End point potentiometrically

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. 

Dissolve 0.35 g of miconazole nitrate in 75 mL of anhydrous acetic acid R, with slight heating, if necessary. Titrate with 0.1 M perchloric acid determining the end point potentiometrically, according the general method (2.2.20). Carry out a blank titration. One milliliter of 0.1 M perchloric acid is equivalent to 47.91 mg of Ci8H15Cl4N304. 

Procedure Weigh accurately about 0.3 g and dissolve in 50 ml of dehydrated pyridine. Titrate with 0.1N tetrabutylammonium hydroxide, determining the end point potentiometrically and protecting the solution and titrant from atmospheric carbon dioxide throughout the determination. Perform a blank determination and make any necessary correction. Each ml of 0.1N tetrabutylammonium hydroxide is equivalent to 0.03388 g of q4HuCIN204S. 

Theory Nitrazepam is a weakly basic compound and hence, it may be titrated conveniently by means of a non-aqueous titration technique and determining the end-point potentiometrically. 

Procedure Dissolve 0.2 g of clonidine hydrochloride in 70 ml of ethanol (96%) and titrate with 0.1M ethanolic sodium hydroxide Vs determining the end-point potentiometrically. Each ml of 0.1 M ethanolic sodium hydroxide Vs is equivalent to 26.66 mg of C9H9C12,N3, HC1. 

The British Pharmacopoeia has adopted the assay of chlorpromazine in acetone, using a non-aqueous titration with perchloric acid and methyl orange as the indicator [4], According to the described procedure, each mL of 0.1 M perchloric acid is equivalent to 0.03189 g of drug substance. For chlorpromazine hydrochloride, the BP has described its titration with NaOH in a 1 10 solution of 0.01 M HCl / ethanol. Determining the end point potentiometrically, each mL of 0.1 M sodium hydroxide is equivalent to 0.03558 g of substance. 

Assay Carry out Method 1 for nonaqueous titration, Appendix VIIIA, using 0.4 g and determining the end-point potentiometrically. Each milliliter of 0.1 M perchloric acid US is equivalent to 25.3 mg of C28H33C1N2-2HC1. 

Dissolve about 1 g of hyoscyamine sulfate, accurately weighed, in 50 mL of glacial acetic acid and titrate with 0.1N perchloric acid VS, determining the end-point potentiometrically. Perform a blank determination, and make any necessary correction. 

Dissolve 0.5 g in 25 ml of anhydrous glacial acetic acid. Carry out the non-aqueous titration method using 0.1M perchloric acid and determining the end-point potentiometrically. 

In many cases, where the colour of the material prevents sharp visual colour changes at the pH end-point, potentiometric titrations must be used. Sometimes a different pH end-point results using phenolphthalein indicator, as opposed to a potentiometric end-point, preventing absolute comparisons between techniques. However, the visual end-point using a suitable indicator such as phenolphthalein usually provides good accuracy and reproducible results on samples analysed by the same technique, provided the colour of the sample does not interfere with end-point detection. 

Suppose we consider some very weakly basic compounds, such as ketimines, phosphines, and oxiranes. A very interesting method of dealing with oxiranes was developed by Durbetaki. The oxirane was reacted with hydro-bromic acid to form the bromohydrln. This type of reaction has long been known using hydrochloric acid, but in that medium the reaction takes approximately three hours. In glacial acetic acid, the reaction is enough to allow you to titrate directly at normal speed. You can get an end point potentiometrically or with an indicator. In fact, if you have a mixture of amine and oxirane, you can get two potentlometric breaks, the first for the amine and the second for the oxirane. Amides, phosphene oxides, triphenyl methanol, and amine oxides are very weak bases and cannot be titrated in glacial acetic acid under ordinary conditions. However, they can be titrated if one uses acetic anhydride as solvent, or if one uses a solvent that is mixed with acetic anhydride. Why does acetic anhydride work There are two reasons. First, it removes the last trace of water from the solution secondly, perchloric acid in the presence of acetic anhydride forms the ion CHsCO". Since this is an extremely reactive substance, one can titrate very weak bases. 

