Titration standardisation

Several variations of the chemical method are in use. In the one described below, a freshly prepared Fehling s solution is standardised by titrating it directly against a standard solution of pure anhydrous glucose when the end-point is reached, I. e., when the cupric salt in the Fehling s solution is completely reduced to cuprous oxide, the supernatant solution becomes completely decolorised. Some difficulty is often experienced at first in determining the end-point of the reaction, but with practice accurate results can be obtained. The titrations should be performed in daylight whenever possible, unless a Special indicator is used (see under Methylene-blue, p. 463). 

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. 

A secondary standard is a substance which may be used for standardisations, and whose content of the active substance has been found by comparison against a primary standard. It follows that a secondary standard solution is a solution in which the concentration of dissolved solute has not been determined from the weight of the compound dissolved but by reaction (titration) of a volume of the solution against a measured volume of a primary standard solution. 

Two excellent methods (utilising acid-base indicators) are available for standardisation. The first is widely employed, but the second is more convenient, less time-consuming, and equally accurate. A third, back-titration, procedure is also available. 

Procedure A with standard hydrochloric acid. Place the standardised (approx. 0.1 M) hydrochloric acid in the burette. Transfer 25 mL of the sodium hydroxide solution into a 250 mL conical flask with the aid of a pipette, dilute with a little water, add 1-2 drops of methyl orange or 3-4 drops of methyl orange-indigo carmine indicator, and titrate with the previously standardised hydrochloric acid. Repeat the titrations until duplicate determinations agree within 0.05 mL of each other. 

Sodium hydroxide. Prepare a solution of approximately 0.5M sodium hydroxide in methylcellosolve. This should be standardised by titration with potassium hydrogenphthalate using the mixed indicator given below. 

Solutions of EDTA of the following concentrations are suitable for most experimental work 0.1M, 0.05M, and 0.01 M. These contain respectively 37.224 g, 18.612g, and 3.7224 g of the dihydrate per litre of solution. As already indicated, the dry analytical grade salt cannot be regarded as a primary standard and the solution must be standardised this can be done by titration of nearly neutralised zinc chloride or zinc sulphate solution prepared from a known weight of zinc pellets, or by titration with a solution made from specially dried lead nitrate. 

The EGTA solution may be standardised by titration of a standard (0.05M) calcium solution, prepared by dissolving 5.00 g calcium carbonate in dilute hydrochloric acid contained in a 1 L graduated flask, and then after neutralising with sodium hydroxide solution diluting to the mark with de-ionised water, use zincon indicator in the presence of Zn-EGTA solution (see below). 

Procedure. Prepare a manganese(II) sulphate solution (approx. 0.05M) by dissolving 11.15 g of the analytical-grade solid in 1 L of de-ionised water standardise the solution by titration with 0.05 M EDTA solution using solochrome black indicator after the addition of 0.25 g of hydroxylammonium chloride — see below. 

Both ammonium and potassium thiocyanates are usually available as deliquescent solids the analytical-grade products are, however, free from chlorides and other interfering substances. An approximately 0.1M solution is, therefore, first prepared, and this is standardised by titration against standard 0.1 JVf silver nitrate. 

Standardisation. Pipette 10.0 mL of the sodium tetraphenylborate solution into a 250 mL beaker and add 90 mL water, 2.5 mL 0.1 M nitric acid, 1.0 mL iron(III) nitrate solution, and 10.0 mL sodium thiocyanate solution. Without delay stir the solution mechanically, then slowly add from a burette 10 drops of mercury(II) nitrate solution. Continue the titration by adding the mercury(II) nitrate solution at a rate of 1-2 drops per second until the colour of the indicator is temporarily discharged. Continue the titration more slowly, but maintain the rapid state of stirring. The end point is arbitrarily defined as the point when the indicator colour is discharged and fails to reappear for 1 minute. Perform at least three titrations, and calculate the mean volume of mercury(II) nitrate solution equivalent to 10.0 mL of the sodium tetraphenylborate solution. 

Procedure A. Prepare an approximately 0.1 JVf solution of ammonium iron(II) sulphate by dissolving about 9.8 g of the solid in 200 mL of sulphuric acid (0.5M) in a 250 mL graduated flask, and then making up to the mark with freshly boiled and cooled distilled water. Standardise the solution by titrating 25 mL portions with standard potassium permanganate solution (0.02M) after the addition of 25 mL sulphuric acid (0.5JVf). 

Method A Standardisation with arsenic (III) oxide. Discussion. The most trustworthy method for standardising cerium(IV) sulphate solutions is with pure arsenic(III) oxide. The reaction between cerium(IV) sulphate solution and arsenic(III) oxide is very slow at the ambient temperature it is necessary to add a trace of osmium tetroxide as catalyst. The arsenic(III) oxide is dissolved in sodium hydroxide solution, the solution acidified with dilute sulphuric acid, and after adding 2 drops of an osmic acid solution prepared by dissolving 0.1 g osmium tetroxide in 40mL of 0.05M sulphuric acid, and the indicator (1-2 drops ferroin or 0.5 mL /V-phenylanthranilic acid), it is titrated with the cerium(IV) sulphate solution to the first sharp colour change orange-red to very pale blue or yellowish-green to purple respectively. 

