9.9 : Standard Solutions

Standard solutions refer to solutions with a precisely known concentration or composition. A primary standard is a highly pure, high molar mass, stable substance that is entirely soluble in water, the most commonly used solvent in analytical chemistry. The primary standard solution can be used to standardize secondary standards, which are substances with known concentrations but are less pure and stable. Standard solutions are essential for achieving accurate and reliable results in analytical procedures, so understanding them is crucial for anyone working in a laboratory setting.

The preparation of a standard solution using a primary standard involves several steps:

1. Weigh the Primary Standard: Use a funnel to carefully transfer the weighed primary standard into a clean and dry volumetric flask. It is crucial to use a weighing balance capable of providing precise measurements.
2. Transfer Standard to Volumetric Flask: Use a funnel to carefully transfer the weighted primary standard into a clean and dry volumetric flask. Ensure all the solid is transferred from the weighing boat to the flask. After the transfer, reweigh the empty weighing boat to confirm no residue remains.
3. Wash the Funnel: Use small quantities of distilled water to wash down any solid material from the funnel into the volumetric flask. This ensures that all of the primary standard is dissolved in the solution.
4. Dissolve the Solid: Swirl the volumetric flask until the solid is completely dissolved. It may be necessary to add water gradually until the solid is completely dissolved.
5. Approach the Calibration Line: Add more distilled water to bring the solution level close to the calibration mark on the neck of the volumetric flask. Ensure the solid is completely dissolved before adding more solvent to avoid exceeding the calibration line.
6. Adjust the Meniscus: Use a pipette to add water drop by drop until the bottom of the meniscus (the curve at the surface of the liquid) aligns precisely with the calibration mark. This step ensures that the solution has the correct volume and concentration.
7. Mix the Solution: Securely close the volumetric flask and invert it several times to ensure complete solution mixing.
8. Label: Label the volumetric flask with the chemical composition, solution concentration, and preparation date. This information helps identify and use the solution correctly.