Measurements of pH

the standard solution can be replaced with a test solution, and OCP of the cell. Ex, should be measured. Assuming the Nernst equation for the glass electrode 

When a number of buffer solutions are available, it is safer to assume that the slope 0 can slightly deviate from Nemstian and, therefore, should be found using two or more standard solutions. If two standard solutions SI and S2 are used, then the Nemstian slope can be found from the following equation  

Then pHx can be defined by an equation similar to Equation 5.10 replacing [F/(2303RT)] with (1/0)  

These days, the use of a pH glass indicator electrode and an Ag/AgCl reference electrode along with a high-resistance electrometer is not common practice. Instead, a combined pH/silver-silver chloride sensor shown in Eigure 5.4 is conunonly used 

FIGURE 5.4 Schematic of a combined pH/silver-silver chloride sensor. (1) Ag/AgCl reference electrode, (2) salt bridge, (3) filling hole, (4) reference electrode solution, (5) internal buffer solution, (6) Ag/AgCl internal electrode, (7) pH-sensitive glass membrane, and (8) Pt wires. 

In Section 12.3, we discussed the use of litmus paper, pH paper, and indicators as means for determining the acidity level, or pH, of a solution. While these have been widely used and are convenient and inexpensive for many applications, they do not provide the laboratory worker with the precision that is often needed. The use of pH paper is the most precise of the three, but it can only determine pH to within 1 pH unit. Often the pH of a solution needs to be measured to the first or second decimal place. 

Since the meter is a sort of modified voltmeter, the measurement is a relative measurement. This means that the voltage developed at the tip of the probe is measured relative to some other voltage. This other voltage is the 

A pH determination using the hydrogen gas electrode requires two measurements of the e.m.f. of a pH cell, std and x, corresponding to the immersion of the electrodes in a solution of known pH and in the unknown solution. The relation is 

However, almost all pH measurements now use the glass electrode and, to allow for slight variations in its pH response, the electrode is calibrated against two pH standards which, if possible, bracket the pH range of the test solutions. The measured e.m.f. of an unknown solution can then be converted directly to a pH measurement. This procedure is based on the operational definition of pH as proposed by Bates and Guggenheim (1960). The equation is 

Alternatively, the pH of an unknown solution may be determined with the aid of indicators whose colour changes have been correlated with pH values of standard solutions. For fuller details of the technique of pH measurement, see Albert and Seijeant (1971). 

Determinations of pH, either by potentiometry or by spectrophotometry, thus require accurately prepared reference solutions of analytical grade reagents as standards. The standards should be well separated on the pH scale but, to minimize the effects of liquid-junction potentials, they should not be less than pH 3 nor greater than pH 11. 

The usual standards for pH measurement are 0.05 m potassium hydrogen phthalate and 0.01 M sodium borate buffers having the assigned values given in Table 3.1. (Strictly, the United States National Bureau of Standards (NBS) standards are based on molalities (moles per kg. 

Blood and urine pH can be measured easily by means of a calibrated glass electrode, whereas pH measurement inside the metabolizing cells is not easily accomplished. Techniques for estimating intracellular pH include glass electrode measurements on homogenates, calorimetric or fluorometric analysis of intracellular distribution of indicator dyes, and microelectrode methods. 

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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. 

Measurement of pH With the availability of inexpensive glass pH electrodes and pH meters, the determination of pH has become one of the most frequent quantitative analytical measurements. The potentiometric determination of pH, however, is not without complications, several of which are discussed in this section. 

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. 

The glass pH electrode has been the most widely used tool for measurement of pH. Optical pH sensing is one of the most well established methods of pH determinations, which is based on measurements of the absorption spectmm of an indicator, either dissolved in the test solution or immobilized on a substrate. 

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. 

Measurement of Acidity or Alkalinity of Explosives. (Measurement of pH value). These tests consist essentially of extrg the sample of expl with w (either cold in the case of liquids such as NG, or hot for solids such as TNT, Tetryl, PA, etc), and detg the acidity of the resulting extr either by titration, colorimetric methods or by means of a pH meter. These tests are described under the individual compds... 

Measurement of pH was performed using a Metrohm model 691 pH meter equipped with a Metrohm combined LL micro pH glass electrode calibrated prior to use with pH = 2 and 9 buffers. The checkers found that adjustment to a lower pH led to product with higher amounts of inorganic impurities. The checkers also found that the use of pH paper results in different pH values as compared to the pH meter. 

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. 

Dissolved inorganic carbon is present as three main species which are H2CO3, HCOs and CO. Analytically we have to approach the carbonate system through measurements of pH, total CO2 or DIC, alkalinity (Aik), and PcOj- In an open carbonate system there are six unknown species H", OH , PcOj/ H2CO3, HCOs, and CO . The four equilibrium constants connecting these species are K, Ki, Kh, and fCw. The values of these equilibrium constants vary with T, P, and S (Millero, 1995). To solve for the six rmknowns we need to measure two of the four analytical parameters (Stumm and Morgan, 1996). Direct measurement of Pco is the best approach, but if that is not possible then the most accurate and precise pair (Dickson, 1993) is Total CO2 by the coulometric method Johnson et al., 1993) and pH by the colorimetric method (Clayton et ah, 1995). 

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. 

Glass pH electrodes are simple to use and maintain. They respond selectively to hydronium ion concentration and provide accurate measurements of pH values between about 0 and 10. They can be small enough to be implanted into blood vessels or even inserted into individual living cells. In precision work, these electrodes are calibrated before each use, because their characteristics change somewhat with time and exposure to solutions. The electrode is dipped into a buffer solution of known pH, and the meter is electronically adjusted until it reads the correct value. 

How can you know whether reactants or products are favored in a reaction at equilibrium The answer depends upon the reaction. For the club soda reaction, a measurement of pH indicates the amount of acid present in a solution. The lower the pH, the more acid is present. 

Measurements of pH were made with a glass electrode on 25 grams of lemon peel macerated in 100 ml. of distilled water in a Waring Blendor, and cleared by centrifugation. 

Covington, A. K., R. G. Bates and R. A. Durst, Definition of pH scales, standard reference values, measurement of pH and related terminology, Pure Appl. Chem., 57, 531 (1985). 

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,... 

Povrozin YA, Markova LI, Tatarets AL, Sidorov VI, Terpetschnig EA, Patsenker LD (2009) Near-infrared, dual-ratiometric fluorescent label for measurement of pH. Anal Biochem 390 136-140... 

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. 

Byme L., Lau K.T., Diamond D., Monitoring of headspace total volatile basic nitrogen from selected fish species using reflectance spectroscopic measurements of pH sensitive films, Analyst 2002 127 (10) 1338-1341. 

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