Substances Assayed Gravimetrically

SUBSTANCES ASSAYED GRAVIMETRICALLY

A good number of pharmaceutical substances may be determined gravimetrically by obtaining their respective difficultly soluble salts as precipitates, weighing to a constant weight and finding the percentage purity of the substance in question.

A few typical examples are cited below so as to expatiate the procedure as well as the theoretical aspects.

1. Sodium Chloride

Materials Required : Sodium chloride : 0.25 g ; 5% w/v silver nitrate in DW (+ 2-3) drops of conc. HNO₃ ; dilute nitric acid (6 N) ; asbestos fibre.

Theory : The following reaction forms the basis for the calculation of the theoretical amount of silver nitrate solution required as well as the purity of the given sample of NaCl. Thus, we have :

As 0.2570 g of NaCl has been used (from experimental data); therefore, the exact amount of AgNO₃ required would be :

Hence, the amount of AgNO₃ solution required theoretically would be 0.7470/0.05 = 14.94 ml.

From above, the percentage purity of the given sample of NaCl may be found as shown below :

The weight of AgCl is found to be 0.6288 g experimentally, or 0.4078 is the ‘gravimetric factor’.

Consequently, the percentage purity of the sample is determined by the formula :

where, W = Wt. of the product of a chemical reaction with the substance under determination,

E = Gravimetric Factor, and

S = Wt. of the sample.

By incorporating the data given above, the amount of sodium chloride present in 100 g of the sample i.e., the percentage purity of NaCl in the given sample may be calculated as follows :

Procedure : Weigh accurately between 0.20 to 0.30 g of sodium chloride and dissolve in 100 ml of DW. Add to it 1 ml of dilute nitric acid gradually with constant stirring. Check and confirm that the resulting solution is acidic with the help of blue litmus paper. Measure out 5.0 ml in excess of the amount of silver nitrate solution calculated on theoretical basis to precipitate all the available chlorine as silver chloride. The requisite quantity of silver nitrate solution must be added in small lots at intervals with constant stirring with a glass rod. Cover the beaker with a watch-glass and boil the contents very gently with occasional stirring (to avoid bumping of the liquid and loss of volume). Stop heating and digest the mixture for 10 minutes so as to agglomerate the precipitate and enhance settling thereby leaving a clear supernatant liquid. Add 2 drops of silver nitrate solution to the hot supernatant liquid in order to confirm whether precipitation is completed. Keep the beaker away from direct sunlight to allow the precipitate to settle.

Take a properly prepared Gooch crucible, heat to constant weight and fit it into the suction flask. Decant most of the supernatant liquid first into the Gooch crucible by applying gentle suction to hasten filtration. Wash the precipitate on the Gooch crucible at least thrice with 15 ml portions of 0.01 N nitric acid.

Test the above filtrate to be free of AgNO₃. Finally wash the precipitate twice with 5 ml portion of DW to get rid of most of the HNO₃ previously retained by the precipitate from the former wash solution. Now, apply vigorous suction to drain out the liquid from the precipitate to the maximum extent. Dry the crucible to a constant weight between 110-120°C in an electric oven until two concurrent weighings are achieved. Thus, the weight of the crucible (tare) must be deducted from the weight of the crucible plus the precipitate to arrive at the weight of silver chloride duly obtained from the sample.

Precautions :

1.           The solution of the substance is usually acidified with HNO₃ to check the precipitation of other substances insoluble in water but soluble in HNO₃ e.g., CO₃^2–, O^2– and PO₄^3–. Besides HNO₃ also helps to coagulate any colloidal AgCl,

2.           The excess of HNO₃ must be avoided to cause solvolysis of silver halides,

3.           Heating should be affected only after the addition of AgNO₃, otherwise Cl₂ may be liberated and lost. Thus, we have :

4.           The precipitation should preferably be carried out in the absence of strong light because AgCl undergoes decomposition in sunlight with loss of Cl₂,

5.           Washing of the precipitate (AgCl) with 0.01 N HNO₃ is always recommended to prevent loss of AgCl by virtue of its return to colloidal condition (peptization) and to get rid of the soluble salts, namely : AgNO and NaNO₃, and

6.           AgCl is significantly volatile on ignition, hence it must always be dried at a comparatively lower temperature.

2. Potassium Alum, KAl(SO₄)₂, 12H₂O

Theory : The percentage of Al in potassium alum can be determined volumetrically by complexometric titration.

However, gravimetric procedure provides a fairly reliable and useful alternative method of analysis for

Al which may be accomplished by :

(a) precipitation from a solution of the aluminium salt by the addition of NH₄OH in the presence of NH₄Cl, and

(b) complexation from a solution of the aluminium salt with 8-hydroxyquinoline (oxine) either from an ammoniacal solution or from acetic acid-acetate buffer.

In the first method, the following reaction takes place :

Al³⁺  +  3OH^–  → Al(OH)₃ ↓

The gelatinous white precipitate of Al(OH)₃ is duly filtered, washed with dilute NH₄NO₃ solution, transformed to the corresponding oxide and finally weighed as Al₂O₃ .

Disadvantages : There are a number of serious disadvantages of this method, namely :

(i) excess of NH₄OH may directly affect the solubility of Al(OH)₃,

(ii) coprecipitation of metal hydroxides that are usually soluble in NH₄OH,

(iii) heated oxide (Al₂O₃) is hygroscopic in nature, and

(iv) hydroxides may not undergo complete thermal decomposition.

Due to the above short-comings, the second method is usually preferred which shall be discussed below :

Equation :

The resulting precipitate of aluminium-oxine complex is crystalline in nature and hence can be filtered conveniently, washed with water and finally dried at 130-150°C to constant weight.

Disadvantages : There are two disadvantages of the metal-oxine-complex method, namely :

(i) aluminium-oxinate is prone to adsorb oxine, and

(ii) lack of selectivity of oxine such that all metals except the alkaline earths (Ba, Mg, Ca, Sr, Be) and alkali (Li, Na, K, Rb, Cs) should be totally absent.

Calculations :

Materials Required : Potassium alum : 0.3 g ; 0.1 N hydrochloric acid : 1.0 ml ; 8-hydroxyquinoline reagent (or oxine-reagent) (25 ml of a 2% w/v solution of oxine in 2 N acetic acid) ; 2 N ammonium acetate (dissolve 15.0 g of ammonium acetate in 20.0 ml of DW, add 0.3 ml of glacial acetic acid and dilute to 100 ml with DW) ; sintered glass crucible No : 3 or 4.

Procedure : Weigh accurately about 0.3 g of potassium alum in a 400-ml beaker, dissolve it in 150 ml of DW containing 1.0 ml of 0.1 N HCl and warm the contents of the beaker to about 60°C. Add the requisite quantity of the oxine reagent and then add a 2 N solution of ammonium acetate gradually from a pipette till precipitation just commences. Add a further portion (50 ml) of ammonium acetate solution with vigorous stirring. Allow the contents of the beaker to stand for 60 minutes with frequent stirring. Filter the precipitate through No : 3 or 4 sintered glass crucible that has been previously dried to a constant weight at 130—150°C. Wash the precipitate throughly with cold DW and dry at 130 to 150°C to constant weight. Each gram of aluminium oxinate is equivalent to 0.05873 g of Al.

3. Cognate Assays

A good deal of pharmaceutical substances are officially assayed gravimetrically as appears in Table 10.2.