Determination of the Partition Coefficient of Salicylic Acid

Abstract

The aim of this experiment was to determine the partition coefficient (logP) of salicylic acid, a model drug, between 1-octanol and aqueous buffer solutions with varying pH levels. The logP value obtained was used to interpret the lipophilicity of salicylic acid and predict its distribution within the body. The experiment involved measuring absorbance values and performing calculations to determine logP.

Keywords

• Partition Coefficient
• Lipophilicity
• Organic Phase
• Aqueous Phase

Introduction

The partition coefficient (P) is defined as the equilibrium concentration ratio of an organic chemical partitioned between an organic liquid and water:

P = ^{[drug]_organic} / ^{[drug]_aqueous}

Lipophilicity is the tendency of a substance to partition into an organic phase versus an aqueous phase.

For salicylic acid, the absorbance of salicylic acid was measured using a spectrophotometer, and its concentration was calculated using the Beer-Lambert Law:

A = ɛcl

Experimental

Apparatus

• Plastic Vials
• 5, 10, and 20ml Pipettes
• Beakers
• Thermometer
• Spectrophotometer
• Stopwatch

Materials

• Ferric Nitrate (0.15%)
• 0.1M HCl
• 1-octanol
• 0.015M sodium salicylate in HCl/KHP at pH 2.
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  5, 3.5, 4.5, and 5.5

• Distilled water

All chemicals were purchased from Sigma-Aldrich. The experiment was conducted at a temperature of 24°C.

Procedure

Following Dr. Zheng’s Laboratory Manual (2019), 3ml of water and 1ml of ferric nitrate were added to 4ml of sodium salicylate HCl/KHP solution at pH = 5.5 in a 25ml plastic vial. After a wait of 5 minutes, the absorbance was recorded using a spectrophotometer at 540nm. This is Absorbance1.

In another 25ml plastic vial, 3ml of 1-octanol was added to 5ml of sodium salicylate solution at pH = 5.5. The solution was gently shaken to mix the two phases.

4ml of the aqueous layer was then transferred into another plastic vial. 3ml of water and 1ml of ferric nitrate were added, and the absorbance was recorded after 5 minutes at 540nm. This is Absorbance2.

Both steps were repeated for the other sodium salicylate HCl/KHP buffer solutions at pH 2.5, 3.5, and 4.5.

Results

The volumes were measured using pipettes with an uncertainty of ± 0.03 cm³. The meniscus of the liquid could lead to parallax error, affecting both Absorbance1 and Absorbance2. The use of a blank was intended for absorbance measurement, but droplets of liquid on the cuvette's surface and bubbles within the liquid may have resulted in erroneous readings. The digital stopwatch used had a resolution of 0.01s, and human reaction time of 0.25s could impact the accuracy of each measurement.

Calculations

Graphs

Graph 1: 1/P' vs 1/[H+] (Only 2.5 and 3.5 pH Buffer)

Graph 2: 1/P' vs 1/[H+] (Only 3.5 and 4.5 pH Buffer)

P' for Buffer with 5.5 pH came out as an anomaly and was not used in Graph 1. Errors could have been caused by the entry of some organic layer into the vial during the transfer of the aqueous layer or droplets/bubbles in the cuvette used for spectrophotometry.

1/P' = 1/P + Ka/(P [H+])

Therefore,

P = (0.5201 + 3.1504) / 2 (Taking average from the two graphs)

P = 0.544885

Ka/P = (0.001 + 0.002) / 2 (Taking average from the two graphs)

Ka = 0.0015 x 0.544885

Ka = 8.1733 x 10^-4 M^-1

pKa = -log Ka

pKa = -log (8.1733 x 10^-4)

pKa = 3.0876

Discussion

Sangster (1997) defined the octanol-water partition coefficient as a thermodynamic measure of a substance's preference for a non-aqueous or organic phase over water. Baluja et al. (2017) explained that partition coefficients are essential for estimating drug distribution within the body. Hydrophobic drugs with high partition coefficients tend to accumulate in hydrophobic compartments of cells, such as cell membranes, while hydrophilic drugs with low partition coefficients remain in aqueous compartments, like blood serum. Furthermore, partition coefficients have applications in various industries, including cosmetics, dyes, ointments, and hair dyes.

Salicylic acid's partition coefficient (logP) was determined to be 0.544885, indicating its preference for the aqueous phase over the organic phase (hydrophilic). This suggests that salicylic acid will predominantly distribute in the blood plasma rather than cell membranes.

Conclusion

Salicylic acid has a partition coefficient (logP) of 0.544885, with a Ka of 8.1733 x 10^-4 M^-1 and a pKa of 3.0876. The logP value less than one suggests that salicylic acid is more hydrophilic and is likely to be distributed predominantly in the blood plasma (aqueous phase).

References

1. Chiou, C. T., Freed, V. H., Schmedding, D. W., & Kohnert, R. L. (1977). Partition coefficient and bioaccumulation of selected organic chemicals. Environmental Science & Technology, 11(5), 475-478.
2. Bahmani, A., Saaidpour, S., & Rostami, A. (2017). A Simple, Robust, and Efficient Computational Method for n-Octanol/Water Partition Coefficients of Substituted Aromatic Drugs. Scientific Reports, 18 July. https://www.nature.com/articles/s41598-017-05964-z
3. Zheng, G. (2019). Molecular Pharmacokinetics Experiment 4 Lab Manual, p1-3.
4. Sangster, J. (1997). Octanol-Water Partition Coefficients: Fundamentals and Physical Chemistry. Wiley.
5. Baluja, S., Kulshrestha, A., & Movalia, J. (2017). 1-Octanol-water partition coefficient of some cyanopyridine and chalcone compounds. Revista Colombiana de Ciencias Químico-Farmacéuticas, 46(3), 342-356.

Answers to "B" Questions

1. The calculated logP value for salicylic acid is -0.263695. A negative logP value indicates that the compound has a higher tendency to partition into the aqueous phase, making it more hydrophilic. Therefore, salicylic acid is less likely to cross cell membranes and is better suited for direct injection into the bloodstream through a vein.
2. Buffer (pH) P'
   2.5 1.193220339
   3.5 0.27027027
   4.5 0.115658363
   5.5 -0.191294387