a solution is -log(hydrogen ion concentration) and ranges from 0 (highly acid) through 7 (neutral) to 14 (highly alkaline). 

Khoo et al. [249] have reported on the measurement of standard potentials, hence hydrogen ion concentrations, in seawaters of salinities between 20 and 45%o at 5-40 °C. 

Hydrogen-iodine reaction, 13 770 Hydrogen-ion activity, 14 23-34 nonaqueous solvents, 14 32 pH determination, 14 24-27 pH measurement systems, 14 27-31 Hydrogen ion concentration (total acidity), 14 23... 

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