0.1 M Phosphate Buffer Preparation Calculation
Introduction
Phosphate buffer is a commonly used buffer solution in biological and chemical experiments. It is essential to prepare a phosphate buffer with a specific concentration, such as 0.1 M, to maintain a stable pH and ionic strength in the reaction mixture. In this article, we will guide you through the calculation and preparation of a 0.1 M phosphate buffer.
Materials
- Monobasic sodium phosphate (NaH2PO4)
- Dibasic sodium phosphate (Na2HPO4)
- Distilled water
Theory
A phosphate buffer is a mixture of two phosphate species: monobasic sodium phosphate (NaH2PO4) and dibasic sodium phosphate (Na2HPO4). The pH of the buffer is determined by the ratio of these two species. The Henderson-Hasselbalch equation is used to calculate the pH of the buffer:
pH = pKa + log [base]/[acid]
where pKa is the acid dissociation constant, [base] is the concentration of the dibasic sodium phosphate, and [acid] is the concentration of the monobasic sodium phosphate.
Calculation
To prepare a 0.1 M phosphate buffer, we need to calculate the amount of NaH2PO4 and Na2HPO4 required.
Step 1: Calculate the amount of NaH2PO4
The molecular weight of NaH2PO4 is 119.98 g/mol. To prepare 1 liter of 0.1 M NaH2PO4, we need:
Weight of NaH2PO4 = Molarity x Molecular weight x Volume (in liters) Weight of NaH2PO4 = 0.1 M x 119.98 g/mol x 1 L = 11.998 g
Step 2: Calculate the amount of Na2HPO4
The molecular weight of Na2HPO4 is 141.96 g/mol. To prepare 1 liter of 0.1 M Na2HPO4, we need:
Weight of Na2HPO4 = Molarity x Molecular weight x Volume (in liters) Weight of Na2HPO4 = 0.1 M x 141.96 g/mol x 1 L = 14.196 g
Step 3: Mix the solutions
To prepare the 0.1 M phosphate buffer, mix the calculated amounts of NaH2PO4 and Na2HPO4 in distilled water. The ratio of NaH2PO4 to Na2HPO4 can be adjusted to achieve the desired pH.
Example: Preparation of 0.1 M phosphate buffer, pH 7.4
To prepare 1 liter of 0.1 M phosphate buffer, pH 7.4, mix:
- 11.998 g NaH2PO4
- 14.196 g Na2HPO4
- Distilled water to a final volume of 1 liter
Tips and Precautions
- Use high-purity reagents to avoid contamination.
- Adjust the ratio of NaH2PO4 to Na2HPO4 to achieve the desired pH.
- Sterilize the buffer solution by autoclaving or filtering before use.
- Store the buffer solution at 4°C to prevent bacterial growth.
By following these calculations and steps, you can prepare a 0.1 M phosphate buffer with the desired pH and ionic strength for your experiments.
