Calculations for titration.

What possible errors can occur during titration that would affect calculations?

• Answer:

  Titration is a common laboratory method of quantitative/chemical analysis which can be used to determine the concentration of a known reactant. Because volume measurements play a key role in titration, it is also known as volumetric analysis. A reagent, called the titrant, of known concentration (a standard solution) and volume is used to react with a measured quantity of reactant (Analyte). Using a calibrated burette to add the titrant, it is possible to determine the exact amount that has been consumed when the endpoint is reached. The endpoint is the point at which the titration is stopped. This is classically a point at which the number of moles of titrant is equal to the number of moles of analyte, or some multiple thereof (as in di- or tri- protic acids). In the classic strong acid-strong base titration the endpoint of a titration is when the pH of the reactant is just about equal to 7, and often when the solution permanently changes color due to an indicator. Good accuracy requires that systematic errors be reduced as far as possible. The use of analytical grade reagents will reduce errors due to purity of reagents such as acid or alkali and the salt used for ionic background. Errors in temperature control are systematic errors. Electrode calibration error is also a systematic error, of particular importance when comparing duplicate titration curves. Error in titre volume. The error in titre volume can be estimated by weighing. It is a good idea to check both the accuracy and precision of a burette. If the weight delivered at a given temperature is measures for a series of volumes the data can be fitted to a straight line; the required error value will then be given by the error on the slope. Begin by preparing your buret, it should be conditioned and filled with titrant solution. You should check for air bubbles and leaks, before proceding with the titration to avoid error in volume. Take an initial volume reading and record it in your notebook. Before beginning a titration, you should always calculate the expected endpoint volume. THis will help you to avoid errorsin recording the data, as it will be obvious if you end up with a massive difference. Prepare the solution to be analyzed by placing it in a clean Erlenmeyer flask or beaker - you need to remove contamination to your results. If your sample is a solid, make sure it is completely dissoloved. Put a magnetic stirrer in the flask and add indicator. Use the buret to deliver a stream of titrant to within a couple of mL of your expected endpoint. You will see the indicator change colour when the titrant hits the solution in the flask, but the colour change disappears upon stirring. Approach the endpoint more slowly and watch the colour of your flask carefully. Use a wash bottle to rinse the sides of the flask and the tip of the buret, to be sure all titrant is mixed in the flask. As you approach the endpoint, you may need to add a partial drop of titrant. You can do this with a rapid spin of a teflon stopcock or by partially opening the stopcock and rinsing the partial drop into the flask with a wash bottle. Ask your college to demonstrate these techniques for you, in the lab. Make sure you know what the endpoint should look like. For phenolphthalein, the endpoint is the first permanent pale pink. The pale pink fades in 10 to 20 minutes. If you think you might have reached the endpoint, you can record the volume reading and add another partial drop. Sometimes it is easier to tell when you have gone past the endpoint. When you have reached the endpoint, read the final volume in the buret and record it in your notebook. paralax error, this is a personal error, in that you misread the scale, because your eye is not at a tangent to the liquid level. Subtract the initial volume to determine the amount of titrant delivered. Use this, the concentration of the titrant, and the stoichiometry of the titration reaction to calculate the number of moles of reactant in your analyte solution.