It is a common mistake to read the buret volume from the bottom-up. Simply read and record the number you see (to the right number of significant figures, and with units). You don’t need to add or subtract your reading from 50.
In this experiment you will place the standard solution into a clean buret. The solution that is placed into the buret becomes known as the titrant . (Note that it does not matter which solution is placed in the buret.) When readying the buret, over-fill the buret above the 0.00 mL mark. The stopcock is then opened to allow the solution to flow out of the tip until the solution level falls just below 0.00 mL. This process ensures that the tip of the buret is filled with the solution and not with an air bubble 17 16 15 Always measure the bottom of the meniscus at eye level. ← 1
(more on this later). It is not critical that the initial volume of the solution be exactly 0.00 mL. However, it is important that the value be between 0.00 and 1.00 mL, and that the exact value be recorded. Once the buret is loaded with the titrant, we can then prepare the analyte solution. This is the solution that will be placed in the Erlenmeyer flask to be analyzed. It is important that the exact volume of this solution be known, as you will see in the example below. It is also critical that an indicator be added to the analyte solution to signal the end point; otherwise, you will not know when to stop the titration and you may “over shoot” the end point. Before the titration is started, the initial volume of titrant in the buret is read to the nearest 0.01 mL, i.e., to two decimal places; you have to estimate one decimal beyond the graduations. The titrant is added slowly to the analyte solution. As the addition continues, a fleeting color change may be noted (as a result of the indicator), especially when the titrant first contacts the analyte solution. It is important that the analyte solution be swirled constantly to ensure thorough mixing. The endpoint is not reached until the entire solution has a permanent color change (does not disappear with swirling). With practice it is possible to determine the endpoint within a single drop of titrant solution. Once the color change is permanent, the first titration trial is complete, and the final volume can be read from the buret. The difference between the final and initial buret reading is the volume of titrant added to the flask. The volume and concentration of the standard are used to calculate the moles of titrant added. The moles of titrant can then be stoichiometrically compared to the moles of analyte present, allowing us to calculate the concentration of the analyte solution. Since titration is an analytical technique, multiple trials are run to ensure consistency of the results. Titrations should be run with a minimum of two trials; however, the more trials the better your results are likely to be. If, after performing a titration, you find that one set of results does not agree with the others, it should be excluded. Ideally, an additional trial should be run to confirm the results. Titration is an
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- Fall '16
- John B Michel
- Mole, Sodium hydroxide, Erlenmeyer flask, Buret, KHP