Method of Analysis for Stearic Acid : Pharmaceutical Guidelines

Method of Analysis for Stearic Acid

Procedure for analysis of Stearic Acid in pharmaceutical quality control laboratory.

1. Description
White powder or white, greasy, flaky crystals or hard masses showing signs of crystallization.

2. Solubility
Soluble in chloroform, in ethanol and in either. Practically insoluble in water.

3. Identification
A. In the assay, the chromatogram obtained with solution (1) shows two principal peaks, which take the same retention time as those of the two principal peaks in the chromatogram obtained with solution (4).
B. Melting point
Limit: Not less than 54°C
C. Congealing temperature
Limit: Not lower than 54°
Melt the substance, at a temperature not more than 20°C above its congealing temperature (Not more than 74°C), and pour it into the inner test-tube to a height of about 50 to 57 mm. Assemble the apparatus with the bulb of the thermometer immersed half-way between the top and bottom of the sample in the test-tube. Fill the bath to almost 20 mm from the top of the tube with a suitable fluid at a temperature 4°C to 5°C below the expected congealing point. When the sample has cooled to about 5°C above its expected congealing point stir it continuously by moving the loop up and down between the top and bottom of the sample. If necessary, congealing may be induced by scratching the inner walls of the test tube with the thermometer or introducing a small amount of the previously congealed substance under examination. Pronounced super cooling may result in deviation from the normal pattern of temperature changes. If it happens repeat the test introducing small fragments of the solid substance under examination at 1°C intervals when the temperature approaches the expected congealing point.
Record the reading of the thermometer and continue stirring only so long as the temperature is falling. Stop the stirring when the temperature is constant or starts to rise slightly. Record the temperature for at least 4 times.
The congealing point will be the mean of not less than four consecutive readings that lie within a range of 0.2°C.

4. Acid value
Limit: Between 200 and 212
Reagent required
Ethanol (95%)
0.1M potassium hydroxide
Phenolphthalein solution.
Dissolve about 10 g of the sample, in 50 ml of a mixture of equal volumes of ethanol (95%) and either previously neutralized with 0.1M potassium hydroxide to phenolphthalein solution. If the sample does not dissolve in the cold solvent, connect the flask with a reflux condenser and warm slowly, with frequent shaking, until the sample dissolves. Add 1 ml of phenolphthalein solution and titrate with 0.1M potassium hydroxide until the solution remains faintly pink after shaking for 30 seconds. Calculate the acid value from the expression.
                         5.61 x n
Acid value = -----------        
n = the number of ml of 0.1M potassium hydroxide required
w = the weight, in g, of the substance

5. Appearance
Heat the substance to be examined to about 75°C. The resulting liquid is not more intensely colored than reference solution Y7 or BY7.

6. Acidity
Melt 5.0 g, shake for 2 min with 10 ml of hot carbon dioxide-free water, cool slowly and filter. To the filtrate add 0.05 ml of methyl orange solution.  No red color develops.

