The determination of pH

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 best known of the M/Mfiy electrodes that are used for the determination of pH are the Sb/SbjOj electrodes, but metals such as bismuth and arsenic filmed with their respective oxide act in a similar manner. Copper in alkaline solutions appears to behave as a pH-dependent Cu/CujO,... 

Soils were further characterized by the determination of pH in water and potassium chloride in the proportions 1 2.5, total organic carbon (TOC) and cation exchange capacity (CEC). Transfer coefficients between soil and plant and enrichment in soils were determined. The results for Pb were represented in the form of relative enrichments for soils (Kabata-Pendias, 1985) and transfer coefficients in plants (Kovalevskii, 1979). 

Although all characteristics of electricity have been used to investigate soil and its properties, only a limited number are used routinely. The most common are those used for the determination of pH, salt content, and soil water content. Of these three, pH is the most common measurement and frequently the first measurement made prior to all other determinations. Although pH can be determined by many methods, for soil, the most common is to use a pH meter and electrode. Conductivity or resistance is used to measure soil salt content, while several different electrical characteristics of soil are used to determine... 

Recently we found that the presence of ions introduces artifacts in the determination of pH by pyranine (54). This effect is related to the relative position of the ion (both cations and anions) in the Hoffmeir series (54). Compared with other ions, AS was found to have only a minimal effect on this shift, which agrees well with the location of NH4+ in the cation Hoffmeir series and of sulfate in the anion series (54). 

Measurement of pH is a potentiometric technique frequently used for measuring the degree of the deterioration of materials that are subjected to natural aging. The determination of pH levels is commonly carried out on ethnographic objects manufactured with parchment or leather, and it is especially relevant in altered paper due to the formation of acidic compounds from the decomposition of the woodpulps and other raw materials, which can induce the hydrolysis of the cellulose and then decrease the resistance and mechanical properties of the document [29]. 

Accurate measurement of pH is critically dependent on good analytical procedures, a fact that may not be appreciated by laboratory personnel [1,2]. The assumption is often made that if the electrode has been calibrated, there will be no variability in pH between laboratories. The pH measurement can erroneously be seen as merely dipping the electrode into the analyte and recording the value. In 1985, Davidson and Gardner [3] drew the following conclusion from their study Interlaboratory Comparisons of the Determination of pH in Poorly Buffered Fresh Waters ... 

In colloidal solutions the measurement of the pH is always delicate, since it depends on the average distance between the oxide particles and the pH electrode. This explains why the latter is a function of the stirring rate of the solution. The best measure is the value obtained without stirring, the solid being far from the electrode and deposited at the bottom of the flask (a centrifugation is sometimes necessary). This measure can be considered as the pH of the bulk solution. This latter value, combined with the characteristics of the solution and the IEP of the oxide, allows calculation of the pH profile from the theory developed by Davis et al. [19]. Devices based on fiber-optic sensors [91] are being developed which might help resolve the difficult problem of the determination of pH of oxides in suspension. 

An example of the second case is the determination of pH or titratable acidity, which are reliably determined by the measurement of the complex equilibrium of the organic compounds that determine their values. Routinely used methods are presented in Table 12.2. 

The second possible cause of erroneous results is the presence of proteins as a general rule, indicator methods are not satisfactory for the determination of pH in protein solutions. The error varies with the nature of the indicator it is usuall less for low molecular weight compounds than for complex molecules. 

An also effective gravimetric method for testing of environmental samples has been reported [78]. This type of sample has been handled by robotic systems for the determination of pH, BOD and suspended solids, using a sophisticated interface that allows the operator to start and stop the robot at any time during the analysis without the need for recovery software [79]. 

ISFETs do not require hydration before use and can be stored indefinitely in the dry state. Despite these many advantages, no ISFET-specific ion electrode appeared on the market until the early 1990s, more than 20 years after its invention. The reason for this delay is that manufacturers were unable to develop the technology to encapsulate the devices to give a product that did not exhibit drift and instability. Several companies now produce ISFETs for the determination of pH, but they are certainly not as routinely used as the glass pH electrode at this time. 

The determination of pH profiles is no problem with the ANTS/DPX assay but more difficult with the pH-sensitive CF and HPTS assays [19, 21]. Determination in BLMs poses no problems and Na NMR spectroscopy should be possible as well. The determination of the ionic strength dependence in BLMs is routine and gives important information on the affinity of the ions to the channels (Section 11.3.6). However, the low conductance often observed at low ionic strength can make the experiments difiicult. For vesicle experiments with varied external ionic strength, osmotic stress can be avoided with high internal ionic strength and compensation of external under-pressure using sucrose [15, 18]. 

Disazo indicators. In this group of indicators are included Congo red, benzopurpurin B, and benzopurpurin 4B, none of which are recommended for use as indicators for the determination of pH. Though the red alkaline form is soluble in water, the blue or violet acid form is insoluble. 

It is a very simple matter to determine B BH+ ratios for, with one-color indicators, it is necessary to measure only the intensity of a color (cf. Chapter Nine on the determination of pH with one-color indicators in unbuffered solutions). One must remember that the solvent may have an effect upon the absorption spectrum and upon the color intensity (absorption coefficient). Such uncertainties may be avoided by the use of comparison solutions having the same composition as the solutions under investigation, or by applying a correction, for the influence of solvent, ascertained by comparing the color of the experimental solutions with that of aqueous solutions. 

Thymolphthalein is not especially suitable for the determination of pH. The acid form is insoluble in water and fading of the color is nearly always observed. When 0.1 c.c. of a 0.1% indicator solution is added to 10 c.c. of a carbonate buffer of pH 10, there appears a light blue color which fades rapidly on standing, due to the flocculation of the acid form. Indicator must be added simultaneously to the buffer and unknown solution and the colors compared immediately if thymolphthalein is used. Even so, results are not always reliable. 

The determination of pH with one-color indicators in unbuffered solutions. 

The writer also has investigated the reliability of the determination of pH by means of indicator papers. These results will be summarized briefly. Contrary to the directions of Haas, the drop should not be permitted to dry. The color of the dried paper is not so distinct, and small differences may be overlooked. Instead of a glass rod, it is better to use a capillary tube in transporting the liquid to the paper. By so doing, 10-20 mm. will be sufficient for a determination. Hardened paper is best as a rule. Schleicher and SchOll for capillary analysis filter paper is also satisfactory. 

The determination of pH is often performed by emf measurements of galvanic cells involving liquid junctions [69ROS], [73BAT], A common setup is a cell made up of a reference half cell (e.g. Ag(s)/AgCl(s) in a solution of constant chloride concentration), a salt bridge, the test solution, and a glass electrode (which encloses a solution of constant acidity and an internal reference half cell) ... 

The Standard Potential of the Quinhydrone Electrode. The quin-hydrone electrode is of interest and importance as a method for the determination of pH values and because the oxidation-reduction relations of quinone and hydroquinone have been extensively studied. It will however receive consideration here because it is an excellent example of the use of cells without40 liquid junctions for the determination of the standard potential of a galvanic cell of a somewhat more complex type than those so far considered. 

For further details concerning the operation of this cell, and much else on the determination of pH, see W M. Clark s The Determination of Hydrogen Ions/ 3rd ed The Williams and Wilkins Co., Baltimore, 1928. 

Many of the details of the method will be familiar from the discussion of the determination of pH values. A suitable galvanic cell for the titration of an acid, HA, with a base is... 

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