Weigh out accurately about 0.2 g sodium oxalate into a 250 mL conical flask and add 25-30 mL 1M sulphuric add. Heat the solution to about 60 °C and then add about 30 mL of the cerium(IV) solution to be standardised dropwise, adding the solution as rapidly as possible consistent with drop formation. Re-heat the solution to 60 °C, and then add a further 10 mL of the cerium(IV) solution. Allow to stand for three minutes, then cool and back-titrate the excess cerium(IV) with the iron(II) solution using ferroin as indicator. 

If it is desired to base the standardisation directly upon arsenic(III) oxide, proceed as follows. Weigh out accurately about 0.20 g of pure arsenic(III) oxide into a conical flask, dissolve it in 10 mL of 1M sodium hydroxide, and add a small excess of dilute sulphuric acid (say, 12-15 mL of 0.5M acid). Mix thoroughly and cautiously. Then add carefully a solution of 2 g of sodium hydrogencarbonate in 50 mL of water, followed by 2 mL of starch solution. Titrate slowly with the iodine solution to the first blue colour. Repeat with two other similar quantities of the oxide. 

C) With a standard solution of iodine. If a standard solution of iodine is available (see Section 10.112), this may be used to standardise the thiosulphate solution. Measure a 25.0 mL portion of the standard iodine solution into a 250 mL conical flask, add about 150 mL distilled water and titrate with the thiosulphate solution, adding 2 mL of starch solution when the liquid is pale yellow in colour. 

Chloride. Experience in this determination may be obtained by the titration of, say, carefully standardised ca 0.005 M hydrochloric acid. 

Procedure. Experience in this titration may be acquired by titration of, say, 5.00 mL of accurately standardised 0.01 M sodium hydroxide solution. Use 50 mL of supporting electrolyte and a current of 30 mA. 

The indicator electrode must be reversible to one or the other of the ions which is being precipitated. Thus in the titration of a potassium iodide solution with standard silver nitrate solution, the electrode must be either a silver electrode or a platinum electrode in the presence of a little iodine (best introduced by adding a little of a freshly prepared alcoholic solution of iodine), i.e. an iodine electrode (reversible to I-). The exercise recommended is the standardisation of silver nitrate solution with pure sodium chloride. 

Dissolve 20 g of tetra-n-butylammonium iodide in 100 mL of dry methanol and pass this solution through the column at a rate of about 5 mL min - L the effluent must be collected in a vessel fitted with a Carbosorb guard tube to protect it from atmospheric carbon dioxide. Then pass 200 mL of dry methanol through the column. Standardise the methanolic solution by carrying out a potentiometric titration of an accurately weighed portion (about 0.3 g) of benzoic acid. Calculate the molarity of the solution and add sufficient dry methanol to make it approximately 0.1M. 

Sulphate has been determined in seawater by photometric titration with hydrochloric acid in dimethyl sulfoxide [223]. The sample (5 ml) is slowly added to dimethyl sulfoxide (230 ml) and titrated with 0.02 M hydrochloric acid (standardised against sulfate) with bromo-cresol green as indicator. Since borate, carbonate, and bicarbonate interfere, a separate determination of alkalinity is necessary. 

They used the titration method of Tsunogai et al. [157]. The titrant solutions were standardised against calcium carbonate of primary standard quality (99.9975% purity) rather than zinc, and the EGTA (Eastman Chemicals) was used without further purification. 

The determination of neomycin by non-aqueous titration has been described by Penau et all2l. Neomycin base is allowed to react with standardised perchloric acid the excess acid is then back-titrated with potassium hydrogen phtha-late using crystal violet as indicator. To determine the neomycin content of the sulphate salt the same authors precipitated the sulphate with benzidine before reacting the neomycin with perchloric acid. The amount of benzidine required to precipitate the sulphate is calculated from the sulphate content which is itself determined by titration with sodium hydroxide. 

The fat to be examined (0-5-0-7 g.) is dissolved in 15 c.c. of chloroform in a dry conical flask (capacity 500 c.c.) and 25 c.c. of the standardised iodine solution are added. If, after a short time, the colour of the solution diminishes to a light brown, it is necessary to add a further 10 c.c. of the iodine solution. After four hours the colour of the solution should still be dark brown. Potassium iodide solution (20 c.c. of 10 per cent solution) is now added and the uncombined iodine still present titrated as above. The calculation is made in accordance with the definition of iodine value . Lard, olive oil, or linseed oil should be examined. 

In usual practice, the volumetric titrations may be accomplished either by direct titration method e.g., assay of HC1 employing NaOH as the titrant, or by residual titration method e.g., assay of ZnO in which case a known-excess-measured volume of standardised solution of H2S04, more than the actual amount chemically equivalent to ZnO, is added to the sample thereupon, the H2S04 which remain unreacted with ZnO is subsequently titrated (sometimes referred to as back titration or residual titration in the text) employing standardized NaOH solution. 

Primary standards are stable chemical compounds that are available in high purity and which can be used to standardise the standard solutions used in titrations. Titrants such as sodium hydroxide or hydrochloric acid cannot be considered as primary standards since their purity is quite variable. So for instance sodium... 

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