Technique G : Buret Use Section 1 : Purpose of Technique A buret is primarily used for titration to determine the concentration of an unknown solution by adding a solution of known concentration. Titration is a type of quantitative analysis and the most common forms are acid-base, precipitation, complexometric, and redox titrations. Because the purpose of titration is to find the concentration of an unknown solution, it is very important that accurate measurements are taken to determine the volume of the known solution added to bring a reaction to an endpoint. The titrant is the solution with known concentration and is placed in the buret. The analyte is the solution of unknown concentration that is being measured. The analyte is usually placed in an Erlenmeyer flask with an indicator. The equivalence point is the exact point where the moles of the titrant solution and the analyte solution are equivalent. The end point is when the indicator in the solution shows a color change. This usually occurs close to and just after the equivalence point is reached. Using a buret as opposed to other measurable glassware, such as pipets and graduated cylinders, makes it easier to control the delivery of and measure the precise amount of solution being delivered. Slight turns of the stopcock located on the buret can control the flow of solution and there are methods to get as close as possible to the end point without going over. Because of its smaller meniscus, a buret is more accurate and precise than a graduated cylinder. A measurement made with a buret should be to the hundredths place (ex: 0.00). The burets generally used in the laboratory titration procedures hold up to 50 millimeters (ml). 10 ml, 25 ml and 1.0 ml burets are also commercially available. Section 2 : Components for Using a Buret Buret Stand Buret clamp Deionized water bottle Erlenmeyer flask Waste beaker Section 3: Setup of a Buret Part 1: Inspect the equipment Check the equipment for broken parts before use. Inspect the glassware and the tip of the buret. As long as the bore of the buret tip is intact, the buret is functional. Part 2: Preparing the Buret Before titration, the buret must be rinsed with laboratory water and with the solution you are titrating with. Rinse the buret with small volumes laboratory water three times. Then, rinse the buret with the titrant solution three times, making sure to let the solution coat the inside of the buret completely and run through the tip. Check for air bubbles in the tip of the buret. Air bubbles trapped in the tip can throw off your measurements. If you see an air bubble, you can get rid of it by filling the buret with a solution, firmly holding it over a sink, and vertically shaking it up and down in one quick motion. Part 3: Setting up the Buret Apparatus After the buret has been rinsed with the laboratory water and the solution, fill the buret with the solution, going above the 0.00 graduation mark. Attach the buret to a stand using the buret clamp. Place the waste beaker under the tip of the buret. Stand at eye level to the 0.00 graduation mark and turn the stopcock slightly to release some of the solution into the waste beaker. Try to get the meniscus to reach the 0.00 line as your initial measurement. It is okay if you go past the 0.00 line. Just make sure to record the initial measurement accurately for what it is. Part 4: Performing the Titration Fill the Erlenmeyer flask with the required amount of the analyte solution and make sure to add indicator . Adding indicator is the only way to visibly see whether you reached the end point or not. Place the Erlenmeyer flask under the tip of the buret. Turn the stopcock of the buret, releasing the titrant solution into the Erlenmeyer flask. Control the flow of the titrant by adjusting the stopcock. Observe the analyte solution for changes in color from the indicator. Swirl the solution in the flask to make sure the titrant is evenly distributed. You know when you are close to the end point if the small bursts of color in the flask persist for more than a few seconds. Slow down the release of titrant into the flask by turning the stopcock as you near this point. Begin adding half drops instead of full drops if necessary. Titration is complete once the analyte solution reaches a uniform color. The end point is reached when the indicator color is faint. A more vibrant color may indicate that the end point was exceeded. If the endpoint was significantly surpassed, you will have to redo the titration. The following photos show the general setup and a faint pink for the endpoint (phenolphthalein is the indicator used in the photos—other indicators may be used in the titration). Section 4 : Important Things to Remember Measure your buret to the nearest 0.01 ml When taking measurements using the buret, make sure to view the meniscus at eye level. You should be looking straight at the line where the meniscus lies. The graduation line when