Question: A student spots an unknown sample on a TLC plate and develops it in pentane solvent. Only one spot, for which the Rf value is 0.05, is observed. Is the unknown a pure compound? What can be done to verify the purity of the sample using TLC?

My answer: It's not possible to tell if the sample is pure. The low Rf value indicates that the sample barely moved up the plate so more than one polar substance could well be sticking to the SiO2. A more polar solvent needs to be use to allow give the sample an opportunity to move up the plate and possibly separate. Purity will be verified if there is no separation.

Please check my answer.

Sheryl

That sounds ok to me.

how would you determine the degree of purity of a sample of sodium thiosulphate?

haha i seriously have the same chem question nearly 3 years after you posted that

Well, determining the degree of purity of sodium thiosulphate can be a piece of cake! One way to do it is by titration. You can titrate your sample with iodine solution and starch indicator. As the iodine reacts with the thiosulphate, it turns the solution blue-black. The amount of iodine needed to reach this color change tells you how much thiosulphate is in the sample. The less iodine you need, the purer your sample is. So, the more "titrationship" between the two, the less impurities in your thiosulphate! Who knew chemistry could be so titrifying?

To determine the degree of purity of a sample of sodium thiosulfate, you can perform a titration with a known concentration of iodine. Here are the step-by-step instructions:

Step 1: Prepare the necessary solutions
- Prepare a standard solution of iodine by dissolving a known amount of iodine in potassium iodide solution. This solution is commonly known as the iodine solution or iodine titrant.
- Prepare a sodium thiosulfate solution by dissolving the sample in water.

Step 2: Perform the titration
- Add a few drops of starch indicator solution to the sodium thiosulfate solution. The starch will turn the solution dark blue.
- Slowly add the standardized iodine solution to the sodium thiosulfate solution while stirring continuously. The iodine will react with the thiosulfate to form iodide ions.
- Continue adding iodine solution until the dark blue color disappears, indicating that all the thiosulfate ions have reacted with the iodine.
- Record the volume of the iodine solution used during the titration.

Step 3: Calculate the purity
- Use the stoichiometry of the reaction between thiosulfate and iodine to calculate the number of moles of thiosulfate in the sample.
- Determine the purity by comparing the number of moles of thiosulfate in the sample to the theoretical amount based on the known concentration of the iodine solution.

Note: It's essential to perform this titration carefully and accurately to obtain reliable results. Additionally, it is always recommended to consult or follow an established laboratory procedure when performing chemical analyses.

To determine the degree of purity of a sample of sodium thiosulphate, you can perform several analytical techniques. Here is one common method using titration:

1. Start by preparing a standard solution of iodine. This can be done by dissolving a known amount of potassium iodate (KIO3) in an acidic solution and then adding excess potassium iodide (KI).

2. Take a known mass of the sodium thiosulphate sample and dissolve it in a specific volume of distilled water.

3. Add a few drops of starch indicator to the sodium thiosulphate solution. The starch will form a blue-black color complex with the iodine.

4. Next, titrate the sodium thiosulphate solution with the prepared iodine solution. Slowly add the iodine solution to the thiosulphate solution while stirring, until a permanent blue color is observed. This indicates the reaction is complete.

5. Record the volume of the iodine solution used in the titration.

6. Repeat the titration for multiple samples of the sodium thiosulphate to ensure accuracy.

The degree of purity of the sodium thiosulphate can be determined by comparing the volume of iodine solution required to react with the sample against the known concentration of the standard iodine solution. A lower volume of iodine solution used indicates a higher purity of the sodium thiosulphate sample.

Other methods to determine purity include spectrophotometry, chromatography (such as TLC, as mentioned in the previous question), or instrumental techniques like Fourier transform infrared spectroscopy (FTIR) and nuclear magnetic resonance (NMR) spectroscopy. The choice of method depends on the available equipment and the specific requirements of the analysis.