Detection of the end point

A solution of iodine in aqueous iodide has an intense yellow to brown colour. One drop of 0.05M iodine solution imparts a perceptible pale yellow colour to 100 mL of water, so that in otherwise colourless solutions iodine can serve as its own indicator. The test is made much more sensitive by the use of a solution of starch as indicator. Starch reacts with iodine in the presence of iodide to form an intensely blue-coloured complex, which is visible at very low concentrations of iodine. The sensitivity of the colour reaction is such that a blue colour is visible when the iodine concentration is 2 x 10 5 M and the iodide concentration is greater than 4x 10 4M at 20 °C. The colour sensitivity decreases with increasing temperature of the solution thus at 50 °C it is about ten times less sensitive than at 25 °C. The sensitivity decreases upon the addition of solvents, such as ethanol no colour is obtained in solutions containing 50 per cent ethanol or more. It cannot be used in a strongly acid medium because hydrolysis of the starch occurs. 

Starches can be separated into two major components, amylose and amylopectin, which exist in different proportions in various plants. Amylose, which is a straight-chain compound and is abundant in potato starch, gives a blue colour with iodine and the chain assumes a spiral form. Amylopectin, which has a branched-chain structure, forms a red-purple product, probably by adsorption. 

The great merit of starch is that it is inexpensive. It possesses the following disadvantages (1) insolubility in cold water (2) instability of suspensions in water (3) it gives a water-insoluble complex with iodine, the formation of which precludes the addition of the indicator early in the titration (for this reason, in titrations of iodine, the starch solution should not be added until just prior to the end point when the colour begins to fade) and (4) there is sometimes a drift end point, which is marked when the solutions are dilute. 

Carbon tetrachloride has been used in certain reactions instead of starch solution. One litre of water at 25 °C will dissolve 0.335 g of iodine, but the same volume of carbon tetrachloride will dissolve about 28.5 g. Iodine is therefore about 85 times as soluble in carbon tetrachloride as it is in water, and the carbon 

Preparation and use of starch solution. Make a paste of 0.1 g of soluble starch with a little water, and pour the paste, with constant stirring, into 100 mL of boiling water, and boil for 1 minute. Allow the solution to cool and add 2-3 g of potassium iodide. Keep the solution in a stoppered bottle. 

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E. Detection of the colour change. With all of the metal ion indicators used in complexometric titrations, detection of the end point of the titration is dependent upon the recognition of a specified change in colour for many observers this can be a difficult task, and for those affected by colour blindness it may be... 

The indicator used in the titration is zincon (Section 10.48) which gives rise to an indirect end point with calcium. Detection of the end point is dependent upon the reaction... 

For the titration of chlorides, fluorescein may be used. This indicator is a very weak acid (Ka = ca lx 10-8) hence even a small amount of other acids reduces the already minute ionisation, thus rendering the detection of the end point (which depends essentially upon the adsorption of the free anion) either impossible or difficult to observe. The optimum pH range is between 7 and 10. Dichlorofluorescein is a stronger acid and may be utilised in slightly acid solutions of pH greater than 4.4 this indicator has the further advantage that it is applicable in more dilute solutions. 

Discussion. Iodine (or tri-iodide ion Ij" = I2 +1-) is readily generated with 100 per cent efficiency by the oxidation of iodide ion at a platinum anode, and can be used for the coulometric titration of antimony (III). The optimum pH is between 7.5 and 8.5, and a complexing agent (e.g. tartrate ion) must be present to prevent hydrolysis and precipitation of the antimony. In solutions more alkaline than pH of about 8.5, disproportionation of iodine to iodide and iodate(I) (hypoiodite) occurs. The reversible character of the iodine-iodide complex renders equivalence point detection easy by both potentiometric and amperometric techniques for macro titrations, the usual visual detection of the end point with starch is possible. 

Apparatus. Set up the apparatus as in Section 14.10 with two small platinum plates connected to apparatus for the amperometric detection of the end point. 

Precipitation titrations are typified by the titration of chloride with silver or vice versa. In this case, interferences with the precipitation reaction may occur because of components in the soil, and the soil itself may interfere with detection of the end point. Thus, complexation reactions are rarely applied directly to soil however, they can be applied to soil extracts. Common environmental titration methods described in the United States Environmental Protection Agency (USEPA) methods are summarized in Table 10.1 [1,2],... 

The titrimetric determination of soil constituents is most commonly applied to a limited number of soil analyses, namely, organic carbon, nitrogen compounds, carbonates, and chlorides. Determination of acid content by titration is generally not done because the titration curves are not amenable to typical titration analysis. Because of the color of soil and the fact that it is a suspension when stirred, it is often necessary to remove the constituent of interest before titration. In other cases, it is possible to do a direct titration using an appropriate indicator. However, even in these cases, detection of the end point is difficult. 

