Making soluble salts using titration involves accurately determining the volumes of acid and alkali needed for complete neutralization and then carrying out the reaction on a larger scale without the indicator, followed by purification.
Titration is a crucial technique in preparing soluble salts via acid-base neutralization because it allows you to find the exact quantities of the acid and alkali that react completely with each other. This ensures that the final salt product is pure and not contaminated with excess acid or alkali.
Why Use Titration?
When you mix an acid and an alkali, they undergo a neutralization reaction:
Acid + Alkali -> Salt + Water
To obtain a pure salt, you need to react exactly the right amount of acid with exactly the right amount of alkali. If you add too much acid or alkali, the final salt will be impure. Titration helps determine these precise volumes.
The Titration Process
The provided reference outlines the key steps for performing the titration:
- Measure the Alkali: Use a pipette to accurately measure a specific volume of the alkali solution into a conical flask. Add a few drops of a suitable indicator, such as phenolphthalein or methyl orange.
- Prepare the Burette: Add the acid solution into a burette and record the starting volume. Ensure the tap and tip are filled and there are no air bubbles.
- Slow Addition: Add the acid very slowly from the bure burette to the conical flask containing the alkali and indicator. Swirl the flask constantly to mix the solutions.
- Endpoint Detection: Continue adding the acid drop by drop until the indicator changes to the appropriate colour. This colour change signals the endpoint of the titration, indicating that the alkali has been completely neutralized by the acid.
- Phenolphthalein: Changes from pink (alkali) to colourless (acidic/neutral).
- Methyl Orange: Changes from yellow (alkali) to orange (acidic/neutral).
- Record Volume: Note the final volume of acid in the burette. The volume of acid used is the starting volume minus the final volume.
- Repeat: Repeat the titration several times to obtain concordant results (volumes that are close to each other). Calculate the average volume of acid required to neutralize the measured volume of alkali.
This average volume provides the stoichiometric ratio – the exact amount of acid needed to react with the initial volume of alkali.
Making the Pure Soluble Salt
Once the exact volumes needed for neutralization are known from the titration, you can make a larger batch of the pure salt without the indicator:
- Scale Up Reaction: Using the volumes determined by titration (e.g., if 25 cm³ of alkali required 20 cm³ of acid), mix these specific volumes of the original acid and alkali solutions in a clean conical flask or beaker. Do not add the indicator at this stage, as it would contaminate the salt.
- Evaporate Water: Gently heat the resulting salt solution (which is typically salt dissolved in water) in an evaporating basin. Heat carefully to evaporate most of the water. Stop heating when crystals just begin to appear around the edge of the basin.
- Crystallization: Remove the evaporating basin from the heat and allow the solution to cool slowly. As it cools, the soluble salt will form crystals. Slow cooling often results in larger, more well-defined crystals.
- Filter and Dry: Filter the crystals to separate them from any remaining liquid (filtrate). Rinse the crystals with a small amount of cold distilled water to remove any surface impurities. Dry the crystals by placing them in a warm oven or leaving them in a desiccator or warm, dry place.
| Step | Purpose |
|---|---|
| Titration | Determine exact volumes for neutralization |
| Neutralization | React acid and alkali without indicator |
| Evaporation | Remove most of the water |
| Crystallization | Form solid salt crystals |
| Filtration/Drying | Separate and purify crystals |
This methodical approach ensures that you obtain a pure sample of the desired soluble salt.
