Titrations with

Andrews deration An important titration for the estimation of reducing agents. The reducing agent is dissolved In concentrated hydrochloric acid and titrated with potassium iodale(V) solution. A drop of carbon tetrachloride is added to the solution and the end point is indicated by the disappearance of the iodine colour from this layer. The reducing agent is oxidized and the iodate reduced to ICl, i.e. a 4-eiectron change. 

Crude oils contain carboxylic acids. These are analyzed by titration with potassium hydroxide and the result of the analysis is expressed in mg of KOH/g crude. 

Experimentally, the aqueous iron(II) is titrated with cerium(IV) in aqueous solution in a burette. The arrangement is shown in Figure 4.6, the platinum indicator electrode changes its potential (with reference to a calomel half-cell as standard) as the solution is titrated. Figure 4.7 shows the graph of the cell e.m.f. against added cerium(IV). At the equivalence point the amount of the added Ce (aq) is equal to the original amount of Fe (aq) hence the amounts of Ce (aq) and Fe (aq) are also equal. Under these conditions the potential of the electrode in the mixture is ( - - f)/2 this, the equivalence point, occurs at the point indicated. 

This provides a method of estimating an iron(lll) salt. After reduction the iron(II) salt is titrated with manganate(VII) solution. 

The liberated iodine is titrated with standard sodium thiosulphate(Vr) solution after acidification to remove the hydroxide ions. 

A method of estimating small amounts of water in organic liquids (and also in some inorganic salts) is that of Karl Fischer. The substance is titrated with a mixture of iodine, sulphur dioxide and pyridine dissolved in methyl alcohol. The essential reaction is ... 

In what way does a solution of hydrogen peroxide react with (a) chlorine water, (b) potassium permanganate solution, (c) potassium dichromate solution, (d) hydrogen sulphide 50 cm of an aqueous solution of hydrogen peroxide were treated with an excess of potassium iodide and dilute sulphuric acid the liberated iodine was titrated with 0.1 M sodium thiosulphate solution and 20.0 cm were required. Calculate the concentration of the hydrogen peroxide solution in g 1" ... 

Addition of silver nitrate to a solution of a chloride in dilute nitric acid gives a white precipitate of silver chloride, AgCl, soluble in ammonia solution. This test may be used for gravimetric or volumetric estimation of chloride the silver chloride can be filtered off, dried and weighed, or the chloride titrated with standard silver nitrate using potassium chromate(VI) or fluorescein as indicator. 

Excess standard acid is added, and the excess (after disappearance of the solid oxide) is estimated by titration with standard potassium manganate(VII). 

Alternatively, a known weight of the pyrolusite may be heated with concentrated hydrochloric acid and the chlorine evolved passed into potassium iodide solution. The iodine liberated is titrated with sodium thiosulphate ... 

In water pollution studies, the oxygen content can be measured by making the water alkaline and shaking a measured volume with an oxygen-free solution containing Mn- (aq). The solution is acidified with sulphuric acid, potassium iodide added and the liberated iodine titrated with sodium thiosulphate. 

IL Molecular Weight of Acids by Titration with Standard Alkali. III. Molecular Weight of Bases by Analysis of Chloroplatinates. 

Molecular Weight of an Acid by Titration with Standard Alkali Solution. 

Weigh out accurately about 2-5 g. of pure powdered succinic acid, transfer to a 100 ml. graduated flask, dissolve in distilled water, make the solution up to the graduation mark and mix well. Now, by means of a pipette, transfer 25 ml. of the solution to a 150 ml. conical flask, add a drop of phenolphthalein solution and titrate with A/ 2 NaOH or KOH solution to obtain consistent results. 

The excess of unchanged acetic anhydride is then hydrolysed by the addition of water, and the total free acetic acid estimated by titration with standard NaOH solution. Simultaneously a control experiment is performed identical with the above except that the alcohol is omitted. The difference in the volumes of NaOH solution required in the two experiments is equivalent to the difference in the amount of acetic add formed, i.e., to the acetic acid used in the actual acetylation. If the molecular weight of the alcohol is known, the number of hydroxyl groups can then be calculated. 

Note, (i) In view of the large volume of. "l/.NaOH solution required in the above titrations, the contents of the flask A after hydrolysis niay alternatively be washed carefully into a 100 ml. graduated flask, and the solution made up to the mark and well mixed. 25 ml. of the-solution are then withdrawn with a pipette, and titrated with the A/.NaOH solution. The 100 ml. flask is then washed out repeatedly with distilled water, and used similarly for the contents of the flask B. 

Take approximately 0 5 g. of the finely powdered mannitol, using either a weighing bottle, or else weighing directly in the flask A as before. Heat on the water-bath for 60 minutes. During the titration with the NaOH solution, a fine precipitation of the hexacetyl-mannitol may occur, but will not affect the titration. The results are excellent. 

