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Experiment #10 Determination of Acetic Acid in Vinegar Laboratory Overview CHEM 1361 August 2010 Gary S. Buckley, Ph.D. Department of Physical Sciences Cameron University
Table of Contents (you may click on any of the topics below to go directly to that topic) Chemical Background Experimental Scheme Standardization of Sodium Hydroxide Determination of Acetic Acid in Vinegar Experimental Notes Refresher on Accuracy, Precision, Average, and Standard Deviation Accuracy and Precision Measures of Accuracy and Precision Accuracy and Precision An Example
Chemical Background Vinegar, by law, is required to have at least 4%, or 4 grams acetic acid per 100 ml of vinegar. In this experiment you will use an analytical method called titration to determine the % acetic acid in a vinegar sample. Acetic acid, written as HC 2 H 3 O 2 or CH 3 COOH, reacts with NaOH according to the following chemical equation: HC 2 H 3 O 2 (aq) + NaOH (aq) HOH (l) + NaC 2 H 3 O 2 (aq) A known volume of vinegar may be reacted with a measured quantity of sodium hydroxide. Since the acetic acid and sodium hydroxide react in a 1:1 mole ratio, knowledge of the number of moles of NaOH used leads directly to the number of moles of acetic acid in the sample of vinegar. The following slides work this out in more detail.
Experimental Scheme NaOH Sodium hydroxide of a known concentration will be delivered from a buret as indicated in the figure to the right. The Erlenmeyer flask will contain the vinegar sample. A few important points: 1. The concentration of the sodium hydroxide must be known. This concentration is determined through a process called standardization. 2. Reaction of the sodium hydroxide and the acetic acid leads to a clear solution there is no way of knowing when the number of moles of sodium hydroxide matches the moles of acetic acid unless: 3. You add an indicator that will change color at that point, called the end point. In this case, phenolphthalein will be added as the indicator. It will change from clear to pink at the end point. Vinegar + phenolphthalein
Standardization of Sodium Hydroxide Making an accurately known concentration of sodium hydroxide from starting reagents is virtually impossible. Solid sodium hydroxide is hygroscopic absorbs water readily from the air so obtaining a mass that is purely sodium hydroxide is not feasible. It is also available as a 50 % solution but the concentration is not known to high accuracy. Thus, to know accurately (say, four significant figures), the concentration of a sodium hydroxide solution it must be reacted with a known number of moles of some other substance. A variety of primary standards substances whose purity is well known is available to carry out such a reaction. In this experiment, the primary standard potassium hydrogen phthalate, KHC 8 H 4 O 4,will be used to determine the concentration of the sodium hydroxide. Potassium hydrogen phthalate is also referred to affectionately as KHP. Be alert it is not potassium, hydrogen, and phosphorous. Its formula is KHC 8 H 4 O 4 and its molar mass is 204.2 g/mol.
Standardization of Sodium Hydroxide (continued) The reaction between KHP and sodium hydroxide is as follows (in net ionic form): HC 8 H 4 O 4- (aq) + OH - (aq) C 8 H 4 O 4 2- (aq) + HOH (l) The KHP mass will be accurately recorded, from which the number of moles may be determined using its molar mass (204.2 g/mol). Using phenolphthalein as the indicator, sodium hydroxide solution will be added until a faint pink end point is reached. At that point, the moles of sodium hydroxide added will equal the number of moles of potassium hydrogen phthalate added. By determining the volume of sodium hydroxide solution added, its molarity may be determined by : M = NaOH mol NaOH delivered to endpoint (equal to mol KHP measured out) L NaOH solution delivered to endpoint
Determination of Acetic Acid in Vinegar Once the sodium hydroxide solution has been standardized, it may be used to determine the quantity of acid in other solutions, in this case acetic acid in vinegar. Acetic acid and sodium hydroxide react according to the equation (net ionic form): HC 2 H 3 O 2 (aq) + OH - (aq) C 2 H 3 O 2 2- (aq) + HOH (l) Since the molarity of the sodium hydroxide solution is known after the standardization, the moles of sodium hydroxide delivered in the titration may be determined. The moles of sodium hydroxide will match the moles of acetic acid at the end point. The molarity of the acetic acid in the vinegar may be determined by dividing the number of moles of acetic acid by the volume (in L) of the vinegar titrated. To get to the % acetic acid in vinegar (g/100 ml) from there, the moles of acetic acid in one liter may be converted to grams and the result divided by 10 to reflect that 100 ml is one-tenth of one liter.
