Volhard Titration
How it Works - Potassium thiocyanate reacts with silver ions to create an insoluble precipitate of silver thiocyanate. It also reacts with the ferric (Fe3+) ions of the indicator solution to create a blood/wine red solution. Silver ions have a greater affinity for the thiocyanate than ferric ions, so the thiocyanate will react preferentially with the silver ions until there are no more of them in solution. When there are no more silver ions in solution, the red color will remain, indicating the endpoint of the titration.
Two reagents are needed for the titration, the Indicator, and the Titrant. They are prepared as follows.
Ferric Ammonium Sulfate Indicator - The indicator solution is easy to prepare. Only a small amount (approximately 1 ml.) of the indicator solution is needed for each test, so any convenient amount can be made depending on your needs. Dissolve about 1 gram of ferric ammonium sulfate into about 10 ml. of distilled water. The amounts are not critical. It is only important that there be sufficient ferric (Fe3+) ions in the solution to combine with excess thiocyanate ions at the endpoint of the titration. The ferric ions form a dark red complex with the thiocyanate ions to indicate the endpoint has been reached. For best results, the indicator solution should be prepared the same way each time, and the amount used should be consistent.
Potassium Thiocyanate Titrant - More care is required when preparing the titrant, as the precise concentration is important to achieve accurate results. The solution should be prepared in a volumetric flask to ensure accuracy. The molecular weight of potassium thiocyanate is 97.181 grams per mole. We want a 0.1 molar solution, so add 9.718 grams of potassium thiocyanate to a 1,000 ml. volumetric flask. Add about 500 ml. of distilled water to the flask and swirl the solution in the flask until all the thiocyanate is dissolved. When it is all dissolved and at room temperature, add distilled water to bring the solution up to the mark on the neck of the flask. This creates a 0.1 molar solution of potassium thiocyanate.
Note: Potassium thiocyanate is hygroscopic to the point of being deliquescent. Be sure to keep the bottle tightly capped and work quickly when weighing a sample as the sample will gain weight the longer it is exposed to any moisture in the air.
Be meticulous when preparing this solution. Any error in this step will be magnified when the solution is used. My inexpensive scales can drift during use, so I weigh the thiocyanate multiple times to achieve an accurate weight. Always allow all solution to return to the temperature specified on your volumetric flask when topping up the volume to the line on the flask. Be sure to thoroughly mix the final solution to ensure the concentration is consistent throughout.
Performing The Titration - Precision is important in titrations. Pipettes and burettes must be clean and dry. Using a quality measuring pipette, add exactly 1 ml. of the solution to be tested to a suitable, clean container. Erlenmeyer flasks are ideal for this work, as they allow you to swirl the solution in the flask to ensure thorough mixing while minimizing the risk of splashing any of the solution out of the flask. A 250 ml. flask is a good size. A beaker can be used, but extra care must be exercised to prevent any loss of solution which would render the results inaccurate. Add about 1 ml. of the indicator solution, and about 100 ml. of distilled water. These quantities are not critical as they do not affect the accuracy of the results, but be consistent from one test to the next.
Fill a burette with the titrant and adjust the level to the zero mark. Begin adding the titrant from the burette to the flask containing the sample, swirling the flask continuously to mix the solutions. At first, you’ll see a red reaction where the titrant contacts the sample, but the color disappears quickly and a precipitate forms as the solutions mix together. As you approach the endpoint, the color remains longer and the additions of titrant should be slowed to avoid overshooting the endpoint. At the end, you should only add the titrant one drop (or less) at a time, thoroughly mixing between each addition. The endpoint is reached when the last addition of titrant causes the solution to remain a slight pink/orange/purple/red color (the color will vary depending on the colors of any other ions in the solution). Read the amount of titrant that has been used from the burette and use it in the following calculations.
Calculations - The titrant contains 0.1 moles of potassium thiocyanate per liter, or .0001 mole per ml. Multiply the number of ml. of titrant used by .0001 to determine the moles of potassium thiocyanate used. The thiocyanate ions combine with the silver ions in a 1:1 relationship as shown in the formula below.
Ag+ (aq) + SCN- (aq) => AgSCN (s)
So the moles of thiocyanate used equals the moles of silver in the solution.
If you used 10 ml. of titrant, you would have used 10 * .0001 = .001 moles thiocyanate = .001 moles Ag. Since our sample was 1 ml., multiply this result by 1,000 to find the number of moles of silver per liter of solution (or multiply by the number of milliliters of solution to find the number of moles of silver in your solution).
The most common strength of HCl here in the US is 32%. This is a 10.2 molar solution, so in each liter of this HCl there are 10.2 moles of HCl. If your HCl is a different strength, look up or calculate the molarity of what you have available.
So let's say you have a liter of solution that contains 1 mole of silver. You would want to add a little under 1 mole of HCl to react with almost all of the silver.
It is worth noting that the resulting solution will be fairly dilute nitric acid. For a stronger result, evaporate the solution prior to adding the HCl.
NOTE: Palladium, lead, and mercury will interfere with this test as they also form insoluble thiocyanate compounds/complexes.
Dave
