1 ACID-BASE TITRATION REFERENCE: Nelson, J., Chemistry: The Central Science, 3rd edition, Prentice-Hall, 1985 This experiment will demonstrate the techniques of volumetric analysis or titration. Here, a quantitative determination of the amount of acid in an unknown sample will be made. Apparatus and Chemicals 500 mL Erlenmeyer flask Wash bottle 250 mL Erlenmeyer flasks (3) 600 mL beaker 1-pt bottles with caps (3) 19 M NaOH Potassium acid phthalate (primary standard) Ring stand Buret clamp Phenolphthalein solution Unknown acid Weighing bottle 50 mL Class A buret Discussion One of the most common and familiar reactions in chemistry is the reaction of an acid with a base. This reaction is termed neutralization, and the essential feature of this process in aqueous solution is the combination of hydronium ions with hydroxide ions to form water: H3O+(aq) + OH−(aq) → 2 H2O(l) In this experiment you will use the above reaction to accurately determine the concentration of a sodium hydroxide solution that you have prepared. The process of determining the concentration of a solution is called standardization. Next you will measure the amount of acid present in an unknown. To do this, you will accurately measure with a buret the volume of your standard base that is required to exactly neutralize the acid present in the unknown. The technique of accurately measuring the volume of a solution required to react with another reagent is termed titration. An indicator solution is used to determine when an acid has exactly neutralized a base, or vice versa. A suitable indicator changes colors at well-defined pH values; if this pH value corresponds to the pH region in which equivalent amounts of acid and base are present during a titration experiment, we can use the color change to determine the stoichiometric point. The color change is termed the end point of the titration. Different indicators change colors at different pH values. Phenolphthalein, for example, changes color from colorless to pink at a pH of about 9; in slightly more acidic solutions it is colorless, while in slightly more alkaline solutions it is pink. In this experiment your solution of NaOH will be standardized by titrating it aginst a very pure sample of potassium acid phthalate, (KHC8H4O4) of known mass. Potassium acid phthalate (henceforth abbreviated as KHP) has only one replaceable acid hydrogen 3043 gKHP = 1. What is the exactly concentration of the NaOH titrant? 0. or any other acid. a sample containing an unknown amount of KHP (or any other acid.2 g/mole.01512 LNaOHtitrated Once the exact molarity of NaOH solution is known. It is a monoprotic acid with the acidic hydrogen bonded to oxygen and has a molecular weight of 204.490 x10 −3 moleNaOH = 0.12 mL of a NaOH solution that was approximately 0. Its structure is shown below. an equal number of moles of base and acid are present at the end point (moles NaOH = moles KHP) or COOH + Na+ + OH− COOK COO− Na+ + HOH COOK Once the endpoint has been reached then the exact molarity can be calculated by dividing the volume of base that was titrated (in liters) into the moles of NaOH present. the base can be used to determine the amount of KHP. 1. COOH COO− K+ In the titration of a base against KHP.2 g / mole At the end point: 1. .1 M.09854MNaOH (0. Moles NaOH / volume of base titrated (L) = M of NaOH in stock bottle Now that the exact concentration of NaOH is known. for that matter) can be determined.490 x 10-3 mole NaOH titrated therefore.09856 M using all digits from first calc) 0.490 x 10-3 mole KHP = 1.3043 g of pure KHP was weighed out and titrated to an end point with 15. present in a known mass of an impure sample.2 (indicated in BOLD).490 x10 −3 moleKHP 204. Example of Standardization 0. 002104molesNaOH L 0.4296 gKHP × 100 = 80.35 mL of 0. Make sure that the buret does not leak and that the stopcock turns freely. hold the card so that the lowest part of the meniscus is even with the “top” of the black colored box. Add 3 mL of stock solution of carbonate-free sodium hydroxide (approximately 19 M) to only one of the pint bottles and shake vigorously for at least 1 minute. . and after cooling under the water tap. 0.5366 gsample Procedure Preparation of approximately 0. the eye must be on a level with the meniscus. this meniscus is concave (∪).002104 moles NaOH = 0. When taking a volume reading involving the meniscus.02135L = 0. transfer to three 1-pint bottles fitted with caps. The water must run freely from the buret without leaving any drops adhering to the sides. Color a 2 × 1 box in solid black ink in the center of the card. Obtain a 3 × 5 card. The sample was dissolved in approximately 100 mL of distilled. when placed in a buret.09854molesNaOH × 0.1 M Sodium hydroxide Wash the 1-pint bottles with caps. To avoid parallax errors when taking reading. degassed water and indicator was added. In the case of water or water solutions. The other two pint bottles containing the CO2free water will be used for the remainder of the experiment.06% KHP in the unknown sample 0.09854 M NaOH solution was titrated into the solution. Heat a total of 1500 mL of distilled water to boiling using no larger than 600 mL beakers or 500 mL flasks. form a curved meniscus at their upper surfaces. What is the percentage of KHP in the original sample? At the end point: 0.5366 g of an KHP sample of unknown purity was massed. Then rinse with at least five 10 mL portions of distilled water. 