Ion-selective electrodes in titrations

Vytras K, Kalous J, Kala bova Z, Remes M (1982) Ion-selective electrodes in titrations involving azo-couphng reactions. Anal Chem Acta 141 163—171 Wang L, Barrington S, Kim JW (2007) Biodegradation of pentyl amine and aniUne from petrochemical wastewater. J Environ Manage 83 191-197. http //www.doi.oig/10.1016/j. jenvman.2006.02.009... 

Vytras K, Remes M, Kubesova-Svobodova H (1981) Coated-wire organic ion-selective electrodes in titrations based on ion-pair formation determination of arenediazonium salts with sodium tetraphenylborate. Anal Chim Acta 124 91-98... 

Fig. 5.17 shows the curves for the potentiometric titration of Ca2f in the range 5 10 3-5 10 2 M with a titrant carrier stream of 5 10 4 M EDTA using a calcium ion-selective electrode each titration is initiated by an abrupt increase in the potential, followed by an S-shaped decrease in which the inflection point marks the end of titration. According to eqn. 5.12, where the titration product is AB , the mixing volume V, the original concentration of A in the sample Cl and the titrant concentration CB, can be calculated. In the experiments in Fig. 5.17 the sample volume was 200/d and/ = 0.84 ml min-1 by... 

In Nebraska, state regulations require that the chemical makeup of animal feed sold in the state be accurately reflected on the labels found on the feed bags. The Nebraska State Agriculture Laboratory is charged with the task of performing the analytical laboratory work required. An example is salt (sodium chloride) content. The method used to analyze the feed for sodium chloride involves a potentio-metric titration. A chloride ion-selective electrode in combination with a saturated calomel reference electrode is used. After dissolving the feed sample, the chloride is titrated with a silver nitrate standard solution. The reaction involves the formation of the insoluble precipitate silver chloride. The electrode monitors the decrease in the chloride concentration as the titration proceeds, ultimately detecting the end point (when the chloride ion concentration is zero). 

Elemental composition Ce 56.85%, Cl 43.15%. In the aqueous phase following acid digestion, cerium may he analyzed by various instrumental techniques (see Cerium). Chloride ion in the solution may be measured by ion chromatography, chloride ion-selective electrode or titration with silver nitrate using potassium chromate indicator. The solution may require appropriate dilution for analysis of both the metal and the chloride anion. 

Finding the End Point Potcntiomctrically Another method for locating the end point of a precipitation titration is to monitor the change in concentration for the analyte or titrant using an ion-selective electrode. The end point can then be found from a visual inspection of the titration curve. A further discussion of potentiome-try is found in Chapter 11. 

Potcntiomctric Titrations In Chapter 9 we noted that one method for determining the equivalence point of an acid-base titration is to follow the change in pH with a pH electrode. The potentiometric determination of equivalence points is feasible for acid-base, complexation, redox, and precipitation titrations, as well as for titrations in aqueous and nonaqueous solvents. Acid-base, complexation, and precipitation potentiometric titrations are usually monitored with an ion-selective electrode that is selective for the analyte, although an electrode that is selective for the titrant or a reaction product also can be used. A redox electrode, such as a Pt wire, and a reference electrode are used for potentiometric redox titrations. More details about potentiometric titrations are found in Chapter 9. 

The titration of AF+ with E in a water-acetonitrile mixture is followed potentiometrically using a fluoride ion-selective electrode. The cumulative formation constant for AlEs is determined from the titration curve. 

The potentiometric micro detection of all aminophenol isomers can be done by titration in two-phase chloroform-water medium (100), or by reaction with iodates or periodates, and the back-titration of excess unreacted compound using a silver amalgam and SCE electrode combination (101). Microamounts of 2-aminophenol can be detected by potentiometric titration with cupric ions using a copper-ion-selective electrode the 3- and... 

The sodium hydroxide is titrated with HCl. In a thermometric titration (92), the sibcate solution is treated first with hydrochloric acid to measure Na20 and then with hydrofluoric acid to determine precipitated Si02. Lower sibca concentrations are measured with the sibcomolybdate colorimetric method or instmmental techniques. X-ray fluorescence, atomic absorption and plasma emission spectroscopies, ion-selective electrodes, and ion chromatography are utilized to detect principal components as weU as trace cationic and anionic impurities. Eourier transform infrared, ft-nmr, laser Raman, and x-ray... 

Perhaps the most precise, reHable, accurate, convenient, selective, inexpensive, and commercially successful electroanalytical techniques are the passive techniques, which include only potentiometry and use of ion-selective electrodes, either direcdy or in potentiometric titrations. Whereas these techniques receive only cursory or no treatment in electrochemistry textbooks, the subject is regularly reviewed and treated (19—22). Reference 22 is especially recommended for novices in the field. Additionally, there is a journal, Ion-Selective Electrode Reviews, devoted solely to the use of ion-selective electrodes. 

