Determination of Salicylsalicylic Acid In Salicylic Using Thin Layer Chromatography SIR: Salicylic acid (SA) is a widely utilized reagent and raw material. A possible impurity in SA is the salicylate ester known as salicylsalicylir wid (SSA, Salysal or Diplosal). Several years ago procedures were publishid for the determination of the purity of SSA (2-4), which, however, involved classical nonspecific met,hods. The nonspecificity of thesc procedures Irecludes the accurate ileterniination of SSA in related compounds. Therefore, a specific thin layer chromatographic method (TLC) was developed to determine this impurity. Sepnration attained hy TLC, coupled with the use of ultraviolet spectrometry, provided accurate qualitative and quant,itative methods of analysis for SSA in SA.
measured at the maximum near 310 mp using 1-cm. cells and a blank of the rluting solvent done. Quantitative results are calculated from t,bese absorbances. An additional memure of specificity may be obtained by running the ultraviolet, curve on an eluted solution that h a s been madc alkaline by the addition of alcoholic hase. In t,his medium the SSA ahsorption maximum will shift to 345 rns instead of the 310 m r observed in wid, while SA itself will show no shift. RESULTS AND DISCUSSION
EXPERIMENTAL
Apparatus. Glass plates (8 by 8 inches) coated with a 250-micron layer of aluminum oxide G (Urinkmann Instruments, Inc., N. Y.) mixed 1:1 with distilled water. Allow to airdry for 1 hour, then place in oven a t 100’ C. for 1 hour and cool prior to use. Long wavelength ultraviolet lamp, Model UVL-22 (Irltraviolet Products, Inc., San Gabriel, Calif.). Spectrophotonleter, Model 202 (Perkin-Elmer Corp., Norwalk, Conn.). Reagents. Developing Solution. A solution of 85 parts petroleum ether, 10 parts ethyl acetate, and 5 parts acetic acid. Eluting Solvent. 0.1N HCI in absolute ethanol. SSA Standard. Prepared by the procedure outlined in the literature ( I ) . SSA Standard Solution. Accurately weixh 200 mg. of SSA standard into a 50-ml. volumetric flayk, dissolve in and make to volume with absolute ethanol. Dilute 5 ml. of this solution to 50 ml. with absolute ethanol to give a solution of 0.4 #g. per #I. SA Solution. In a 25-ml. volumedric flask weigh accurately a 5-gram sample of SA. Dissolve in and make to volume with ahsolute ethanol. Alkaline Potassium Permanganate Solution. A 1% aqueous solut,ion of potassium permanganate A.R. containing 1% sodium hydroxidr.. Procedure. QUALITATIVE.On a prepared plat,e place 3 spots of 20 PI. each of the SA solution, 3 of I O PI. each, and 3 of 5 #I. earh of the standard SSA solution. After drying, develop the plate in the devcloping t,ank until
Acid
Figure 1 . Thin layer chromatogram of two sampler of .SA each containing 0.05y0SSA For photographic dority the spots were retouched, but the size, shape, podtion, ond relolive intensity were unaltered
the front is about inch from the top of the plate. After drying, examine the plate undcr the long wavelength ultraviolet lami) and spray with alkaline permanganate, if desired. The size and color intensity of the SSA spot ohtained from 5 MI. of the standard SSA solution and that obtained from 10 PI. of the standard SSA solution correspond to SA samples containing 0.05% and 0.1% 55.4, respectively. Thus it is possible to estimate the SSA content of the SA as O.l%. Other percentages could be spotted as desired to conform to the level of SSA in the SA sample. QUANTITATIVE. For quantitative determinations, the standard and sample solutions are prepared as above, hut 10 spots of 20 p l , each of sample and 10 of 25 #I. each of the standard are aiqilied to separate platen. After development and drying, the SSA spots are outlined under the ultraviolet lamp, then scraped into a small sintered glass funnel. The SSA is eluted from the alumina with the arid-alcohol solvent and made to volume i n a 25-ml. volumetric flask. The abmrbances of sample and standard solutions are then
Qualitativr estimations proved to be reproducihlc and accurate. Quantitative detortnination on solutions containing 0.25, 0.5, and 1% SSA in SA were reproducible within limits of *5%. Recoveries were consistently in t,he order of 80% when compared to external (nonchrornatographed) standards and loO’%o when compared to internal It (chromatographed) standards. should be possible to do quantitative recoveries below 0.2% by using cells with longer cell paths than the 1 cm. used in this work. With this system the standard SSA solution gave an R, value at the leading edge of the spot of about 0.72, while small amounts of SA gave a R, of about 0.28. With the predominantly SA samples used here, the R, values were closer (0.70 for SSA and 0.63 for SA), hut resolution was still obtained. Successful analyses were carried out on various commercial samples with excellent duplication of results. I n all cases, results were in the order of 0.0570 SSA or less. LITERATURE CITED
ROBERT
w.BAILEY
The Norwirh Pharmacal Co. Norwich, N. Y. RECEIVED for review May 25, 1964. Accepted June 22, 1964. VOL. 36. NO.
io.
SEPTEMBER 1964
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