REFERENCES: Nelson, J., Chemistry:The Central Science, 3rd Edition, Prentice-Hall, 1985

Transcription

1 AID AND BASE TITRATION Titration Page 1 of 5 REFERENES: Nelson, J., hemistry:the entral Science, 3rd Edition, Prentice-all, 1985 This experiment will demonstrate the techniques of volumetric analysis or titration. ere, a quantitative determination of the amount of acid in an unknown sample will be made. Apparatus and hemicals 500 ml Erlenmeyer flask 3 1 pint bottles with caps 50 ml lass A buret Phenolphthalein solution Buret clamp Potassium acid phthalate (primary standard) Ring stand ml Erlenmeyer flasks 19 M NaO Weighing bottle Balance Unknown acid Wash bottle 600 ml beaker 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: 3 O + (aq) + O (aq) 2 2 O(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 p values; if this p value corresponds to the p region in which equivalent amounts of acid and base are present during a trtration 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 p values. Phenolphthalein, for example, changes color from colorless to pink at a p of about 9; in slightly more acidic solutions it is colorless, while in slightly more alkaline solutions it is pink.

2 Titration Page 2 of 5 In this experiment your solution of NaO will be standardized by titrating it against a very pure sample of potassium acid phthalate, (K 8 4 O 4 ) of known mass. Potassium acid phthalate (henceforth abbreviated as KP) has only one replaceable acid hydrogen (indicated in BOLD). Its structure is shown below. It is a monoprotic acid with the acidic hydrogen bonded to oxygen and has a molecular weight of g/mole. O O O - K + Or OO OOK O (in shorthand notation) In the titration of a base against KP, an equal number of moles of base and acid are present at the end point (moles NaO = moles KP) or OO OOK + Na + + O - OO OOK Na Once the end point 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 NaO present. moles NaO volume of base titrated (L) = M of NaO in stock bottle Now that the exact concentration of NaO is known, a sample containing an unknown amount of KP (or any other acid, for that matter) can then be determined. Example of Standardization g of pure KP was weighed out and titrated to an end point with ml of a NaO solution that was approximately 0.1 M. What is the exact concentration of the NaO titrant? At the end point: therefore, g KP g/mole = x 10 3 mole KP x 10 3 mole KP = x 10 3 mole NaO titrated x 10 3 mole NaO L of NaO titrated = M NaO exactly

3 Titration Page 3 of 5 Once the exact molarity of the NaO solution is known, the base can be used to determine the amount of KP or any other acid present in a known mass of an impure sample. Example: The percentage determination of an acid in an unknown sample g of an KP sample of unknown purity was massed. The sample was dissolved in approximately 100 ml of distilled, degassed water and indicator was added. The end point was reached after ml of M NaO solution was titrated into the solution. What is the percentage of KP in the original sample? At the end point: therefore, moles/l NaO x L = moles NaO moles NaO = moles JP = g KP ( g KP g sample) * 100% = 80.06% KP in the unknown sample Procedure Preparation of approximately M Sodium hydroxide. eat a total of 1500 ml of distilled water to boiling using no larger than 600 ml beakers, and after cooling under the water tap, transfer to three 1-pint bottles fitted with caps. 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. The other 2 pint bottles containing the O 2 - free water will be used for the remainder of the experiment. Preparation of a buret for use. lean a 50 ml, class A buret with soap solution and a buret brush and thoroughly rinse with tap water. Then rinse with at least five 10 ml portions of distilled water. The water must run freely from the buret without leaving any drops adhering to the sides. Make sure that the buret does not leak and that the stopcock turns freely. Reading a buret. All liquids, when placed in a buret, form a curved meniscus at their upper surfaces. In the case of water or water solutions, this meniscus is concave ( ), and the most accurate buret readings are obtained by observing the position of the lowest point on the meniscus on the graduated scales. To avoid parallax errors when taking readings, the eye must be on a level with the meniscus. Obtain a 3 5 card. olor a 2 1 box in solid black ink in the center of the card. When taking a volume reading involving the meniscus, hold the card so that the lowest part of the meniscus is even with the "top" of the black colored box.