looking directly at it should not form an oval. In the left photo above, note that the inscribed circle at the 1.00 ml mark looks like an oval. In the right photo above, note the inscribed circle at the 20.00 ml mark looks like an oval, the 22.00 ml mark looks like a line, and the 24 ml mark again looks like an oval. If the mark closest to the meniscus is a line, it means that your eye is at the correct height to properly read the buret. People are not generally good at estimating tenths, but are good at splitting by halves. Use this technique to estimate the final 0.01 ml reading between the tenths of a milliliter mark. Mentally draw a line ½ way between the two marks. This will be the 0.05 ml between the two tenth milliliter marks. Repeat this process to split the distance between the 0.05 ml mark and the inscribed mark. This will be a 0.025 ml increment. Evaluate where the meniscus is now relative to your estimates and round either up or down to the final reading. For example, if your meniscus is between 22.30 ml and 22.40 ml, then your buret should be evaluated like this: 22.30 ______________ 22.325 - - - - - - - - - - - - - 22.35 ------------------------ 22.375 - - - - - - - - - - - - - 22.40 ______________ Now you should be able to accurately estimate where the meniscus falls on the buret to the nearest 0.01 ml. Use your information If you are doing multiple titrations, there is nothing wrong with doing calculations to estimate the volumes for your second, third, and fourth titrations. Recognize that your calculation is just an estimate and may not reflect the actual results of the titration procedure. The only way to know the actual concentration of the analyte is by doing the titration multiple times. For example: You are performing an acid-base titration between hydrochloric acid (HCl) and sodium hydroxide (NaOH). The concentration of the sodium hydroxide is 2.1043 M and you are trying to determine the concentration of 15.00 ml of HCl. After your first titration, your final buret measurement is at 17.35 ml. Use this calculated estimate as a guide for the next titrations. HCl+NaOH→H 2 O+NaCl \(M_{\mathrm{HCl}}=\frac{M_{\mathrm{NaOH}} V_{\mathrm{NaOH}}}{V_{\mathrm{HCl}}}\) \(M_{H C l}=\frac{(2.1043 M \mathrm{NaOH})(17.35 \mathrm{~mL} \mathrm{NaOH})}{15.00 \mathrm{~mL} \mathrm{HCl}}\) \(M_{H C l}=2.434 M \mathrm{HCl}\) For your second titration, you decide to use a 20.00 ml volume of HCl instead of a 15.00 ml volume. Using a little algebra, the amount of 2.1043 M NaOH that is expected to be added is given by: \(V_{\mathrm{NaOH}}=\frac{M_{\mathrm{HCl}} V_{\mathrm{HCl}}}{M_{\mathrm{NaOH}}}\) For our example: \(V_{\mathrm{NaOH}}=\frac{2.434 \mathrm{MHCl} \times 20.00 \mathrm{ml} \mathrm{HCl} \text { solution }}{2.1043 \mathrm{MNaOH}}\) For our example: \(V_{\mathrm{NaOH}}=23.13 \mathrm{ml}\) NaOH titrant (estimated) Therefore, for your second titration, dispense down about 20-22 ml of titrant before carefully adding the last drop to the endpoint. Adding titrant by half-drops If you think you are nearing the end point, instead of allowing the buret to release full drops one at a time, you can create half-drops. To do this, close the stopcock until the buret is no longer releasing the titrant. Carefully open the stopcock slightly so that a drop forms at the tip of the buret, but not enough to fall. Turn the stopcock off. Use a deionized water bottle, and wash the tip of the buret so that the drop of titrant falls into the titration. This is considered a half-drop. Remaining fluid on the tip of the buret at this point should be entirely laboratory water. The next full drop that the buret releases from the tip will actually be another half drop of titrant (combined with the remaining water on the tip). You can alternate back and forth to add half-drop quantities of titrant. First titration quick method Often, you have no idea where the first titration endpoint occurs. This makes it quite easy to accidentally exceed the endpoint and add too much titrant, with resulting accuracy error. One technique to work around this is to reserve a little bit of analyte solution. Pour a small volume of analyte into out into a clean, small beaker. It is considered reserve because if you accidentally over shoot the end point, you can use reserve to save your analysis. The titrant can be added relatively quickly until the indicator changes color. The following photos show how this works. A small amount of analyte is temporarily transferred to a small beaker. The titration is rapidly done dropwise until the indicator changes. The reserve is returned to the Erlenmeyer beaker to change the indicator back, and you now know you are close to the endpoint. Now, use the half-drop technique described earlier to finish the titration. R emember to return the reserve to the Erlenmeyer flask regardless if it were needed . After you have completed one titration, subsequent volumes can be estimated as was described above.