Introduction

Sodium thiosulfate (Na2S2O3) is a widely employed reagent in various chemical processes and analytical laboratories. When preparing a 0.025 N sodium thiosulfate solution, it is important to follow specific steps to ensure accuracy and consistency. This guide will walk you through the process, including the calculation of necessary amounts and the preparation procedure.

Calculation of Solute Amount

The first step in preparing a 0.025 N Na2S2O3.5H2O solution is to calculate the amount of solute required. Sodium thiosulfate pentahydrate (Na2S2O3.5H2O) is typically used because it is readily available.

The molecular weight of Na2S2O3.5H2O is 248 g/mol. The equivalent weight of sodium thiosulfate is the same as its molecular weight because it is a monoprotic base. Therefore:

Equivalent weight of sodium thiosulfate Molecular weight of Na2S2O3.5H2O 248 g/mol

The normality of the required solution is 0.025 N. Normality is defined as the number of gram equivalents of the solute per liter of solution. To determine the amount of sodium thiosulfate needed per liter, use the formula:

Amount of solute required per 1 L Equivalent weight × Normality

Amount 248 g/mol × 0.025 N 6.2 g

Given that 1 L of 0.025 N Na2S2O3.5H2O solution requires 6.2 g of Na2S2O3.5H2O:

Preparation Procedure for 1 L Solution

Weigh out 6.2 g of sodium thiosulfate pentahydrate (Na2S2O3.5H2O) on a precision scale. Dissolve the weighed sodium thiosulfate in a small amount of distilled water and mix thoroughly. Transfer the solution to a volumetric flask of 1 L capacity. Fill the flask with distilled water up to the calibration mark and mix thoroughly to ensure homogeneity.

Preparation Procedure for 500 mL Solution

For a 500 mL solution, you will need half the amount of solute calculated for 1 L. Thus, the amount of sodium thiosulfate required would be:

Amount of solute required for 500 mL 6.2 g / 2 3.1 g

Importance of Standardization

It is crucial to note that sodium thiosulfate pentahydrate is not a primary standard due to its susceptibility to air oxidation and contamination. Therefore, it must be standardized to determine its exact concentration. Standardization can be achieved using a standard solution of potassium dichromate (K2Cr2O7) and an indicator such as potassium iodide (KI) and starch.

Conclusion

Preparing a 0.025 N sodium thiosulfate solution involves precise calculations and careful preparation steps. Ensure you handle the reagents with care and use appropriate safety measures. Appropriate standardization is vital to guarantee accurate results in your chemical analyses. By following this guide, you can confidently prepare the solution for your needs.