Indicator adsorption

Fajans method The titration of Cl" with Ag using fluorescein as an adsorption indicator. At the end point the precipitate becomes red. 

Table 11.30 lists standard solutions for precipitation titrations and Table 11.31 lists specific reagents as indicators, adsorption indicators, and protective colloids for precipitation titrations. 

Titration methods using adsorption indicators, based on the precipitation of insoluble iodides, have also been proposed (81—84). The sensitivity of these methods is less than that for the thiosulfate titration. Electrometric titration of the reaction between iodine and thiosulfate (85) was not found to be practicable for routine deterrninations of minute quantities of iodine. 

Eosin [15086-94-9] (tetrabromofluorescein), C2QHgBi40, made by the bromination of fluorescein, is both a dye and an adsorption indicator. Eosin Y /7 7372-87-17, C2QH Br40 2Na, the disodium salt, is a biological stain. 475 -Dibromofluorescein [596-03-2], C2QH2QBr20, is used in D C Orange No. 

Thus an almost complete separation is theoretically possible. The separation is feasible in practice if the point at which the iodide precipitation is complete can be detected. This may be done (a) by the use of an adsorption indicator (see Section 10.75(c)), or (b) by a potentiometric method with a silver electrode (see Chapter 15). 

Precipitation of silver bromide will occur when the concentration of the bromide ion in the solution is 2.0 x 103 times the iodide concentration. The separation is therefore not so complete as in the case of chloride and iodide, but can nevertheless be effected with fair accuracy with the aid of adsorption indicators (Section 10.75(c)). 

The following conditions will govern the choice of a suitable adsorption indicator ... 

A disadvantage of adsorption indicators is that silver halides are sensitised to the action of light by a layer of adsorbed dyestuff. For this reason, titrations should be carried out with a minimum exposure to sunlight. When using adsorption indicators, only 2 x 10-4 to 3 x 10 3 mol of dye per mol of silver halide is added this small concentration is used so that an appreciable fraction of the added indicator is actually adsorbed on the precipitate. 

Other dyestuffs have been recommended as adsorption indicators for the titration of halides and other ions. Thus cyanide ion may be titrated with standard silver nitrate solution using diphenylcarbazide as adsorption indicator (see Section 10.44) the precipitate is pale violet at the end point. A selection of adsorption indicators, their properties and uses, is given in Table 10.8. 

Table 10.8 Selected adsorption indicators properties and uses Indicator Use...

Discussion. Very pure silver can be obtained commercially, and a standard solution can be prepared by dissolving a known weight (say, 10.787 g) in nitric acid in a conical flask having a funnel in the neck to prevent mechanical loss, and making up to a known volume (say, 1 L for a 0.1 M solution). The presence of acid must, however, be avoided in determinations with potassium chromate as indicator or in determinations employing adsorption indicators. It is therefore preferable to employ a neutral solution prepared by dissolving silver nitrate (relative molecular mass, 169.87) in water. 

Either the Mohr titration or the adsorption indicator method may be used for the determination of chlorides in neutral solution by titration with standard 0.1M silver nitrate. If the solution is acid, neutralisation may be effected with chloride-free calcium carbonate, sodium tetraborate, or sodium hydrogencarbonate. Mineral acid may also be removed by neutralising most ofthe acid with ammonia solution and then adding an excess of ammonium acetate. Titration of the neutral solution, prepared with calcium carbonate, by the adsorption indicator method is rendered easier by the addition of 5 mL of 2 per cent dextrin solution this offsets the coagulating effect of the calcium ion. If the solution is basic, it may be neutralised with chloride-free nitric acid, using phenolphthalein as indicator. 

Similar remarks apply to the determination of bromides the Mohr titration can be used, and the most suitable adsorption indicator is eosin which can be used in dilute solutions and even in the presence of 0.1 M nitric acid, but in general, acetic (ethanoic) acid solutions are preferred. Fluorescein may be used but is subject to the same limitations as experienced with chlorides [Section 10.77(b)], With eosin indicator, the silver bromide flocculates approximately 1 per cent before the equivalence point and the local development of a red colour becomes more and more pronounced with the addition of silver nitrate solution at the end point the precipitate assumes a magenta colour. 