Potentiometric Titrations. If one wishes to analyze electroactive analytes that are not ions or for which ion-selective electrodes are not available, two problems arise. First, the working electrodes, such as silver, platinum, mercury, etc, are not selective. Second, metallic electrodes may exhibit mixed potentials, which may arise from a variety of causes. For example, silver may exchange electrons with redox couples in solution, sense Ag" via electron exchange with the external circuit, or tarnish to produce pH-sensitive oxide sites or Ag2S sites that are sensitive to sulfide and haUde. On the other... 

The reference electrode contributes heavily to the economics of electroanalytical chemistry. Companies that sell and service electroanalytical instmmentation are few in number and small in size, or they are parts of much larger companies. One suppHer of electroanalytical instmmentation is Princeton AppHed Research Corp. (PARC) of Princeton, Newjersey. PARC is a subsidiary of EG G Instmments, Inc. Among the many suppHers of ion-selective electrodes are Orion (Boston, Massachusetts), Corning (Corning, New York), and Ingold (Wilmington, Massachusetts). Brinkmann Instmments, Inc. (Westbury, New York) is a useful suppHer of titration equipment. 

Anionic surfactants like alkanesulfonates can also be determined by poten-tiometric titration with ion-selective electrodes. Recently, a special PVC electrode was offered as a high-sense surfactant electrode in combination with the new titrant l,3-didecyl-2-methylimidazolium chloride [20]. This one-phase... 

Electrochemical analytical techniques are a class of titration methods which in turn can be subdivided into potentiometric titrations using ion-selective electrodes and polarographic methods. Polarographic methods are based on the suppression of the overpotential associated with oxygen or other species in the polarographic cell caused by surfactants or on the effect of surfactants on the capacitance of the electrode. One example of this latter case is the method based on the interference of anionic surfactants with cationic surfactants, or vice versa, on the capacitance of a mercury drop electrode. This interference can be used in the one-phase titration of sulfates without indicator to determine the endpoint... 

The methods most commonly used to detect hydrogen sulfide in environmental samples include GC/FPD, gas chromatography with electrochemical detection (GC/ECD), iodometric methods, the methylene blue colorimetric or spectrophotometric method, the spot method using paper or tiles impregnated with lead acetate or mercuric chloride, ion chromatography with conductivity, and potentiometric titration with a sulfide ion-selective electrode. Details of commonly used analytical methods for several types of environmental samples are presented in Table 6-2. 

Potentiometric titration with a sulfide ion-selective electrode as an indicator has been used to measure hydrogen sulfide in the air at ppb levels (Ehman 1976). The method has been shown to have very good accuracy and precision. No interference could be found from nitrogen dioxide, sulfur dioxide, or ozone. 

Calcium-selective electrodes have long been in use for the estimation of calcium concentrations - early applications included their use in complexometric titrations, especially of calcium in the presence of magnesium (42). Subsequently they have found use in a variety of systems, particularly for determining stability constants. Examples include determinations for ligands such as chloride, nitrate, acetate, and malonate (mal) (43), several diazacrown ethers (44,45), and methyl aldofuranosides (46). Other applications have included the estimation of Ca2+ levels in blood plasma (47) and in human hair (where the results compared satisfactorily with those from neutron activation analysis) (48). Ion-selective electrodes based on carboxylic polyether ionophores are mentioned in Section IV.B below. Though calcium-selective electrodes are convenient they are not particularly sensitive, and have slow response times. 

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. 

In addition, potentiometric titration methods exist in which an electrode other than an ion-selective electrode is used. A simple platinum wire surface can be used as the indicator electrode when an oxidation-reduction reaction occurs in the titration vessel. An example is the reaction of Ce(IV) with Fe(II) ... 

Ion-selective electrodes are particularly useful for monitoring the disappearance of an ion during a titration. In many cases it is not necessary to calibrate the instrument because there is often a significant change in the potential at the end-point of a titration. However, some electrodes have a slow response time and care must be taken to ensure that titration is not performed too quickly. 

Ion-selective electrodes have certain undoubted advantages (a) they do not affect the test solution (b) they are portable (c) they are suitable both for direct determinations and as sensors in titrations (< ) they are not expensive. 

Analytical determinations with the fluoride ion-selective electrode These are based either on direct potentiometry of fluorides [37, 84, 85, 88, 430] or on titration determinations of either fluorides or of other ions and also on titrations with fluoride ions as indicator. The advantages of potentiometry with an ISE over other analytical methods for determining fluorides were pointed out by Crosby etai [67], Further comparison studies [42, 56, 191, 433] came to the same conclusions, confirmed also by a study of 16 methods [365]. Fluoride ions are titrated either with La (for concentrations greater than 1 mM) or Th (in the concentration range 0.2-1 mM F ) [13, 102, 103, 113,233, 234]. Titration with fluoride ions can be used for the determination of Al with formation of the AIF4 complex up to nanomolar concentrations, especially in ethanol-water mixtures [25] (see also [267,384]). Precipitation titrations can also be used to determine La, Th and UOJ [241, 384] as well as Li in... 

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