7. Nickel
Limit: Not more than 1 ppm
Test solution: Weigh 0.100 g (m) of the substance being examined into a suitable high-pressure-resistant digestion vessel (fluoropolymer or quartz glass), add 6 ml of nitric acid and 2 ml of strong hydrogen peroxide solution. Prepare a blank in the same manner. Place the closed vessels in a laboratory microwave oven and digest with an appropriate program, e.g. 250 W for 10 minutes; 600 W for 5 minutes; 400 W for 5 minutes; 250 W for 7 minutes. Allow the digestion vessels to cool before opening. Quantitatively transfer to flasks, dilute to 25 ml with water and mix.
Standard solutions: To 10.0 ml of nickel standard solution (10 ppm Ni) add 1.0 ml of nitric acid, 2.0 ml of strong hydrogen peroxide solution and dilute to 20 ml with water. In four volumetric flasks, introduce 20 ml, 50 ml, 100 ml and 150 ml of this solution, add 6 ml of nitric acid to each flask, dilute with water to 25 ml and mix to obtain calibration solutions containing respectively 4 ng/ml, 10 ng/ml, 20 ng/ml and 30 ng/ml (ppb) of nickel.
Zero solution: Dilute 6.0 ml of nitric acid in water and dilute to 25 ml with the same solvent.
Method: Prepare mixtures of 1 volume of a 0.50% w/v solution of magnesium nitrate and 2 volumes of the blank solution, the test solution, the calibration solutions and the zero solution, respectively.
Determine the absorbance of all test solutions at 232.0 nm using a suitable graphite furnace atomic absorption spectrometer equipped with a Zeeman background compensation system, a pyrolytically-coated tube with the platform, and a nickel hollow-cathode lamp. Maintain the drying temperature of the furnace at 100°C for 10 seconds after a 10 seconds ramp, the ashing temperature at 1400°C for 10 seconds after a 20 seconds ramp, and the atomisation temperature at 2500°C for 5 seconds. Use the zero solution to set the instrument to zero. Determine the calibration function from the readings of the calibration solutions. With the external calibration, determine the concentrations of the test solution and the blank solution from the corresponding absorptions. If necessary, dilute with the zero solution to obtain a reading within the calibrated absorbance range (dilution factor f).
Calculate the content in µg/g (ppm) of Ni in the sample using the expression:
       C x f
=  ----------
       m x 40

c = measured concentration in nanograms per millilitre
f = dilution factor
m = weight of the substance being examined, in g

8. Iodine value
Limit: Not more than 4.0
Reagent required
Carbon tetrachloride
Iodine monochloride solution
Potassium iodide solution
0.1M sodium thiosulphate
Starch solution
Weigh accurately about 0.2 g of the substance in a dry 500-ml iodine flask, add 10ml of carbon tetrachloride and dissolve. Add 20 ml of iodine monochloride solution insert the stopper and allow standing in a dark at a temperature between 15°C and 25°C for 30 minutes.  Place 15 ml of potassium iodide solution in the cup top, carefully remove the stopper, rinse the stopper and the sides of the flask with 100 ml of water, shake and titrate with 0.1M sodium thiosulphate using starch solution, added towards the end of the titration, as indicator. Note the number of ml required (a). Repeat the operation omitting the substance being examined and note the number of ml required (b).
                             1.269 x (b - a) x M
Iodine value = ----------------------------
                                   0.1 x W
W = Weight, in g, of the substance
M = Molarity of sodium thiosulphate

9. Mineral acid
Limit: No red color is produced.
Reagent required
Methyl orange solution
Take 5g of the sample in a test tube. Melt the sample. Add an equal volume of the melted substance of hot water. Shake for 2 minutes. Cool and filter. To the filtrate add 0.05 ml of methyl orange solution. No red precipitate is produced.

10. Heavy metals
Limit: Not more than 20 ppm.
Reagent required
Lead standard solution (20 ppm Pb).
Nitric acid AR
Sulphuric acid AR
Hydrochloric acid AR
Ammonia solution AR
Dilute ammonia solution
Dilute acetic acid solution
Hydrogen sulfide solution (freshly prepared)
Standard preparation: Into a 50 ml clean and dried Nessler’s cylinder, pipette out 1 ml of lead standard solution (20 ppm Pb) and dilute with water to 25 ml. Adjust pH between 3.0 and 4.0 with dilute acetic acid or dilute ammonia solution. Dilute with water to about 35 ml and mix.
Sample preparation: Weigh accurately about 2.0 gm of sample in a clean and previously ignited silica crucible, add sufficient sulphuric acid to moisten the sample and ignite carefully at a low temperature until the sample is thoroughly charred. Add to this charred mass 2 ml of nitric acid and 5 drops of sulphuric acid and heat cautiously until white fumes are no longer evolved. Ignite the sample in a muffle furnace, at 500°C to 600°C until the carbon is completely burnt off. Allow it to Cool and add 4 ml of hydrochloric acid. Cover the crucible, keep it in a water bath and allow the sample to digest for 15 minutes. Remove the cover and evaporate slowly to dryness on a water bath. Moisten the residue with 1 drop of hydrochloric acid. Add 10 ml of water and digest for another 2 minutes. Add ammonia solution drops wise until the solution becomes just alkaline to litmus paper. Dilute with water to 25 ml and adjust with dilute acetic acid to a pH between 3.0 and 4.0. Filter, if necessary, rinse the crucible and filter with 10 ml of water. Combine the filtrate and washings in a clean and dried Nessler’s cylinder and dilute with water to about 35 ml and mix.
To each of the cylinder containing the standard solution and sample solution, respectively add 10 ml of freshly prepared hydrogen sulfide solution, mix, dilute to 50 ml with water and allow to stand for 5 minutes. View the contents of both the cylinders downwards over a white surface and compare the color intensity of sample solution with that of the standard solution. The color produced with the test preparation is not more intense than that of the standard preparation.