0.4296 g KHP therefore. Reading a buret All liquids.002104 moles KHP = 0. Preparation of a buret for use Clean a 50 mL class A buret with soap solution and a buret brush and thoroughly rinse with tap water. and the most accurate buret readings are obtained by observing the position of the lowest point on the meniscus on the graduated scales.3 Example: The percentage determination of an acid in an unknown sample. The end point was reached after 21. Record the masses and label the three flasks in order to distinguish them from one another. Discard each portion. Do not weigh the flasks. Add to each flask two drops of phenolphthalein indicator solution. This coloration disappears with swirling. Rinse the previously cleaned buret with at least four 5 mL portions of the approximately 0. As the sodium hydroxide solution is added. A sheet of white paper below the flask will help you to see the color change. accurately weigh to four significant figures. The end point is reached when one drop of the sodium hydroxide solution turns the entire solution in the flask from colorless to pink. Allow the buret to stand for at least 30 seconds before reading the exact position of the meniscus. Completely fill the buret with the solution and remove the air from the tip by running out some of the liquid into an empty beaker. a pink color appears where the drops of the base come in contact with the solution. Remove any hanging drop from the buret tip by touching it to the side of the beaker used for the washings.1 M sodium hydroxide solution that you have prepared. at which time the sodium hydroxide should be added drop by drop. Make sure that the lower part of the meniscus is at the zero mark or slightly lower. Record the buret reading. Figure 1 Level of meniscus Swirl the flask continuously until one drop of titrant causes a color change throughout the entire solution. Slowly add the sodium hydroxide solution to one of your flasks of KHP solution while gently swirling the contents of the flask as illustrated in Figure 1. the color disappears more slowly. . As the end point is approached.6 grams each of pure KHP into 250 mL Erlenmeyer flasks. Do not return any of the washings to the bottle. Record the initial buret reading.4 STANDARDIZATION OF SODIUM HYDROXIDE SOLUTION Mass from a weighing bottle (your instructor will show you how to use a weighing bottle if you do not already know) triplicate samples of between 0. This solution should remain pink when it is swirled. Repeat this procedure with the other two samples. Remove any hanging drop from the buret tip by touching it to the side of the flask and wash down the sides of the flask with a stream of water from the wash bottle. Add to each sample about 100 mL of CO2-free distilled water and warm gently with swirling until the salt is completely dissolved.4 and 0. It is most important that the flask be swirled constantly throughout the entire titration. Allow the titrated solution to stand for at least 1 minute so the buret will drain properly. If one result is noticeably different from the others. Test your results by computing the average deviation from the mean.0 percent.75 Weigh by difference (from the unknown bottle) one portion of the sample to four significant figures and place it in one clean 250 mL flasks. ANALYSIS OF AN UNKNOWN ACID Calculate the approximate mass of unknown that should be taken to require about 30 mL of your standardized sodium hydroxide assuming that your unknown sample is 75% KHP. Example of Sample Size How much mass of unknown that is approximately 75% by mass KHP will be required if 30. Record the initial and final buret readings for each trial. Add 100 mL CO2-free water and two drops phenolphthalein indicator to each flask. .03000 L)(0. perform an additional titration. Titrate with your standard sodium hydroxide solution to the faintest visible shade of pink as described above in the standardization procedure. The average of the three acceptable determinations is taken as the molarity of the sodium hydroxide. Weigh by difference the unknown for trials 2 and 3 and place in clean. and has a history of problems and/or possible error in preparation discard it and titrate another sample. Dissolve the sample in 100 mL CO2-free distilled water and add two drops of phenolphthalein indicator solution. For best results the three determinations should agree within 1.0%.002956molesKHP)(204.80 g approximately 0. labeled 250 mL Erlenmeyer flasks. calculate how much sample should be used for trials 2 and 3. Calculate the percentage of potassium acid phthalate (KHP) in the samples.5 From the data you obtain in the three titrations. Dissolve the samples thoroughly and titrate. If the first trial requires less than 20 mL of NaOH to reach the end point. calculate the molarity of the sodium hydroxide solution to four significant figures. label your stock bottle of NaOH solution with this average value. If any result is more than two standard deviations away from the mean. Now.002956 moles KHP (0.002956 moles NaOH = 0. The three determinations should agree within 1. The sample size should be about the amount determined by the above computation. If they do not.09854 M NaOH should be used? (0.09854 M NaOH) = 0. mL of 0. Compute the standard deviation of your results.223g / mol ) = 0. the standization should be repeated until agreement is reached. Calculate average NaOH Molarity: Calculate standard deviation: .6 Acid-Base Titration Report Sheet Name_____________________________________ Standarization of NaOH Solution Trial 1 Mass of bottle + KHP before removing trial mass Mass of bottle + KHP after removing trial mass Final buret reading Initial buret reading mL of NaOH used for trial Molarity of NaOH for trial (show work below) Trial 2 Trial 3 Sample calculation for calculation of NaOH molarity for one trial. after removing mass Mass of unknown for each trial Final buret reading Initial buret reading ML of NaOH used for each trial Molarity of NaOH Moles of NaOH used for each trial (show calc) Trial 2 Trial 3 Calculation for moles of NaOH used for one trial Trial 1 Mass of KHP in Unknown for each trial Trial 2 Trial 3 Calculation for mass of KHP in Unknown for one trial Trial 1 Percent KHP in Unknown for each trial Trial 2 Trial 3 Calculation of percent KHP for one trial Calculation of Average Percent KHP ________________________________________________________________________ Calculation of standard deviation . before removing mass Mass of bottle + Unk.7 Analysis of Unknown Acid Unknown No:__________ Name_____________________________________ Trial 1 Mass of bottle + Unk.