Standardisation, volumetric solutions

Once the factor for a solution is known (i.e. once the solution has been standardised), multiplication of the experimentally determined volume by the factor will yield what the volume would have been if the solution had been precisely the nominal molarity (i.e. if the factor had been 1.000). In practice, very few volumetric solutions are factor 1.000 this is due, in the main, to the time that would be taken to weigh out a sample to four decimal places. Volumetric solutions are usually prepared by weighing out approximately the desired weight of sample, then standardising the resulting solution against a solution of known concentration. 

Laboratory reagents intended for prolonged use should be marked with the preparation date and the signature of the person who prepared them. The expiry date of unstable reagents and culture media should be indicated on the label, together with specific storage conditions. In addition, for volumetric solutions, the last date of standardisation and the last current factor should be indicated. 

For the determination of acetanilide, to an aliquot part of the filtrate from the iodide precipitation add sodium sulphite and sodium bicarbonate in slight excess add 2 drops of acetic anhydride and extract with three 60-ml portions of chloroform. Evaporate the chloroform to low bulk, add 10 ml of dilute sulphuric acid and evaporate the rest of the chloroform. Add 20 ml of water and digest on a water-bath for one hour. Add 10 ml of concentrated hydrochloric acid and titrate with 0 1 N potassium bromate-bromide, adding the volumetric solution very slowly to the well-shaken mixture until a faint yellow colour remains. Standardise the bromate-bromide solution against pure acetanilide. 

The sodium nitrite volumetric solution is preferably standardised against a specimen of pure sulphanilamide. In the absence of apparatus for the electrometric end-point, starch-iodide paste may be used as external indicator. It is necessary to titrate slowly and to keep the temperature of the reaction mixture below 15°. 

Pipette 25.0 cm of the Mnl2 solution into suitable containers eg two to four petri dishes (9 cm diameter) and place in reproducible positions under a low pressure mercury lamp. Irradiate for a period (say 30-60 minutes) and collect the solutions quantitatively in a 250 cm volumetric flask. Titrate an aliquot against standardised Na2S203 solution using starch indicator (Sec. 11.4.4). 

The ability of the solid chlorates(V) to provide oxygen led to their use in matches and fireworks. Bromates(V) and iodates(V) are used in quantitative volumetric analysis. Potassium hydrogen diiodate(V), KHflOjlj, is used to standardise solutions of sodium thiosulphate(Vf) since in the presence of excess potassium iodide and acid, the reaction... 

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. 

Pure ferric oxide is recommended for the standardising of permanganate solutions for volumetric analysis. It is dissolved in hydrochloric acid, reduced with stannous chloride, and titrated with permanganate.11... 

Ferrous ammonium sulphate is a stable salt at ordinary temperatures, and its solutions in the cold do not readily oxidise. Hence it is largely used in the laboratory for standardising solutions of potassium permanganate for volumetric analysis. Its solubility in water is as follows —4... 

However, 0.1 (N) approx NaOH can also be standardised versus 0.1 (N) accurately prepared oxalic acid which is a primary standard using phenolpthalein indicator. Equivalent weight of Oxalic acid (HgCgO. 2H2O) is 63 [63 g in one litre gives 1(N) or 6.3 g in 1 litre gives O.l(N)]. Therefore 1.575 gm is to be weighed accurately in 250 ml volumetric flask and volume made upto the mark with distilled water to prepare 250 ml of O.l(N) oxalic acid solution. Then a known volume say 25 ml of this acid will be taken by pipette plus (1 drop phenolpthalein) and volume of NaOH required to neutralise from burette is noted in ml at the end point when pink colour just appears with one drop of NaOH. 

All the volumetric methods described in later chapters include an instruction such as titrate with 0.1 M hydrochloric acid . The subsequent calculation then includes a term C, the exact concentration of titrant in mol/1. Why not use the known value, 0.1 Because a reagent which is nominally 0.1 M or 1.0 M may actually be slightly weaker or stronger than the nominal concentration. In some laboratories it is the practice to prepare solutions that are exactly 0.1 M, 1.0M, etc., but this seems unnecessarily laborious. In order to prepare such a solution it is necessary first to prepare a solution of approximately the desired concentration, standardise it very accurately, then dilute or strengthen it, and finally restandardise it to make sure the adjusted solution is correct. The adjustment is unnecessary when insertion into the equation of the concentration found by the first standardisation is all that is needed. 