C.HsNH, + CHjCO-O-COCHa = C.R NHCOCHs -h CHjCOOH then hydrolysed with water and the total free acetic acid estimated by titration with standard NaOH solution, the result being compared with that obtained in a control or blank experiment. 

The acetylations may alternatively be performed in 100 ml. conical flasks, and the contents after dilution and hydrolysis with water can then be carefully washed into 250 ml. beakers for the titration with alkali. The calculation is precisely similar to that of the first method. Excellent results are r>btalned. 

Method. An amino-acid such as glycine, NHjCH,COOH, cannot be estimated by direct titration with standard alkali solution, owing to the opposing effects of the basic and the acidic groups. If, however, the amino-acid is first... 

Add a known volume ofo oaM.AgNOj solution (in excess) and boil the solution until the silver chloride has coagulated. Filter through a conical 5 cm. funnel, ensuring that the filter-paper does not protrude above the r m of the funnel. Wash the silver chloride and the filter-paper several times with a fine jet of distilled water. To the united filtrate and washings add i ml. of saturated ferric alum solution. The solution should be almost colourless if it is more than faintly coloured, add a few drops of concentrated nitric acid. Then titrate with 0 02M-ammonium thiocyanate solution until the permanent colour of ferric thiocyanate is just perceptible. (Alternatively the chloride may be determined potentiometrically.)... 

Remove 25 ml. by means of a pipette, add a few drops of phenolphthalein the colour is pink. Now add very cautiously, drop by drop, dilute acetic acid (say Mjioo) until the pink colour has just not disappeared. It is important not to add too much acid, otherwise the casein will be precipitated. Now add 5 ml. of neutralised formalin (see p. 464) and then titrate with Mj 10 NaOH solution. Note the amount required. 

The method is based on the conversion of urea to amnionium carbonate and the estimation of the latter by titration with standard acid. For this purpose, two equal quantities of urea (or urine) are measured out into two flasks A and B. A is treated with 10 ml. of a strong urease preparation and some phenol-phthalein, warm water is added and the mixture is adjusted by the addition of V/io HCl from a burette A until the red colour is just discharged. This brings the mixture to about pH 8 (the optimum for urease) and also prevents loss of ammonia. 

The contents of B, which act as a control, are treated with mercuric chloride in order to inhibit the action of the enzyme, and then 10 ml. of urease solution are added. The solution is diluted with water and ammonium chloride added (in order to balance the ammonium chloride subsequently formed in A). Meth) l-red is then added and the solution is titrated with Mj 10 HCl from a second burette B until a bright red colour is obtained. 

The solution in A is now treated with mercuric chloride and methyl-red, and then titrated with Ml 10 HCl until its colour matches that of the solution in B. The difference in the volume of HCl run in from the burettes A and Bi is a measure of the amount of urea present. 

Hydrazine hydrate may be titrated with standard acid using methyl orange as indicator or, alternatively, against standard iodine solution with starch as indicator. In the latter case about 0-1 g., accurately weighed, of the hydrazine hydrate solution is diluted with about 100 ml. of water, 2-3 drops of starch indicator added, and immediately before titration 6 g. of sodium bicarbonate are introduced. Rapid titration with iodine gives a satisfactory end point. 

The cyanide content may be determined by titration with standard silver nitrate solution. ... 

Although two isomeric esters are theoretically possible, the main product is the 2-ester (formulated above) traces of the isomeric l-ester are eliminated during purification. These derivatives possess a free carboxyl group their equivalent weights may therefore be determined by titration with standard alkali and thus serve as an additional check upon the identity of the compound. 

It is essential to standardise the alcoholic potassium hydroxide solution immediately before use by titration with standard 0-5N or 0-25N hydrochloric or sulphuric acid using phenolphthalein as indicator. 

These are crystalline compounds with sharp melting points, and possess the further advantage that their equivalent weights may be determined by dissolving in dilute alcohol and titrating with standard alkali. Nitro-phenols, however, give unsatisfactory derivatives. 

The procedure is to pass purified hydrogen through a hot solution of the pure acid chloride in toluene or xylene in the presence of the catalyst the exit gases are bubbled through water to absorb the hydrogen chloride, and the solution is titrated with standard alkali from time to time so that the reduction may be stopped when the theoretical quantity of hydrogen chloride has been evolved. Further reduction would lead to the corresponding alcohol and hydrocarbon ... 

The number of ethylenic linkages In a given compound can be established with accuracy by quantitative titration with perbenzoic acid. A solution of the substance ajid excess of perbenzoic acid in chloroform is allowed to stand for several hours at a low temperature and the amount of unreacted perbenzoic acid in solution is determined a blank experiment is run simultaneously. 

Determine the methylamine content of the commercial solution by titration with standard acid using methyl orange as indicator. Adjust the quantity of methyl-amine solution in accordance with the methylamine content for some commercial samples, the figure may be 33-40 per cent. 

The compound is employed inter alia as an indicator In titrations with potassium dichromate and ceric siilphate solutions. 

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