While measuring the potassium concentration in a drug formulation, the technologist took
10 𝜇𝑙 of the sample and mixed it with 90 𝜇𝑙 of test buffer. From the resulting solution, he took
5 𝜇𝑙 and measured its total potassium content as 25 ng. What would be the concentration of
potassium in the drug formulation?
A. 50 ng/ 𝜇𝑙
B. 25 ng/ 𝜇𝑙
C. 75 ng/ 𝜇𝑙
D. 100 ng/ 𝜇𝑙

Measuring Potassium in Drug Formulations

Laboratory technicians often use dilution methods to analyze the concentration of elements like potassium in drug formulations. Let’s walk through a real-world example to calculate the original potassium concentration.

🧪 Problem Statement

A technologist:

• Took 10 µL of a drug sample
• Mixed it with 90 µL of test buffer (1:10 dilution)
• From that mixture, took 5 µL and measured 25 ng of potassium

What is the original potassium concentration in the drug formulation?

Step-by-Step Calculation

Step 1: Determine the concentration in the measured volume

The 25 ng of potassium was present in 5 µL of the diluted sample:

Concentration in diluted solution = 25 ng / 5 µL = 5 ng/µL

Step 2: Account for the dilution factor

The original solution was diluted 10 times:

Original concentration = 5 ng/µL × 10 = 50 ng/µL

Final Answer: A. 50 ng/µL

Key Takeaways

• Always identify the dilution ratio in stepwise measurements.
• Multiply back by the dilution factor to determine the original concentration.
• This method is essential in pharmaceutical labs for accurate quantification.