ACID – BASE TITRATIONS Determination of Purity of Potassium Acid Phthalate Experiment No. 2
I.
Objectives 1.) To perform an acid-base titration 2.) To prepare prepare the necessary necessary chemicals chemicals needed needed for the experiment experiment 3.) To determine the relative concentrations of the hydrochloric acid and sodium hydroxide solutions 4.) To standardize the sodium sodium hydroxide solution with with potassium potassium acid phthalate phthalate 5.) To determine determine the purity purity of the potassiu potassium m acid phthalate phthalate
II. Resul Results ts and Discus Discussio sion n Table 1. Measurement of the Relative Strengths of Acid and Base Trials Final Reading HCl (mL) Initial Reading HCl (mL) Volume HCl used (mL)
I 18.00 0.00 18.00
II 36.00 18.00 18.00
I II 18.00 0.00 18.00
Final Reading NaOH (mL) Initial Reading NaOH (mL) Volume NaOH used (mL)
14.50 0.00 14.50
29.10 14.50 14.60
43.60 29.10 14.50
Acid to Base Ratio Average Acid/Base Ratio
1.24137931
1.232876712 1.238545111
1.24137931
Base to Acid Ratio Average Base/Acid Ratio
0.805555555
0.811111111 0.807407407
0.805555555
Table 1 shows the amounts of HCl used and the corresponding amounts of NaOH needed to achieve the end point; in this case, the slight pink coloration in the solution. The acid to base and base to acid ra tios were also computed. Table 2. Standardization of Sodium Hydroxide Primary Standard Used: Potassium Acid Phthalate (KHP) Formula Mass of Primary Standard: 204.2212 g/mol % Purity of Primary Standard: 99.8% Trials Weight of Standard KHP (g) Corrected Weight of KHP (g) Final Reading NaOH (mL)
I 0.3150 0.314370
II 0.3318 0.3311364
III 0.3325 0.331835
16.80
34.60
40.2
Initial Reading NaOH (mL) Volume NaOH (mL) Normality of NaOH Average Normality of NaOH
0.00 16.80 0.091628587
16.8 17.80 0.091093229 0.092581351
23.1 17.1 0.095022238
To determine determine the unknown unknown concentratio concentration n of a known known reactant, reactant, titration titration is used. It is a comm common on meth method od in chem chemic ical al anal analys ysis is,, wher wheree in a reag reagen ent, t, call called ed the the titr titran ant, t, of know known n concentration and volume is used to react with a solution of the analyte whose concentration is unknown. The titration setup involves the titrant placed in a calibrated burette and the analyte placed in a flask just below the burette tip. The titrant is then ran until the experimental end point is reached. The end point signals the end of titration. The physical indication that the end point has been reached is the change of the color of the solution to a slightly pink hue that persists for about a minute or more. The end point is achieved through the help of an indicator, or that which will physically indicate the end point. In this case, phenolphthalein indicator was used. This end point is also more or less the same with the equivalence point, where the moles of the titrant is equal to the moles of the analyte. Since Since standa standard rd soluti solutions ons play play an import important ant role role in titime titimetry try,, standa standardi rdizat zation ion was per perfo forme rmed d in this this part part of the the expe experi rime ment nt to dete determ rmin inee the the conc concen entr trat atio ion n of NaOH NaOH.. In standardization, the concentration of a volumetric solution is determined by titrating it against a carefully measured quantity of a primary or a secondary standard or an exactly known volume of another solution (Skoog, 2004). In this experiment, the titrant to be standardized was used to titrate a weighed amount of a primary standard. In this case, the volumetric solution was NaOH while the primary standard that was used was KHP. The average normality that was calculated from the data shows that it is approximately near the 0.1 N NaOH that we had prepared. There could be minimal errors in this experiment, particula particularly rly because these errors are indetermin indeterminate. ate. For example, example, there is a slight slight difference difference between the change in indicator color and the actual equivalence point of the titration. Our inabi inabilit lity y to obser observe ve and detect detect little little diffe differen rences ces in the physic physical al change changess also also affect affectss our our determination of the end point. Table 3. Analysis of KHP sample Trials Weight of KHP sample (g) Final Reading NaOH (mL) Initial Reading NaOH (mL) Volume NaOH (mL)
I 0.3347 8.30 0.00 8.30
II 0.3521 17.7 8.30 9.40
III 0.3941 29.40 17.70 11.70
mg KHP in sample % KHP in sample Average % KHP in sample
156.928719 46.88638154
177.726501 50.47614343 51.1645507
221.212772 56.13112713
This part of the experiment aims to determine the purity of the KHP and the actual amount of KHP that there is in the samples. KHP or potassium acid phthalate is the primary standard that was used and to which the NaOH was compared against. The results showed that the average percent purity of KHP in the three samples was only approximately equal to 51.16%. This result is much less than that of the stated percent purity of the primary standard which is
99.8%. This large difference may be due to a couple of systematic, random and gross errors, all of which affect the results.
