Burets • A buret is a long cylindrical tube with a valve at the bottom end, which is used for making very precise volumetric measurements. • When liquid is dispensed through the valve at the bottom of the buret, the difference between the final volume reading and the initial volume reading measures the volume of liquid which has come out of the buret.
What Does a Buret Look Like? Buret stopcock (note that the buret tip is completely full of liquid, with no air bubbles in the liquid) Top of the buret, showing the liquid’s meniscus A typical buret setup
Reading a Buret • The burets that we will be using are marked at 0.0 mL on the top line, and 50.0 mL on the bottom line, with 0.1 mL increments. • This means that the line on the meniscus must be read to the nearest 0.01 mL to ensure that the correct number of significant figures have been recorded.
Rinsing the Buret • A buret must be scrupulously clean in order to obtain precise and reproducible results. • Rinse the buret well with tap water, and then rinse it twice with 5 mL portions of deionized water. • Rinse the buret twice with 5 mL portions of the solution with which the buret will be filled. • If the buret is rinsed with water, and then filled with the solution immediately, the solution will be diluted slightly by the residual water in the buret. Rinsing the buret with the solution will ensure that the buret is wet with whatever solution it is being filled with, so the concentration of the solution will not change when the buret is filled. • Make sure that some of the water and the wash solution go through the buret tip.
Filling the Buret • After the buret has been rinsed, attach it to a buret clamp on a ring stand. • Fill the buret with the solution which will be measured (use a funnel if necessary). Fill the buret above the 0.0 mL line. • Check to make sure that the buret is not leaking.
Getting Rid of Air Bubbles • If the buret tip contains air bubbles, this will affect the volume measurement. Open the valve to allow some of the solution to run out of the buret, until there are no more bubbles of air in the buret tip.
Acid-Base Titrations • A titration is a procedure that is often used for determining the concentration of a solution. Most commonly, a standard solution of known concentration is reacted with a solution of unknown concentration. • By measuring the volume of standard solution that reacts with a known volume of the unknown solution, the concentration can be calculated from the reaction stoichiometry. • A common example of this process is an acid-base titration, in which an acid or base of unknown concentration reacts with a base or acid of known concentration in a neutralization reaction: HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l)
Acid-Base Titrations • The solution of unknown concentration which is being titrated is placed in an Erlenmeyer flask, which can be easily swirled to ensure complete mixing. • The titrant, which will react with the unknown solution, is placed in the buret. This is a solution of known concentration (or with a concentration that is determined by another series of titrations known as standardization). • The valve at the bottom of the buret is opened, and the titrant is added to the unknown solution, causing a chemical reaction to take place. The addition is stopped when the number of moles of titrant which has been added is equal to the number of moles of reactant in the solution which is being titrated.
The Equivalence Point • In the preceding example, at the equivalence point, the number of moles of NaOH added from the buret equals the number of moles of HCl which were initially present. • The concentration of the unknown HCl solution can be calculated from the reaction stoichiometry:
End Points and Indicators • The end point of the titration, where we experimentally estimate the equivalence point to be, is usually signaled by the color change of an acid-base indicator, such as phenolphthalein. • Phenolphthalein is colorless in an acidic environment, but pink in a basic environment. • Once the end point of the titration is reached, one excess drop of the NaOH solution turns the phenolphthalein pink.
Calculations: Initial and Final Readings • In this example, 15.00 mL of HCl of unknown concentration is being titrated with 0.5115 M NaOH. • Shown below are the initial and final readings on the buret. The difference between the initial and final readings is the volume of NaOH titrant which has been dispensed from the buret. Initial reading 2.38 mL Final reading 18.51 mL Volume of titrant = final reading – initial reading = 18.51 mL – 2.38 mL = 16.13 mL
Calculations: Moles of Titrant • Once the volume of the titrant and its concentration are known, the number of moles of titrant which have been used can be determined. 0.01613 L × 0.511 mol/L NaOH = 0.008251 mol NaOH
Calculations: Moles of Unknown • The reaction stoichiometry is then used to determine the number of moles of HCl initially present: HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l)
Calculations: Molarity of the Unknown • Once the number of moles of HCl are known, dividing by the initial volume of HCl (in this example, 15.00 mL) gives us the concentration of the solution:
Titration of Vinegar • In this experiment, the concentration of acetic acid, HC2H3O2, in commercial vinegar will be determined by titration against a standard solution of NaOH. The equation for this neutralization reaction is: HC2H3O2(aq) + NaOH(aq) NaC2H3O2(aq) + H2O(l) • From the known concentration of the sodium hydroxide, and the volume of the solution that emerges from the buret, the number of moles of NaOH can be determined, which allows the number of moles of HC2H3O2 originally present and the concentration and weight percent of the acid to be determined.