End point (potentiometric) The apparent equivalence point of a titration at which a relatively large potential change is observed. 

Dissolve about 01 g in 25 ml of water, add 25 ml of glacial acetic acid and 25 ml of dilute hydrochloric acid and titrate with 0-02N ceric ammonium sulphate, determining the end-point potentiometrically using a platinum/calomel electrode system. 1 ml 0-02N = 0 004221 g... 

The most obvious sensor for an acid-base titration is a pH electrode.For example, Table 9.5 lists values for the pH and volume of titrant obtained during the titration of a weak acid with NaOH. The resulting titration curve, which is called a potentiometric titration curve, is shown in Figure 9.13a. The simplest method for finding the end point is to visually locate the inflection point of the titration curve. This is also the least accurate method, particularly if the titration curve s slope at the equivalence point is small. 

Initial attempts at developing precipitation titration methods were limited by a poor end point signal. Finding the end point by looking for the first addition of titrant that does not yield additional precipitate is cumbersome at best. The feasibility of precipitation titrimetry improved with the development of visual indicators and potentiometric ion-selective electrodes. 

The following experiments may he used to illustrate the application of titrimetry to quantitative, qtmlitative, or characterization problems. Experiments are grouped into four categories based on the type of reaction (acid-base, complexation, redox, and precipitation). A brief description is included with each experiment providing details such as the type of sample analyzed, the method for locating end points, or the analysis of data. Additional experiments emphasizing potentiometric electrodes are found in Chapter 11. 

This experiment outlines a potentiometric titration for determining the valency of copper in superconductors in place of the visual end point used in the preceding experiment of Harris, Hill, and Hewston. The analysis of several different superconducting materials is described. 

End Point Determination Adding a mediator solves the problem of maintaining 100% current efficiency, but does not solve the problem of determining when the analyte s electrolysis is complete. Using the same example, once all the Fe + has been oxidized current continues to flow as a result of the oxidation of Ce + and, eventually, the oxidation of 1T20. What is needed is a means of indicating when the oxidation of Fe + is complete. In this respect it is convenient to treat a controlled-current coulometric analysis as if electrolysis of the analyte occurs only as a result of its reaction with the mediator. A reaction between an analyte and a mediator, such as that shown in reaction 11.31, is identical to that encountered in a redox titration. Thus, the same end points that are used in redox titrimetry (see Chapter 9), such as visual indicators, and potentiometric and conductometric measurements, may be used to signal the end point of a controlled-current coulometric analysis. For example, ferroin may be used to provide a visual end point for the Ce -mediated coulometric analysis for Fe +. 

Description of the Method. The concentration of Cr207 in a sample is determined by a coulometric redox titration using Fe + as a mediator and electrogenerated Fe + as the "titrant." The end point of the coulometric redox titration is determined potentiometrically. 

Scale of Operation Coulometric methods of analysis can be used to analyze small absolute amounts of analyte. In controlled-current coulometry, for example, the moles of analyte consumed during an exhaustive electrolysis is given by equation 11.32. An electrolysis carried out with a constant current of 100 pA for 100 s, therefore, consumes only 1 X 10 mol of analyte if = 1. For an analyte with a molecular weight of 100 g/mol, 1 X 10 mol corresponds to only 10 pg. The concentration of analyte in the electrochemical cell, however, must be sufficient to allow an accurate determination of the end point. When using visual end points, coulometric titrations require solution concentrations greater than 10 M and, as with conventional titrations, are limited to major and minor analytes. A coulometric titration to a preset potentiometric end point is feasible even with solution concentrations of 10 M, making possible the analysis of trace analytes. 

Performing the titration to a potentiometric end point, rather than to a colored end point, has been shown to be the more accurate method. Since other carbonyl containing compounds also react to form the oxime and release hydrochloric acid, this test is not specific for benzaldehyde. 