Investigating the reaction between hydroxylammonium chloride and iodine Weigh out accurately about 0.86 g of purest HO.NH3.CI and make up to 250 cm in a volumetric flask. Pipette 25.0 cm aliquot of the solution into a conical flask and dilute to about 150 cm Warm the solution on a hot plate, fitted with a magnetic stirrer. Add 0.15 g purest MgO (used to avoid acidity of the solution) and add slowly (standardised) 0.05 M iodine solution from a burette keeping the suspension warm. When the colour of iodine fades slowly, add 2 cm of freshly prepared starch solution to the conical flask and continue the titration slowly until the blue colour of the starch/iodine adsorption complex persists for 30 seconds. From the average of two concordant titres, deduce the molar ratio I2 to (HO.NH3) and hence an equation representing the reaction, based on changes in oxidation numbers. 

Weigh out accurately 1 g purest potassium nitrite, dissolve in boiled-out water and make up to 250 cm in a volumetric flask. Deliver from a burette 30.0 cm of standardised 0.02 M permanganate solution into a 600 cm beaker. Add 30 cm of 1 5 v/v of sulphuric acid solution and dilute to about 300 cm. Heat the beaker to 40 C and add slowly, while stirring magnetically, 25.0 cm of the freshly prepared nitrite solution, which is placed in a burette. Ensure that the tip of the burette is below the surface of the... 

Because of the uncertainty in the exact water content of the purest sodium thiosulphate and because of the instability of its solutions due to traces of carbon dioxide in distilled water etc., it is essential to standardise solutions before use. Any turbidity in solutions due to the deposition of sulphur necessitates discarding the solution. The purest potassium iodate is >99.9% pure and can be dried at 120 C to remove any moisture. Weigh accurately about 3.5g of dried purest iodate, dissolve in water and make up to 1 dm in a volumetric flask. Pipette 25.0 cm of the solution into a conical flzisk, add 2 g of purest KI and shake well before adding 5 cm IM sulphuric acid solution. Titrate the liberated iodine with the thiosulphate solution as in Sec.7.3.1. Repeat to obtain concordant results and calculate the molar concentration of the thiosulphate solution. 

Weigh out accurately about 0.9 g of the blue crystals. Dissolve in dilute sulphuric acid and make up to 2S0 cm with the acid in a volumetric flask. Pipette 25.0 cm of the solution into a conical flask, add an equal volume of the acid and titrate with standardised (about 0.02 M permanganate) until a drop produces a faint permanent pink colour superimposed on a faint yellow. Repeat to obtain concordant results and calculate from the average titre the molar concentration of VOSO4 and its mass. Hence calculate x in the formula. 

Prepare 0.025 M L ascorbic acid solution by weighing accurately about 1.1 g of the purest solid, dissolving it and making up the solution in a 250 cm volumetric flask using boiled-out distilled deionised water. Pipette 10 cm of the solution into a conical flask, acidify with dilute sulphuric acid and add a few drops of N-phenylanthranilic acid indicator. Titrate with standardised freshly prepared Mn(lll) sulphate solution (as above) until the colour changes sharply to violet. Repeat to obtain concordant results and calculate, from the average titre, the ascorbic acid Mn(llI) molar ratio. 

Weigh out accurately 8.2 g purest K3[Fe(CN)6] into a beaker, dissolve in water and transfer quantitatively to a volumetric flask and make up to 250 cm. Pipette 25.0 cm of the solution into a 250 cm conical flask. Add 2 g K1 dissolved in 20 cm water and 15 cm of a solution containing 2 g ZnS04.7H20. Note the liberation of iodine. Titrate immediately with standardised 0.05 M diiosulphate solution until the colour is pale yellow. Then add 2 cm of freshly prepared starch solution and continue titration until the blue colour has just disappeared. Repeat to obtain concordant results. Calculate from the average litre the concentration of hexacyanoferrate(III) in mol dm. ... 

Prepare a zinc ion solution by dissolving 0.66 g of purest zinc sulphate heptahydrate in water, slightly acidified and made up to 250 cm in a volumetric flask. Pipette 25.0 cm of the solution into a conical flask, add 5-6 drops of Solochrome T indicator and 10 cm of buffer solution (pH 10). Titrate with standardised 0.01 M EDTA solution until the purple red colour loses any trace of red and changes to blue. Remat to obtain concordant results and calculate the concentration of Zn in mol.dm. (Prepare the indicator solution by adding 0.2 g of the pure indicator to 15 cm of triethanolamine and 5 cm of edianol). 

This very accurate coulometer makes use of the reaction e + 2l2 I. The cathode and anode compartments contain Pt electrodes, and are separated by a column of 10% KI solution a standardised solution of I2 in KI is introduced to cover the cathode. During electrolysis, iodine is formed at the anode and dissolves in the KI solution, while iodine is reduced to iodide at the cathode. After the experiment both the loss and gain can be determined volumetrically. 

---