Experimental Notes The primary piece of lab apparatus used in this experiment is called a buret (or burette), pictured to the right. Its function is to provide a means of accurately determining the volume of a solution delivered. At the bottom of the buret is a stopcock that allows solution to flow or blocks its flow depending on its orientation. The positions of the stopcock are illustrated below. Flow is blocked if the stopcock is turned horizontally. Solution flows if the stopcock: If the stopcock is turned is turned vertically. Filling the buret is a simple matter of using a funnel to pour your solution through the top WHILE THE STOPCOCK IS CLOSED. After filling, allow some solution to pass through the stopcock to a waste container to remove air bubbles from the tip.
Experimental Notes (continued) Reading the Buret To attain the full benefit of using a buret, it must be read correctly. Notice the volumes increase as you go down the buret. When making a reading, you are simply recording the volume as indicated on the buret don t try to make it harder by subtracting from 50 or anything. Furthermore, notice how the buret is marked to the 0.1-mL place. This suggests that EVERY reading must be to the.01-ml place we always estimate one digit on an analog device. Aqueous solutions inside of the buret show the meniscus discussed in Experiment #2. The reading is always taken at the bottom of the meniscus. In this case the reading would be 1.40 ml notice the 0 is included to indicate the meniscus was right at the line. You will notice the table in the lab book is set up with the final volume first followed by the initial volume. This helps see that the determination of the volume delivered is made by subtracting the initial volume reading from the final volume reading. Meniscus
Refresher on Accuracy, Precision, Average, and Standard Deviation The following few slides are an encore presentation of the discussion of some topics discussed in Experiment #2. In both your standardization process and your determination of the % acetic acid in vinegar, you will be able to find an average and a standard deviation since you will do multiple trials of each. The average is linked to the accuracy of the results how close your results are to the real value which, by the way, we do not really know. We are simply trying to determine it experimentally. The standard deviation is linked to the precision of the results how close your repetitions are to each other. A small standard deviation indicates a consistent experimental technique, though it does not say anything about the accuracy.
Accuracy and Precision Measurements in the laboratory are an attempt to find the real value of a physical quantity. Two terms used in relation to measurement are: Accuracy the nearness of the measured value to the real value Precision the nearness of repeated measurements to each other Note that a measured result may be accurate, but not precise; not accurate, but precise; both accurate and precise; or neither accurate nor precise. The next couple of slides give one method for considering accuracy and precision.
Measures of Accuracy and Precision Average or mean the average, sometimes called the mean, is simply the sum of repeated attempts to measure the same quantity divided by the total number of attempts. Standard deviation the standard deviation is an indication of the precision of repeated measurements. If one takes N measurements of the same physical quantity, the standard deviation, s, is given by: s N i 1 ( x x) N i 1 2 Where x i represents the i th measurement and x represents the average of all of the measurements.
Accuracy and Precision An Example Suppose you had measured the molarity of a solution (doesn t matter if you know what molarity is yet or not) and got the three values of 0.1042 M, 0.1004 M, and 0.1033 M. The average is given by: x 0.1042M 0.1004M 0.1033M 3 0.1026M The standard deviation would be calculated as: s 2 2 2 (0.1042 0.1026) (0.1004 0.1026) (0.1033 0.1026) 3 1 0.0020M The result would be reported as 0.1026 ± 0.0020 M, or more properly as 0.103 ± 0.002 M as we try to keep one significant figure in the standard deviation and round the average to match that number of decimal places.