Understanding Normality
* Normality (N) is a measure of concentration that relates to the number of equivalents of a solute per liter of solution.
* For sodium thiosulfate, one equivalent is equal to one mole because it reacts with one mole of iodine (I2) in a typical titration reaction.
* Therefore, a 0.1 N Na2S2O3 solution is equivalent to a 0.1 M (molar) solution.
Materials:
* Sodium thiosulfate pentahydrate (Na2S2O3•5H2O) crystals
* Distilled or deionized water
* Volumetric flask (appropriate size, e.g., 100 mL, 250 mL, etc.)
* Analytical balance
* Stirring rod or magnetic stirrer
* Beaker
Procedure:
1. Calculate the Mass of Na2S2O3•5H2O:
* Molecular weight of Na2S2O3•5H2O = 248.18 g/mol
* Desired normality = 0.1 N
* Volume of solution (adjust based on your needs) = Let's assume 250 mL (0.25 L)
* Mass of Na2S2O3•5H2O = (Normality x Molecular weight x Volume) / 1000
* Mass = (0.1 N x 248.18 g/mol x 0.25 L) / 1000
* Mass = 0.62045 g
2. Weigh the Sodium Thiosulfate:
* Carefully weigh out 0.62045 g of sodium thiosulfate pentahydrate using an analytical balance.
3. Dissolve in Water:
* Transfer the weighed sodium thiosulfate crystals to a clean beaker.
* Add about 100-150 mL of distilled or deionized water to the beaker.
* Stir gently with a stirring rod or magnetic stirrer until the crystals dissolve completely.
4. Transfer to a Volumetric Flask:
* Carefully transfer the solution to a clean and dry 250 mL volumetric flask.
* Use a funnel to avoid spills and rinse the beaker with a small amount of water to ensure all of the solution is transferred.
5. Add Water to the Mark:
* Add distilled water to the flask until the bottom of the meniscus (the curved surface of the liquid) aligns with the calibration mark on the neck of the flask.
* Use a dropper to add water carefully near the end.
6. Mix Thoroughly:
* Stopper the flask and invert it several times to thoroughly mix the solution.
7. Label and Store:
* Label the flask with the solution's concentration (0.1 N Na2S2O3), date of preparation, and any other relevant information.
* Store the solution in a dark, cool place to prevent degradation.
Important Notes:
* Purity of Sodium Thiosulfate: Ensure that you are using sodium thiosulfate pentahydrate of high purity. Impurities can affect the accuracy of your solution.
* Stability: Sodium thiosulfate solutions are susceptible to decomposition, especially when exposed to light, heat, or acidic conditions. Prepare fresh solutions whenever possible.
* Standardization: It's a good practice to standardize your sodium thiosulfate solution using a known primary standard, such as potassium iodate (KIO3), to determine its exact concentration. This is particularly important for analytical applications.
* Safety: Always wear appropriate safety goggles and gloves when handling chemicals.