The oxidative titration of chlorpromazine with ceric sulfate or KBrOj-KBr in acid solution has been described, with the end point being determined by a dead-stop end point technique [55]. A similar method involving visual or potentiometric detection of the end point was also reported [56]. 

Because the end point (formation of a permanent brownish coloration) is not very sharp, the back titration method (Ref 4) was used successfully by Dr Fedoroff at the Keystone Ordn Works, Meadville, Pa and improved at PicArsn (Ref 9a). More recently, an electrometric method for detection of the end point was proposed (Refs 5, 6, 7, 8 and 11 Refs 6 7 are discussions of instrumental design and not primarily about FeS04 to detn nitrate). 

Figure 19.10—Karl Fischer method for determination of water. The conventional burette titration with visual detection of the end point leads to imprecise results. Thus, a cell containing two small platinum electrodes is used. As long as no iodide is present in the solution, the current between the electrodes is weak. When excess iodide is present in the solution at the instant the equivalence point is reached, a significant current is registered.

If both the acid and alkali are strong electrolytes, the resultant solution will be neutral (pH 7). If on the other hand either the acid or alkali is a weak electrolyte the resultant solution will be slightly alkaline or acidic, respectively. In either case, detection of the end-point requires accurate measurement of pH. This can be achieved either by using an indicator dye, or by measuring the pH with a glass electrode (described in Chapter 7). 

The detection of the end-point in titrations involving EDTA is most commonly achieved using a metal-ion indicator, i.e. a compound that changes its colour when it complexes with a particular metal ion. The structures of selected metal-ion indicators are shown in Fig. 23.3 and the properties of a variety of metal-ion indicators are given in Table 23.2. 

One experimental system for the determination of analytes in ultrasonically levitated samples is an automated set-up that can be used in conjunction with spectrometric detection using the method of standard additions and/or microtitration with spectroscopic detection of the end-point. Although the potential of this combined approach remains to be explored, acoustic levitation appears to be a highly promising choice for microanalyses [124]. 

The chemical reaction is of the iodide-iodine type the ozone produces iodine quantitatively from the potassium iodide solution. Addition of sodium thiosulfate to the potassium iodide solution prevents volatilization of the iodine (S) and allows accurate amperometric detection of the end point by means of a pair of sensing electrodes... 

Pyridine combines with both products. A Karl Fischer titration unit is available (A.H. Thomas Co.) for detection of the end point by means of platinum electrodes and a pH meter. A.H. Thomas Co. supplies two solutions which are to be mixed (I2 in CH3OH and SO2 in pyridine) as well as a stabilized single solution (also available from Fisher and MCB). Methyl Cellosolve has been suggested as solvent in place of methanol, A similar reagent utilizing a solution of bromine and sulfur dioxide in chloroform has been described. ... 

In Section 11-2 we described acid-base titrations and the use of indicators to tell us when to stop a titration. Detection of the end point in an acid-base titration is only one of the important uses of indicators. 

DETECTION OF THE END POINT INDICATORS—THEY ARE CHELATING AGENTS... 

Fig. 13.11 Sequence of events taking place during a titration with kinetic detection of the end-point. The scale of the time axis is not uniform along its length, vo, vi,. vn denote the tine periods during which the titrant is delivered. Ro, Ri,. Rn represent the time periods during which reaction-rate measurements take place. (Reproduced from [75] with permission of Pergamon Press Ltd).

FI - 13.12 Flow charts of ths ovsrall control and two main subroutines for titration method with kinetic detection of the end-point. (Reproduced from [75] with permission of Pergamon Press Ltd). 

A new method for the determination sulfldes is based on titration by precipitation and fluorimetric detection of the end-point [102]. The indicator is a dye derived from acridine, the fluorescence of which is statically quenched by sulfide The concentrations measured range between 1 and 10 mM by titration with silver nitrate. 

Detection of the end-point depends on the fact that anionic surfactants react with some cationic dyestuffs, and cationic surfactants with some anionic dyestuffs, to form salts which, while not particularly insoluble in water, can be extracted with chloroform. Over the last 50 years, many methods have been proposed which are all variants on the general idea of titrating an anionic with a cationic, or vice versa, in a chloroform-water system in the presence of an ionic dyestuff, the end-point being disclosed by the migration of colour from water to chloroform or vice versa. One of these variants [5] was developed at the behest of the Commission Internationale d Analyses, and a critical review [6] was published two years later. These two papers repay close study. 

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