Thiocyanates may also be determined using adsorption indicators in exactly similar manner to chlorides and bromides, but an iron(III) salt indicator is usually preferred (Section 10.82). 

DETERMINATION OF MIXTURES OF HALIDES WITH ADSORPTION INDICATORS... 

Numerous adsorption indicators have been suggested for various purposes, but a full treatment is outside the scope of this work. 

Discussion. The method is applicable to the determination of a mixture of two salts having the same anion (e.g. sodium chloride and potassium chloride) or the same cation (e.g. potassium chloride and potassium bromide). For example, to determine the amount of sodium and potassium chlorides in a mixture of the two salts, a known weight (Wj g) of the solid mixture is taken, and the total chloride is determined with standard 0.1 M silver nitrate, using Mohr s method or an adsorption indicator. Let w2 g of silver nitrate be required for the complete precipitation of Wj g of the mixture, which contains xg of NaCl and yg of KC1. Then ... 

Now suppose that the determination of potassium chloride and potassium bromide in a mixture is desired. The total halide is determined by Mohr s method or with an adsorption indicator. Let the weight of the mixture be w3 g and w4 g, be the weight of silver nitrate required for complete precipitation,... 

In the second method diphenylcarbazide is employed as an adsorption indicator. The end-point is marked by the pink colour becoming pale violet (almost colourless) on the colloidal precipitate in dilute solution (ca 0.01 M) before the opalescence is visible. In 0.1M solutions, the colour change is observed on the precipitated particles of silver cyanoargentate. 

Diphenylcarbazide as adsorption indicator, 358 as colorimetric reagent, 687 Diphenylthiocarbazone see Dithizone Direct reading emission spectrometer 775 Dispensers (liquid) 84 Displacement titrations 278 borate ion with a strong acid, 278 carbonate ion with a strong acid, 278 choice of indicators for, 279, 280 Dissociation (ionisation) constant 23, 31 calculations involving, 34 D. of for a complex ion, (v) 602 for an indicator, (s) 718 of polyprotic acids, 33 values for acids and bases in water, (T) 832 true or thermodynamic, 23 Distribution coefficient 162, 195 and per cent extraction, 165 Distribution ratio 162 Dithiol 693, 695, 697 Dithizone 171, 178... 

The end points of precipitation titrations can be variously detected. An indicator exhibiting a pronounced colour change with the first excess of the titrant may be used. The Mohr method, involving the formation of red silver chromate with the appearance of an excess of silver ions, is an important example of this procedure, whilst the Volhard method, which uses the ferric thiocyanate colour as an indication of the presence of excess thiocyanate ions, is another. A series of indicators known as adsorption indicators have also been utilized. These consist of organic dyes such as fluorescein which are used in silver nitrate titrations. When the equivalence point is passed the excess silver ions are adsorbed on the precipitate to give a positively charged surface which attracts and adsorbs fluoresceinate ions. This adsorption is accompanied by the appearance of a red colour on the precipitate surface. Finally, the electroanalytical methods described in Chapter 6 may be used to scan the solution for metal ions. Table 5.12 includes some examples of substances determined by silver titrations and Table 5.13 some miscellaneous precipitation methods. Other examples have already been mentioned under complexometric titrations. 

Argentometric Determination op Chlorine and Bromine by means op Adsorption Indicators... 

Cyanide present in Edetate Disodium is assayed by titration with AgN03 in neutral solution employing dimethylaminobenzylidenerhodamine as an adsorption indicator with a colour change from yellow to orange. A few typical examples are illustrated below ... 

Adsorption-coprecipitation phenomenon using fluorescein, dichlorofluorescein and tetrabromofluo-rescein (eosin) essentially impart the fluoresceinate ion that is absorbed on the AgCl particles. At the equivalence point, the AgCl particles change from white to pink due to the coprecipitation of silver fluoresceinate. In short, the adsorption indicator method is quite rapid and capable of providing very accurate results for the estimation of Cl with AgN03. 

Furthermore, Br. I and SCN ions can also be titrated with AgN03 employing eosin as an adsorption indicator. 

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