The measurement of pH

Conventional pH meters can be used to measure acidity in many partially aqueous and non-aqueous media. The glass electrode responds in a reproducible way to hydrogen ions in media which contain at least a few per cent of water and also in certain anhydrous solvents. In the calomel electrode —KCl bridge system, the junction potential between the aqueous and non-aqueous medium may be large. However, this does not preclude the accurate measurement of acidity in media of constant solvent composition provided the junction potential does not vary with acidity. 

An approximate method of measuring pH is also available (Bates et al., 1963). If a pH meter is standardized using aqueous buffers, the meter reading, pH(R), obtained in a 

Some values of 8 for partially aqueous mixtures are given in Table 6.7 (Douheret, 1967 and 1968 Reynaud, 1969 Lesquibe and Reynaud, 1970), for organic component concentrations up to limits recommended by Lesquibe and Reynaud (1970) from studies of ion-pair formation. Other approaches to the determination of 8 values have been described Ongetal., 1964 Van Veen, 1971). 

Comparisons have been made of the pH of aqueous buffers with pH meter readings (pH(R)) for the same concentrations of these buffers in partially aqueous solution. The differences, pH — pH(R), for various concentrations of acetate/ acetic acid and ammonia/ammonium buffers in water and in 50% (v/v) ethanol-water (Gottschalk, 1959) are fairly constant at 0.89—0.92 and 0.24—0.27, respectively, when the pH meter is standardized against aqueous buffers. These differences are not 8 values as defined above, and tables of this kind have not been included in this chapter. 

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Glass membrane pH electrodes are often available in a combination form that includes both the indicator and the reference electrode. The use of a single electrode greatly simplifies the measurement of pH. An example of a typical combination electrode is shown in Figure 11.12. 

The measurement of pH using the operational ceU assumes that no residual Hquid-junction potential is present when a standard buffer is compared to a solution of unknown pH. Although this may never be stricdy tme, especially for complex matrices, the residual Hquid-junction potential can be minimised by the appropriate choice of a salt-bridge solution and caHbration buffer solutions. 

Other problems occur in the measurement of pH in unbuffered, low ionic strength media such as wet deposition (acid rain) and natural freshwaters (see Airpollution Groundwatermonitoring) (13). In these cases, studies have demonstrated that the principal sources of the measurement errors are associated with the performance of the reference electrode Hquid junction, changes in the sample pH during storage, and the nature of the standards used in caHbration. Considerable care must be exercised in all aspects of the measurement process to assure the quaHty of the pH values on these types of samples. 

An important application of the Nernst equation is the measurement of pH (and, through pFI, acidity constants). The pH of a solution can be measured electro-... 

The measurement of pH is further complicated by the effect of high concentrations of sucrose (such as 60 Brix or 60%w/w) on hydrogen ion activity. 

Figure 17-3 shows the range of pH and hydronium ion concentrations. The measurement of pH is a routine operation in most laboratories. Litmus paper, which turns red when dipped in acidic solution and blue when dipped in basic solution, gives a quick, qualitative indication of acidity. As Figure 17-4 shows, approximate measures of pH can be done using pH paper. Universal pH paper displays a range of colors in response to different pH values and is accurate to about 0.5 pH unit. For quantitative pH determinations, scientists use pH meters. 

The dihydroxyaniline-squaraine chromophore was used by Akkaya and Isgor in the fluorescent chemosensor 30 for the measurement of pH [90]. This chemosensor, having the molar absorptivity about 200,000 M em 1 and quantum yield 0.2,... 

Molecular rotors with a dual emission band, such as DMABN or A/,A/-dimethyl-[4-(2-pyrimidin-4-yl-vinyl)-phenyl]-amine (DMA-2,4 38, Fig. 13) [64], allow to use the ratio between LE and TICT emission to eliminate instrument- and experiment-dependent factors analogous to (10). One example is the measurement of pH with the TICT probe p-A,A-dimethylaminobenzoic acid 39 [69]. The use of such an intensity ratio requires calibration with solvent gradients, and influences of solvent polarity may cause solvatochromic shifts and adversely influence the calibration. Probes with dual emission bands often have points in their emission spectra that are independent from the solvent properties, analogous to isosbestic points in absorption spectra. Emission at these wavelengths can be used as an internal calibration reference. 

A. Smit, M. Pollard, P. Cleaton-Jones, and A. Preston, A comparison of three electrodes for the measurement of pH in small volumes. Caries Res. 31, 55-59 (1997). 

Methods in which the cell potential for the sample solution is compared with that for one or more standards are rapid, simple and readily automated. The measurement of pH is the most common application of this type, one or more buffer solutions serving to calibrate the pH-meter (potentiometer). In all such measurements, calibration involves the evaluation of the constant in the equation... 

If there is a change in pH of the medium due to reaction and this change corresponds to the change in concentration of the reactant species (e.g. H+), the measurement of pH as a function of time can also be used to follow the progress of the reaction. 

Which of the following factors will affect the measurement of pH using a glass electrode ... 

A large number of sensitive fluorescent probes are now available for the measurement of pH and of the concentration of ions. 

L. Brown, P. J. Hailing, G. A. Johnston, and C. J. Suckling, Water insoluble indicators for the measurement of pH in water immiscible solvents, Tetrahedron Lett. 31, 5799-5802 (1990). 

