Solve the question below: An analysis is carried out on codeine linctus stated to contain 0.3% w/v of codeine phosphate. The mobile phase consists of 0.1 M acetic acid/methanol (40 : 60) and 0.01 M octane sulphonic acid, and chromatography is carried out on a reverse-phase column with UV monitoring at 285 nm. A one-point calibration was carried out against a calibration standard containing ca 0.06% w/v codeine phosphate. The following data were obtained: • Weight of linctus analysed = 12.7063 g • Density of linctus = 1.25 g/ml • The linctus is diluted to 50 ml with water prior to analysis • Area of codeine peak obtained by analysis of the linctus = 86 983 • Area of codeine phosphate calibration peak = 84 732 • Percentage of w/v of codeine phosphate in calibration standard = 0.06047. Why is the octane sulphonic acid included in the mobile phase? Calculate the percentage of w/v of codeine phosphate in the linctus

The octane sulphonic acid is included in the mobile phase to act as an ion-pairing agent. It helps to improve the retention and resolution of the analyte (codeine phosphate) on the reverse-phase column during chromatography.

Now, let's calculate the percentage of w/v of codeine phosphate in the linctus.

First, we need to calculate the response factor (RF) from the calibration standard. The response factor is the ratio of the area of the codeine phosphate peak to the concentration of the codeine phosphate in the calibration standard.

RF = Area of codeine phosphate calibration peak / Concentration of codeine phosphate in calibration standard RF = 84732 / 0.06047 RF = 1401285.7

Next, we calculate the concentration of codeine phosphate in the linctus using the area of the codeine peak obtained by analysis of the linctus and the response factor.

Concentration of codeine phosphate in linctus = Area of codeine peak / RF Concentration of codeine phosphate in linctus = 86983 / 1401285.7 Concentration of codeine phosphate in linctus = 0.0621 w/v

However, this is the concentration in the diluted linctus. The linctus was diluted to 50 ml, so we need to adjust for this dilution. The original weight of the linctus analysed was 12.7063 g, and the density of the linctus is 1.25 g/ml, so the original volume of the linctus was 12.7063 g / 1.25 g/ml = 10.16504 ml.

Therefore, the percentage of w/v of codeine phosphate in the original linctus is:

Percentage of w/v of codeine phosphate in linctus = (Concentration of codeine phosphate in linctus * Volume after dilution) / Original volume of linctus Percentage of w/v of codeine phosphate in linctus = (0.0621 * 50) / 10.16504 Percentage of w/v of codeine phosphate in linctus = 0.3061

So, the percentage of w/v of codeine phosphate in the linctus is approximately 0.3061%.