Philip J. Cox Schwl of Pharmacy Robert Gordon's Institute of Technology Schoolhill. Aberdeen AB9 IFR Scotland, United Kingdom
II
Preparation of Variable Purity Samples for Use - - - in Disodium Edetate Volumetric Analysis
Although frequently non-specific, volumetric analysis is one techniaue used in the ~ u r i t vdetermination of chemicals intended-for pharmaceuiical "se. Disodium edetate titrations are performed to analyze the particular metal ion concentration of a substance and hence the concentration of metal salt present. In teaching laboratories, samples analyzed by this method are often common commercial chemicals or commercial pharmaceutical preparations. T h e purity levels of these samples are well known, the metal salts being, for example, "at least 99%" or "not less than 99.5%" pure, and acceptable purity levels of pharmaceutical preparations are readily ~btainable.'-~ T h e student in the laboratory may therefore realize what t h e acceptable result of a n analysis should be. If subsequent experimental data leads t o poor results, the calculation and substitution of expected experimental data can become a temptation. T o prevent such a situation arising the purity levels of samples may be varied by dilution. T h e diluent chosen should b e a substance t h a t will not interfere with the analytical procedure, consideration of p H control being particularly important. Purity levels of samples described here are -90% of t h e labeled amount; other levels may be obtained by varying t h e dilution.
Preparation of Samples Crystalline Salts To 180 e of crvstalline metal salt add 20 e of sucrose.. mind to a powder using a mortar and pestle, and mix thoroughly. Examples inrludp zinc sulfate (ZnS04.7Hr01, mngnesium sulfate (MgSOl 7H20j,and p0ta.h alum IAIKISO&I~H?O).Individual samplesirrs for analysis range from 0.2-0.6 g.
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Ointments To 180 g of ointment add 20 g of emulsifying ointment B.P. or soft naaffin (white oetrolatum).melt. and mix earefullvwith s flat soatula ;mil cold. Examplcs include XI& undeclyenare o k m e n t andieam rinol ointment, individual sample sims for analysis heing 2 g and G g, respectively.
Tablets Powder 200 tablets (previouslyweighed) using a mortar and pestle until the narticle size is similar to that of a commercial samole of anhydnui lactme. Add to the powdera weight of the lactose such that the final powder weight = 10~9 tonginal tablet aelght, and mlr. Examples iurlude calcium lactare tablets, magnesium trisilicate tablets. and aluminum hydroxide tablets. A weight of powder equivalent to four tablets is used for each complete analysis. Dusting Powders To 180 e of dustine nowder add 20 e of starch and mix. Examnles are ~teriliihle m a i z e i k h fabsorhahie dustine nowderl for an&is
A commercial fwd mixer, in this case a SunbeammMimaster, may he used to mix all the powder samples mentioned, optimum mixing times being 5-10 min. All final mixtures are examined for homogeneitv. bv analwine samoles taken from different arts of the ointment or powder bed, before ;se in the teaching lahorkory. Discussion Individual students are given the commercial sample and a diluted sample to analyze. As bath samples are similar in appearance the student can no longer assume that any particular result obtained is "good" or "poor," the metal ion concentration of the sample being correctly analyzed only by accurate titration. All examples of sample preparations quoted may be analyzed for metal ion content "sine disodium edetate aceordine "to orocedures . Alternatively, brief accounts of the described in standard analytical procedures are available on request.
'Council of the Pharmaceutical Society of Great Britain, President Kerr, J. P., "British Pharmaceutical Codex," The Pharmaceutical Press, London, 1913. 2The British Pharmacopoeia Commision, Chairman Hartley, F., "British Pharmacopoeia," (H.M.S.O.), University Printing House, Cambridee. 1973. The l k w d States Pharmacopeial Convenrion, President Moyer, John H..'The L'nited States Pharmacopria," XIX Ed., Mack Publishing Companv. Easton. Pa., 1975.
Volume 54. Number 11. November 1977 / 717