Although in this text they are not described as such, many of the applications described in section 3.5.6 fall into the category of luminescent signalling. For instance, the detection of in section 3.S.6.3, the measurement of pH in section 3.5.6.4, and the use of lanthanide chelates in DNA studies outlined in section 3.5.6.9 all use the change in fluorescence for both the signalling and quantification of the target. 

The measurement of pH is one of the most common tests performed in a chemical laboratory since many chemical processes and properties are pH dependent. Examples of these processes are the kinetics of chemical reactions, the spectrum of certain dyes, as well as the solubility and/or bioavailability of many chemicals. 

The measurement of pH is carried out using a sensing electrode, which is sensitive to hydrogen-ion activity and a reference electrode. Combination electrodes incorporating both of these electrodes are also suitable for most applications. Separate reference and sensing electrodes are normally used only for high-precision research applications. 

Semiaqueous or Nonaqueous Solutions. Although the measurement of pH in mixed solvents (e.g., water/organic solvent) is not recommended, for a solution containing more than 5% water, the classical definition of a pH measurement may still apply. In nonaqueous solution, only relative pH values can be obtained. Measurements taken in nonaqueous or partly aqueous solutions require the electrode to be frequently rehydrated (i.e soaked in water or an acidic buffer). Between measurements and after use with a nonaqueous solvent (which is immiscible with water), the electrode should first be rinsed with a solvent, which is miscible with water as well as the analyte solvent, then rinsed with water. Another potential problem with this type of medium is the risk of precipitation of the KC1 electrolyte in the junction between the reference electrode and the measuring solution. To minimize this problem, the reference electrolyte and the sample solution should be matched for mobility and solubility. For example, LiCl in ethanol or LiCl in acetic acid are often used as the reference electrode electrolyte for nonaqueous measurements. 

The measurement of pH is also used to control NH3 feeding in glutamic acid fermentation [2]. 

Table 9-2 lists pA-., values for common buffers that are widely used in biochemistry. The measurement of pH with glass electrodes, and the buffers used by the U.S. National Institute of Standards and Technology to define the pH scale, are described in Chapter 15. 

To identify systematic errors in the measurement of pH of rainwater, a careful study was conducted with 17 laboratories.19 Eight samples were provided to each laboratory, along with instructions on how to conduct the measurements. Each laboratory used two buffers to standardize pH meters. Sixteen laboratories successfully measured the pH of Unknown A (within 0.02 pH unit), which was 4.008 at 25°C. One lab whose measurement was 0.04 pH unit low had a faulty commercial standard buffer. 

Final adjustment with HC1 may be needed if the pH is more than one unit away from a pfCa value. When two buffering materials are present, the composition should be calculated independently for each. The measurement of pH should always be done with great care because it is easy to make errors. Everything depends upon the reliability of the standard buffers used to calibrate the pH meter.7 Often, especially during isolation of small compounds, it is desirable to work in the neutral pH region with volatile buffers, e.g., trimethylamine and C02 or ammonium bicarbonate,... 

One important application of the Nernst equation is the measurement of pH (and, through pH, acidity constants). The pH of a solution can be measured electrochemically with a device called a pH meter. The technique makes use of a cell in which one electrode is sensitive to the H30+ concentration and the second electrode serves as a reference. An electrode sensitive to the concentration of a particular ion is called an ion-selective electrode. One combination is a hydrogen electrode connected through a salt bridge to a calomel electrode. The reduction half-reaction for the calomel electrode is... 

The measurement of pH in cheese making is extremely important to control fermentation/acid production and hence the final quality. While there are no standard methods available for measuring cheese pH, there have been few methods reported in the literature. One method involves preparing a slurry of 10 g of grated cheese in water and measuring the pH potentiometrically (Fox et al., 2004a). However, this method may alter the balance between colloidal and soluble calcium phosphate and hence it is preferable to measure the pH of the cheese directly. The quinhydrone electrode method (Marshall, 1992) measures the pH directly. The potential (mV) created by a paste of cheese and quinhydrone in saturated KC1 is measured and used to determine the pH at a particular temperature. 

Chemical sensing is part of an information-acquisition process in which an insight is obtained about the chemical composition of the system in real-time. In this process, an amplified electrical signal results from the interaction between some chemical species and the sensor. Generally, the interaction consists of two steps recognition and amplification. One common example is the measurement of pH with a glass electrode (Fig. 1.1). 

The measurement of pH is one of the most important measurements in water chemistry. The value of pH defines the types and the rates of chemical reactions in water, and the fate and bio availability of the living organisms. Together with temperature measurements, pH values serve as groundwater well stabilization indicators. 

Finally, the measurement of pH(X) of unknown solutions, the actual subject of pH measurement, is carried out in practical cells with transference containing -> glass electrodes. Calibration procedures for such practical cells are recommended so that the unknown pH(X) is traced back to pH(SS), pH(PS) and to the defined pH. 

C. The potentiometric determination of pH The most advanced and precise method of the measurement of pH is based on the measurement of the electromotive force (e.m.f.) of an electrochemical cell, which contains the solution of the unknown pH as electrolyte, and two electrodes. The electrodes are connected to the terminals of an electronic voltmeter, most often called simply a pH-meter. If properly calibrated with a suitable buffer of a known pH, the pH of the unknown solution can be read directly from the scale. 

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