How to Calculate Potency of Any Solution Using a UV Spectrophotometer

In chemical, pharmaceutical, and cosmetic laboratories, it’s often necessary to determine the actual concentration or potency of an active ingredient in a solution. While advanced methods like HPLC or GC are precise, a UV–Vis spectrophotometer provides a simple, quick, and cost-effective alternative for routine potency checks.

Why UV Spectrophotometry Works

Many compounds absorb UV or visible light at specific wavelengths (λmax). According to the Beer–Lambert Law:

Where:

• $A$ = absorbance

• $\varepsilon$ = molar absorptivity (constant for the compound at a given wavelength)

• $b$ = path length of the cuvette (usually 1 cm)

• $C$ = concentration

Key idea: For the same compound at the same wavelength, absorbance is directly proportional to concentration.

Step-by-Step Procedure

1. Prepare a Reference Standard

• Accurately weigh a known amount of the compound.

• Dissolve in suitable solvent to make a standard solution of known concentration.

• Measure absorbance at the compound’s λmax.

2. Prepare the Sample Solution

• Weigh or measure the solution or sample.

• Dilute in solvent as needed to fall within the linear range of absorbance (0.1–1.5 preferred).

• Filter if necessary.

• Measure absorbance at the same λmax as the standard.

3. Calculate Concentration or Potency

Single-point formula:

Where:

• $A_s$ = absorbance of sample

• $A_{std}$ = absorbance of standard

• $C_{std}$ = concentration of standard

• $V_s$ = total volume of sample solution

• DF = dilution factor

Convert to % w/w if you started with a weighed sample:

Example (General)

Suppose:

• Standard: 1 mg/mL, Abs = 1.200

• Sample solution (after dilution): Abs = 1.176

• Sample volume = 100 mL, dilution factor = 5

This simple calculation gives the actual potency of the compound in the solution.

Advantages and Limitations

Advantages:

• Quick, simple, and inexpensive.

• Requires only a UV–Vis spectrophotometer.

• Suitable for routine laboratory QC.

Limitations:

• Less specific than HPLC; other UV-absorbing compounds may interfere.

• Requires method validation (linearity, precision, recovery).

• Best for solutions where the matrix does not significantly absorb at the measurement wavelength.

Conclusion

UV spectrophotometry is a versatile and accessible tool for potency determination of any compound in solution. With proper preparation, calibration, and validation, labs can quickly ensure their products or solutions meet target concentrations without relying on more expensive instruments.