4 STANDARDIZATION OF SODIUM YDROXIDE SOLUTION Titration Page 4 of 5 Mass from a weighing bottle (your lab instructor will show you how to use a weighing bottle if you do not already know) triplicate samples of between 0.4 and 0.6 grams each of pure KP into 250 ml Erlenmeyer flasks; accurately weigh to four significant figures. Do not weigh the flasks. Record the masses and label the three flasks in order to distinguish them from one another. Add to each sample about 100 ml of O 2 -free distilled water and warm gently with swirling until the salt is completely dissolved. Add to each flask two drops of phenolphthalein indicator solution. Rinse the previously cleaned buret with at least four 5 ml portions of the approximately M sodium hydroxide solution that you have prepared. Discard each portion. Do not return any of the wash.ings to the bottle. ompletely fill the buret with the solution and remove the air from the tip by running out some of the liquid into an empty beaker. Make sure that the lower part of the meniscus is near the zero mark. Allow the buret to stand for at least 30 seconds before reading the exact position of the meniscus. Remove any hanging drop from the buret tip by touching it to the side of the beaker used for the washings. Record the initial buret reading. Slowly add the sodium hydroxide solution to one of your flasks of KP solution while gently swirling the contents of the flask as illustrated in Figure 1. As the sodium hydroxide solution is added, a pink color appears where the drops of the base come in contact with the solution. This coloration disappears with swirling. As the end point is approached, the color disappears more slowly, at which time the sodium hydroxide should be added drop by drop. It is most important that the flask be swirled constantly throughout the entire titration. The end point is reached when one drop of the sodium hydroxide solution turns the entire solution in the flask from colorless to pink. This solution should remain pink when it is swirled. Allow the titrated solution to stand for at least 1 minute so the buret will drain properly. 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. Record the buret reading. Repeat this procedure with the other two samples. Level of meniscus Pull the stopcock in against the taper each time you turn it A sheet of white paper below the flask will help you to see the color change. Swirl the flask continuosly until one drop of titrant causes a color change thruout the entire solution From the data you obtain in the three titrations, calculate the molarity of the sodium hydroxide solution to four significant figures.

5 Titration Page 5 of 5 The three determinations should agree within 1.0 percent. If they do not, the standardization should be repeated until agreement is reached. The average of the three acceptable determinations is taken as the molarity of the sodium hydroxide. Now, label your stock bottle of NaO solution with this average value. ANALYSIS OF AN UNKNOWN AID alculate 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 percent KP. Weigh by difference (from a weighing bottle) triplicate portions of the sample to four significant figures and place them in three separate clean 250 ml flasks. The sample size should be about the amount determined by the above computation. Dissolve the sample in 100 ml Of O 2 -free distilled water and add two drops of phenolphthalein indicator solution. Titrate with your standard sodium hydroxide solution to the faintest visible shade of pink as described above in the standardization procedure. alculate the percentage of potassium acid phthalate (KP) in the samples. For best results the three determination should agree with 1.0 percent. ompute the standard deviation of your results. Test your results by computing the average deviation from the mean. If one result is noticeably different from the others, perform an additional titration. If any result is more than two standard deviations away from the mean, and has a history of problems and/or possible error in preparation discard it and titrate another sample.

6 Titration Experiment Report Sheet Name: Standardization of NaO Solution Trial 1 Trial 2 Trial 3 Mass of bottle + KP Mass of bottle Mass of KP used Final buret reading Initial buret reading ml of NaO used Molarity of NaO Average Molarity: Standard deviation: Show calculations for Molarity and standard deviation

7 Analysis of Unknown Acid Name: Unknown Number: Trial 1 Trial 2 Trial 3 Mass of bottle + Unknown Mass of bottle Mass of Unknown used Final buret reading Initial buret reading ml of NaO used Molarity of NaO used Moles of NaO used Mass of KP in Unknown Percent KP in Unknown Average Percent KP: Standard deviation: Show calculations of percent KP and standard deviation.