Mohr method

Mohr method Titration of Cl with Ag in the presence of added Cr04 ". A red precipitate forms at the end point. 

A mixture containing only KCl and NaBr is analyzed by the Mohr method. A 0.3172-g sample is dissolved in 50 mL of water and titrated to the Ag2Cr04 end point, requiring 36.85 mL of 0.1120 M AgNOa. A blank titration requires 0.71 mL of titrant to reach the same end point. Report the %w/w KCl and NaBr in the sample. 

The stress, L, determined using the Modified Mohr method effeetively aeeounts for all the applied stresses and allows a direet eomparison to a materials strength property to be made (Norton, 1996), as was established for the Distortion Energy Theory for duetile materials. The set of expressions to determine the effeetive or maximum stress are shown below and involve all three prineipal stresses (Dowling, 1993) ... 

Discussion. The Mohr method cannot be applied to the titration of iodides (or of thiocyanates), because of adsorption phenomena and the difficulty of distinguishing the colour change of the potassium chromate. Eosin is a suitable... 

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. 

Ag+ preferentially reacts with the analyte to form a soluble salt or complex. During this addition, Ag+ reacts with the analyte only, and not the indicator. But when all the analyte is completely consumed by Ag+ and no more of it is left in the solution, addition of an excess drop of silver nitrate titrant produces an instant change in color because of its reaction with the silver-sensitive indicator. Some of the indicators used in the argentometric titrations are potassium chromate or dichlorofluorescein in chloride analysis and p -dime thy la m i nobe nzalrho da n i nc in cyanide analysis. Silver nitrate reacts with potassium chromate to form red silver chromate at the end point. This is an example of precipitation indicator, where the first excess of silver ion combines with the indicator chromate ion to form a bright red solid. This is also known as Mohr method. 

Fractional precipitation The calculation as to which of two sparingly soluble salts will be precipitated under given experimental conditions may be also made with the aid of the solubility product principle. An example of great practical importance is the Mohr method for the estimation of halides. In this process a solution of chloride ions is titrated with a standard solution of silver nitrate, a small quantity of potassium chromate being added to serve as an indicator. Here two sparingly soluble salts may be formed, viz. silver chloride (a white precipitate) and silver chromate (which is red) ... 

For most of the potassium chloride-water-THF work, data were obtained in the same manner as the other two systems except that potassium chloride was analyzed by the Mohr method (5). At low concentrations of THF, however, a solid precipitate was observed. To ascertain its composition, a mixture of the three components with total mixture composition corresponding to a point within the solid-liquid, two-phase region of the preliminary graph was brought to equilibrium and the... 

For detailed explanations of the chemistry of specific prreipitation titrations the reader is referred to other sources. These include methods such as the Mohr method for halides using the silver chromate end point, the Fajans adsorption indicator method, the Volhard method for FeSCN end point, the titration of halide mixtures with the attendant solid-solution and adsorption effects, the titration of fluoride with thorium(IV), and the titration of sulfate with barium ion. 

The formation of a second, highly colored precipitate is the basis of the Mohr method of endpoint detection. Chloride and bromide ions are titrated with standard silver nitrate using chromate ion as indicator, the endpoint being indicated by the appearance of brick-red silver chromate. ... 

Because silver chromate is more soluble, the Ksp value (soluble product constant) of silver chromate is not exceeded until the precipitation of Cl is complete. The endpoint can be corrected by using the Mohr method to standardize the silver nitrate solution against pure sodium chloride. 

The Mohr method was first described in 1865 by K. F. Mohr, a German pharmaceutical chemist who was a pioneer in the development of titrimetry. Because Cr(VI) has been discovered to be a carcinogen, the Mohr method is now. seldom used. 

Industrial grade NaCl has a content of 92-98%. The precipitation titration can be conducted using 0.1 N AgNO, as the titrant and 5% K,Ci<) as the indicator (the Mohr method). The sample chloride solution should be buffered with calcium carbonate to a pH between 6.3 and 7.2 in order to avoid any interference from other... 

A weighed ampoule is broken in a closed flask containing aqueous sodium hydroxide solution. Chlorine is determined by the Mohr method, and selenium is weighed as the metal after reducing an aliquot with hydroxylammo-... 

Indicators Reacting with the Titrant. There are several examples of an indicator forming a colored compound with a titrant. The Mohr method for determining chloride serves as an example. The chloride is titrated with standard silver nitrate solution. A soluble chromate salt is added as the indicator. This produces a yellow solution. When the precipitation of the chloride is complete, the first excess of Ag reacts with the indicator to precipitate red silver chromate ... 

Sodium chloride levels are routinely determined in canned vegetables and legumes. Titrimetric methods are most commonly used with the sodium chloride first isolated by either ashing at 500-550°C, followed by aqueous dissolution of the ash and titration with silver nitrate solution (Mohr method), or by boiling the food in dilute nitric acid, adding excess silver nitrate and back-titration with potassium thiocyanate (Volhard method). Of these, the latter method is generally more accurate than the former method however, it is also more time-consuming. 

A 30 ml aliquot was analyzed for Br by the Mohr method. Exchange capacity = meq/g of dry resin. 

Mix 0 4 g with 2 g of ammonium chloride in a platinum crucible and heat carefully, without melting the residual chloride, until fuming has ceased. Allow to cool, add 2 g of ammonium chloride, mix, again heat carefully until fuming has ceased and then heat for a further one hour. Allow to cool, dissolve the residue in 50 ml of water and titrate with O IN silver nitrate by the Mohr method (p, 288). 1 ml 0 1 N = 0 01386 g KCIO4. 

Several different kinds of reactions other than acid-base reactions can be used for titrations. The oldest titration is a precipitation titration of chloride with silver nitrate, the Mohr method dating from 1856 ... 

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