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
Deionized water bottle
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.