Solution potentiometric monitoring

A related form of an automatic potentiometric titrator is instrumentation that permits the maintenance of the acidity or basicity of a solution over a period of time. Such devices are known as pH-stats, and find application in kinetic studies of hydrolysis reactions. The general approach is (by either manual or automatic means) to add either acid or base such that the pH in the solution is maintained constant over a period of time. Normally the amount of acid or base added as a function of time is sought in order that kinetic measurements may be made for the system. In its simplest form the acidity of the solution is monitored with a pH meter and controlled at a preselected value by the addition of acid or base from a burette the quantity delivered as a function of time is recorded in a notebook. Obviously for the fast reactions this becomes difficult and dependent on the dexterity of the individual. 

Finding the End Point Potentiometrically Another method for locating the end point of a redox titration is to use an appropriate electrode to monitor the change in electrochemical potential as titrant is added to a solution of analyte. The end point can then be found from a visual inspection of the titration curve. The simplest experimental design (Figure 9.38) consists of a Pt indicator electrode whose potential is governed by the analyte s or titrant s redox half-reaction, and a reference electrode that has a fixed potential. A further discussion of potentiometry is found in Chapter 11. 

The potentiometric method also surpasses the others for speed, simplicity, precision and accuracy as indicated in Table 12.1. Furthermore, it is particularly suited to the continuous monitoring of fluoride levels in drinking water. The spectrophotometric methods are lengthy because of the time required to develop a stable colour (up to 1 hour), the alizarin red-S complex being especially poor in this respect. It was noted, however, that for the three bleaching methods (1-3) the rate of change of absorbance by the blank closely followed that of solutions containing fluoride, i.e. the difference between the blank and a sample absorbance is nearly constant. 

As mentioned previously, electroanalytical techniques that measure or monitor electrode potential utilize the galvanic cell concept and come under the general heading of potentiometry. Examples include pH electrodes, ion-selective electrodes, and potentiometric titrations, each of which will be described in this section. In these techniques, a pair of electrodes are immersed, the potential (voltage) of one of the electrodes is measured relative to the other, and the concentration of an analyte in the solution into which the electrodes are dipped is determined. One of the immersed electrodes is called the indicator electrode and the other is called the reference electrode. Often, these two electrodes are housed together in one probe. Such a probe is called a combination electrode. 

Worked Example 3.5. A new means of extracting nickel from its ore is being investigated. The first step is to crush the rock to powder, roast it, and then extract soluble nickel species (as Ni " ") into an aqueous solution. The activity, a(Ni +), is monitored by a potentiometric method, where a wire of pure nickel metal functions as an electrode and is immersed in aliquot samples taken from the plant. This wire monitors the electrode potential NP+,Ni.If 2+,Ni =-0-230 V, what is if afNi " ) = 10... 

Figure 4.6 Schematic representation of the apparatus required when monitoring a precipitation process via a potentiometric titration. The salt bridge is impregnated with a saturated solution of KNO3.

An additional point worth mentioning is that the potentiometric method can monitor several partially soluble salts at once. For example, if a solution contains chloride, bromide and iodide ions, then a plot of emf against the volume of cation (e.g. Ag ) will contain three inflection points (see Figure 4.8), one for each of the three silver halides. for Agl is smaller than that for AgCl, while (AgBr) has an intermediate value, so the first inflection point represents the precipitation of Agl, the second represents formation of AgBr and the third represents the formation of insoluble AgCl. ... 

The equilibrium solubility of an Fe oxide can be approached from two directions -precipitation and dissolution. The first method involves precipitating the oxide from a supersaturated solution of ions with stepwise or continuous addition of base und using potentiometric measurements to monitor pH and calculate Fej- in equilibrium with the solid phase until no further systematic change is detected. Alternatively the oxide is allowed to dissolve in an undersaturated solution, with simultaneous measurement of pH and Fejuntil equilibrium is reached. It is essential that neither a phase transformation nor recrystallization (formation of larger crystals) occurs during the experiment this may happen with ferrihydrite which transforms (at room temperature) to a more condensed, less soluble phase. A discussion of the details of these methods is given by Feitknecht and Schindler (1963) and by Schindler (1963). 

