Question

24.

Kagel and Farwell report a reversed-phase HPLC method for determining the concentration of acetylsalicylic acid (ASA) and caffeine (CAF)

in analgesic tablets using salicylic acid (SA) as an internal standard.

23

A

series of standards was prepared by adding known amounts of acetyl

-

salicylic acid and caffeine to 250-mL Erlenmeyer flasks and adding 100

mL of methanol. A 10.00-mL aliquot of a standard solution of salicylic

acid was then added to each. The following results are obtained for a

typical set of standard solutions.

milligrams of

peak height ratios for

standard

mg ratio of peaks

ASA CAF ASA/SA CAF/SA

1 200.0 20.0 20.5 10.6

2 250.0 40.0 25.1 23.0

3 300.0 60.0 30.9 36.8

A sample of an analgesic tablet was placed in a 250-mL Erlenmeyer

flask and dissolved in 100 mL of methanol. After adding a 10.00-mL

portion of the internal standard, the solution was filtered. Analysis of

the sample gave a peak height ratio of 23.2 for ASA and of 17.9 for

CAF.

(a)

Determine the milligrams of ASA and CAF in the tablet.

(b)

Why was it necessary to filter the sample?

(c)

The directions indicate that approximately 100 mL of methanol is

used to dissolve the standards and samples. Why is it not necessary

to measure this volume more precisely?

(d)

In the presence of moisture, ASA decomposes to SA and acetic acid.

What complication might this present for this analysis? How might

you evaluate whether this is a problem?