III. III. Conclu Conclusio sion n It has already been said that chemical analysis is extremely important and useful in many ways. Titration allows us to analyze the samples which are actually representatives of the bulk quantity. This particular technique allows us to compare the samples on standard materials of accurately known composition (Skoog, 2004). Through titration, we have standardized NaOH and determined the actual amount of KHP in the given samples. Titration is a very important procedure where we can verify the quality and quantity of the components of the samples. Errors and large differences may occur, but these errors are always present and usually unavoidable. unavoidable.
IV. Questions Questions and and Problems Problems 1.) In the preparation preparation of 0.1N 0.1N NaOH, why why is it necessary to use carbonate-free base? It is necessary that the NaOH solution is carbonate-free so that the carbonate ions will not react with HCl or with the KHP. 2.) What will be the the resulting concentration concentration of sodium sodium hydroxide when the tip of the burette is not filled completely? Is it higher, lower or no change? Explain your answer. If the tip of the burette is not filled completely (ie. has bubbles), then the volume you have measured is larger than the actual volume that has been ran in the analyte because the bubbles take up a space which has been included in your reading. So if you remove the bubbles, you will see that the volume decreases. In effect, if you have a volume volume that that is actual actually ly larger larger than the actual, actual, you will get wrong wrong answer answerss in the molarities of NaOH and amount of KHP present in the samples. 3.) Write the net ionic ionic reaction of titrant titrant and the secondary standard standard used in the experiment. NaOH + HCl NaCl + H2O 4.) Why is potassium potassium acid phthalate used used as a primary standard? What are its its properties which are considered to be important? KHP was used as primary standard because it has the following properties: high purity purity where established established methods methods for confirming confirming its purity purity is available available;; atmospher atmospheric ic stability; absence of hydrate water so that the composition of the solid does not change with variations in humidity; modest cost; reasonable solubility in the titration medium; reasonably large molar mass so that the relative error associated with weighing the standard is minimized. 5.) Calculate the concentration (molarity) of NaOH given that 0.5123 g of KHP requires 21.06 mL of NaOH. 0.5123 g KHP x 1 mol KHP x 1mol NaOH NaOH = 2.509 x 10 -3 mol NaOH 204.2212 g 1 mol KHP
2.509 x 10 -3 mol NaOH = 0.119 M NaOH 0.02106 L NaOH V. Refe Refere ren nces ces
[1] Skoog DA, West DM, Holler, FJ, Crouch SR.2004.Fundamentals of Analytical Chemistry.Singapore.Brooks/Cole.pp91-170. [2] Kelley MT.1959.Analytical Chemistry.New York.Ergmon Press.pp14-36. [3] http://www.wikipedia.com/ http://www.wikipedia.com/
VI. Sample Sample Calcul Calculation ationss 1.) Acid Acid to to Base Base Ratio Ratio = vol acid = 18.00 mL = 1.24137931 vol base 14.50 mL 2.) Averag Averagee Acid Acid to Base Base Ratio Ratio = 1.24137931 + 1.232876712 + 1.24137931 = 1.238545111 3 3.) Base Base to to Acid Acid Ratio Ratio = vol base = 14.50 mL = 0.805555555 vol acid 18.0 mL 4.) Averag Averagee Base Base to Acid Acid Ratio Ratio = 0.805555555 + 0.811111111 + 0.805555555 = 0.807407407 3 5.) Correc Corrected ted Weig Weight ht of KHP KHP = weight KHP x % purity = 0.3158 x 0.998 = 0.31437 g 6.) Normal Normality ity of NaOH NaOH = corrected weight KHP x n = 0.31437 g x 1 . = 0.091628587 N NaOH MW KHP x Vol NaOH 204.22 g/mol x 0.0168 L 7.) 7.) mg KHP KHP in in samp sample le = average [NaOH] x Vol NaOH used x MW KHP = 0.092581351 mol x 0.0083 0.0083 L x 204,221.2 mg . = 156.9287192 156.9287192 mg mg KHP L mol 8.) 8.) % KHP KHP in in sam sampl plee = weight above . x 100 = 156.9287192 mg KHP x 100 = 46.8863816 Weight measured KHP 334. 7 mg