During the past 40 years there have been numerous exciting extensions of electrochemistry to the field of analytical chemistry. A series of selective-ion potentiometric electrodes have been developed, such that most of the common ionic species can be quantitatively monitored in aqueous solution. A highly effective electrolytic moisture analyzer provides continuous online assays for water in gases. Another practical development has been the voltammetric membrane electrode for dioxygen (02), which responds linearly to the partial pressure of 02, either in the gas phase or in solution. The use of an immobilized enzyme (glucose oxidase) on an electrode sensor to assay glucose in blood is another extension of electrochemistry to practical analysis. 

Although all potentiometric measurements (except those involving membrane electrodes) ultimately are based on a redox couple, the method can be applied to oxidation-reduction processes, acid-base processes, precipitation processes, and metal ion complexation processes. Measurements that involve a component of a redox couple require that either the oxidized or reduced conjugate of the species to be measured be maintained at a constant and known activity at the electrode. If the goal is to measure the activity of silver ion in a solution, then a silver wire coupled to the appropriate reference electrodes makes an ideal potentiometric system. Likewise, if the goal is to monitor iron(UI) concentrations with a platinum electrode, a known concentration of... 

Table 2.1 summarizes a number of redox couples that are well behaved in aqueous solutions and provide a means for monitoring the indicated species by potentiometric measurements. This can be either in the form of monitoring a titration or as a direct absolute measurement of activity. Although the tabulations of standard potentials12 17 imply that the listing should be much more comprehensive, most of the couples tabulated are not well behaved in an electrochemical sense and do not provide a Nemstian response under normal laboratory conditions. The vast majority of the data tabulated is based on other than electrochemical measurements. 

Reference to Table 4.1 indicates that olefins can be determined by the electrochemical generation in situ of halogens. Bromine is effective for both olefins and sulfur compounds and is the basis for an automatic coulometric titrator for continuous analysis of petroleum streams.17 The basic principle of this instrument is a potentiometric sensing system that monitors bromine concentration in a continuously introduced sample stream. The bromine in the solution reacts with the sample components and causes a decrease in the concentration of bromine. When this decrease is sensed by the potentiometric detection electrodes, the electrolysis current producing bromine adjusts itself to maintain the bromine concentration. Because the sample is introduced at a constant rate, the electrolysis current becomes directly proportional to the concentration of the sample component. Thus, the instrument records the electrolysis current as concentration of sample component and provides a continuous monitor for olefins or sulfur in petroleum streams. 

The solution composition is monitored potentiometrically using ion-selective electrodes or conductiometrically using a platinum electrode. No attempt is made to control the solution composition. 

There are a number of factors that determine whether a protonic acid can initiate polymerization of alkenes. Their acidity (pKa), and therefore the basicity of the resulting counteranion, determines the efficiency of initiation. Although reliable pKa values of acids stronger than sulfuric or hy-droiodic (pKa < -9) are difficult to obtain in aqueous solutions due to their nearly complete dissociation, the pKa values of acetic acid (4.75) and trichloroacetic acid (0.7) in water provide useful references. Conductometric and potentiometric estimates of the pK values of selected protonic acids in various organic solvents are summarized in Table 11 in descending acid strength. These values are not very precise, however, because the amount of moisture in each system was not monitored precisely. 

Direct potentiometric measurements are used to complete chemical analyses of species for which an indicator electrode is available. The technique is simple, requiring only a comparison of the voltage developed by the measuring cell in the test solution with its voltage when immersed in a standard solution of the analyte. If the electrode response is specific for the analyte and independent of the matrix, no preliminary steps are required. In addition, although discontinuous measurements are mainly carried out, direct potentiometry is readily adapted to continuous and automatic monitoring. 

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