11. Sulphated ash
Limit: Not more than 0.1%
Heat a silica crucible to redness for 10 min., allow cooling in a desiccator and weighing. Place about 1.00 g of sample in the silica crucible, moisten with sulphuric acid, ignite gently, again moisten with sulphuric acid and ignite at about 800°C, cool, weigh again, ignite for 15 min. and repeat this procedure until two successive weighing does not differ by more than 0.5 mg.
                                       W3 – W1
% Sulphated Ash  =   --------------- X 100
                                       W2 – W1
W1 = Weight of empty platinum crucible
W2 = Weight of crucible + sample
W3 = Weight of crucible + residue (After ignition)

12. Assay
Limit: Not less than 40.0% of C18H36O2 the sum of content of C18H36O2 and C16H32O2 is not less than 90.0%
Reagent required
Sodium chloride saturated solution                
Anhydrous sodium sulfate               
Boron trifluoride solution
Chromatographic conditions
Column: Glass column (1.5 m x 4mm), packed with acid washed silanised diatomaceous support (100 to 120 mesh) coated with 15% w/w of diethylene glycol succinate polyster.

Flow rate
40 ml/min
Injection volume
2 –3 ml
Column temperature
Injector port temperature
Detector temperature

Solution (1) preparation: Take 0.1g of the sample in a clean, Iodine flask; add 5ml of boron trifluoride solution. Heat the solution under a reflux condenser for 15 minutes.  Cool and transfer to a separating funnel with the aid of 10 ml of hexane.  Add 10ml of saturated solution of sodium chloride and 10 ml of water.  Shake discard the lower aqueous layer and dry the upper layer over anhydrous sodium sulfate.
Solution (2) preparation: Take 5 ml of a 1% w/v solution of nonadecanoic acid (internal standard) in toluene to a mixture of 50 mg of Stearic acid WRS and 50mg of palmitic acid WRS and evaporate to dryness. Add 5 ml of boron trifluoride solution and complete the procedure described for solution (1) beginning with the words “heat under reflux condenser…
Solution (3) preparation: Weigh 0.1g of the sample, add 5ml of the internal standard solution and evaporate to dryness. Add 5ml of boron trifluoride solution and complete the procedure described for solution (1) beginning at the words “heat under reflux condenser…
Solution (4) preparation: Weigh accurately about 50 mg of Stearic acid WRS and 50 mg of palmitic acid WRS and dissolve in toluene. Evaporate it to dryness and to this add 5 ml of boron trifluoride solution and complete the procedure as described for solution (1) beginning with the words “heat under reflux condenser ………”

Inject equal volume of all solutions. Calculate the content of Stearic acid, C18H36O2 and palmitic acid, C16H32O2; from the declared content of C18H36O2 in Stearic acid WRS and the declared content of C16H32O2 in palmitic acid.

Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of Pharmaceutical Guidelines, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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