For the primary stage (phosphoric) V) acid as a monoprotic acid), methyl orange, bromocresol green, or Congo red may be used as indicators. The secondary stage of phosphoric) V) acid is very weak (see acid Ka = 1 x 10 7 in Fig. 10.4) and the only suitable simple indicator is thymolphthalein (see Section 10.14) with phenolphthalein the error may be several per cent. A mixed indicator composed of phenolphthalein (3 parts) and 1-naphtholphthalein (1 part) is very satisfactory for the determination of the end point of phosphoric(V) acid as a diprotic acid (see Section 10.9). The experimental neutralisation curve of 50 mL of 0.1M phosphoric(V) acid with 0.1M potassium hydroxide, determined by potentiometric titration, is shown in Fig. 10.6. 

The various relationships concerning the interconversion between un-ionised and ionised or different resonant forms of indicators referred to in Section 10.7 apply equally well to those indicators used for non-aqueous titrations. However, in this type of titration the colour change exhibited by an indicator at the end point is not always the same for different titrations as it depends upon the nature of the titrand to which it has been added. The colour corresponding to the correct end point may be established by carrying out a potentiometric titration while simultaneously observing the colour change of the indicator. The appropriate colour corresponds to the inflexion point of the titration curve (see Section 15.18). 

In acid-base titrations the end point is generally detected by a pH-sensitive indicator. In the EDTA titration a metal ion-sensitive indicator (abbreviated, to metal indicator or metal-ion indicator) is often employed to detect changes of pM. Such indicators (which contain types of chelate groupings and generally possess resonance systems typical of dyestuffs) form complexes with specific metal ions, which differ in colour from the free indicator and produce a sudden colour change at the equivalence point. The end point of the titration can also be evaluated by other methods including potentiometric, amperometric, and spectrophotometric techniques. 

The hydrogen ions thus set free can be titrated with a standard solution of sodium hydroxide using an acid-base indicator or a potentiometric end point alternatively, an iodate-iodide mixture is added as well as the EDTA solution and the liberated iodine is titrated with a standard thiosulphate solution. 

Variamine blue (C.I. 37255). The end point in an EDTA titration may sometimes be detected by changes in redox potential, and hence by the use of appropriate redox indicators. An excellent example is variamine blue (4-methoxy-4 -aminodiphenylamine), which may be employed in the complexometric titration of iron(III). When a mixture of iron(II) and (III) is titrated with EDTA the latter disappears first. As soon as an amount of the complexing agent equivalent to the concentration of iron(III) has been added, pFe(III) increases abruptly and consequently there is a sudden decrease in the redox potential (compare Section 2.33) the end point can therefore be detected either potentiometrically or with a redox indicator (10.91). The stability constant of the iron(III) complex FeY- (EDTA = Na2H2Y) is about 1025 and that of the iron(II) complex FeY2 - is 1014 approximate calculations show that the change of redox potential is about 600 millivolts at pH = 2 and that this will be almost independent of the concentration of iron(II) present. The jump in redox potential will also be obtained if no iron(II) salt is actually added, since the extremely minute amount of iron(II) necessary is always present in any pure iron(III) salt. 

Conductimetric measurements can also be used to ascertain the end-point in many titrations, but such use is limited to comparatively simple systems in which there are no excessive amounts of reagents present. Thus, many oxidation titrations which require the presence of relatively large amounts of acid are not suited to conductimetric titration. Conductimetric titrations have been largely superseded by potentiometric procedures (see Chapter 15), but there are occasions when the conductimetric method can be advantageous.14... 

It may be noted that very weak acids, such as boric acid and phenol, which cannot be titrated potentiometrically in aqueous solution, can be titrated conductimetrically with relative ease. Mixtures of certain acids can be titrated more accurately by conductimetric than by potentiometric (pH) methods. Thus mixtures of hydrochloric acid (or any other strong acid) and acetic (ethanoic) acid (or any other weak acid of comparable strength) can be titrated with a weak base (e.g. aqueous ammonia) or with a strong base (e.g. sodium hydroxide) reasonably satisfactory end points are obtained. 

---