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EXPERIMENT 6Colorimetric Determination of pHDEL MUNDOLARNSEEINTRODUCTIONColorimetry any technique by which an unknown color is evaluated in terms of standard colors the technique may be visual, photoelectric, or indirect by means of spectrophotometryhttp://www.answers.com/topic/colorimetry#ixzz1MK1Yv3SlpH ndicators also called acIdbase IndIcators pH indicators are usually weak acids or weak bases that change their color depending on their dissociation (protonation) state pH indicators can be used to check pH of the solutionhttp://www.ph-meter.info/pH-measurements-indicatorsIndicator Lower pH coIorpH Range(transition intervaI)Higher pH coIorThymoI bIue Red 1.2 - 2.8 YeIIowBromophenoI bIue YeIIow 3.0 - 4.6 PurpIeChIorophenoI red YeIIow 4.8 - 6.4 VioIetBromothymoI bIue YeIIow 6.0 - 7.6 BIuePhenoI red YeIIow 6.8 - 8.4 Reduffer Solutions A buffer solution is one in which the pH of the solution is "resistant" to small additions of either a strong acid or strong base. uffers consist of a weak acid and its conjugate base or vice versa, in relatively equal and "large" quantities.http://www.chem.purdue.edu/gchelp/howtosolveit/equilibrium/buffers.htmMclvaineuffer System A citrate/phosphate buffer system that can be volumetrically set for pH in a wide range (2.2 to 8)http://www.biochemlab.cn/shiji/peizhi/20993.htmlHenderson-Hasselbalch Equation An equation expressing the pH of a buffer solution as a function of the concentration of the weak acid or base and the salt components of the buffer.http://medical-dictionary.thefreedictionary.com/Henderson-Hasselbalch+equationColorimetric Analysis Uses the variation as a means of determining the pH since the intensity of the color of a solution changes with its concentration or pHhttp://www.inc.bme.hu/en/subjects/genchem/phdet2.pdfhttp://www.inc.bme.hu/en/subjects/genchem/phdet2.pdf y comparing the intensity of the colour of a solution of unknown concentration (or pH) with the intensities of solutions of known concentrations (or pH), the concentration of an unknown solution may be determinedEXPERIMENTObjective To be able to determine the pH of an unknown solution colorimetrically To be able to calculate the ionization constant of a weak acid!ART A:!reparation of uffer SolutionsA set of Mclvaine buffers were accurately prepared in test tubes of uniform sizes labeled according to their respective pH levels.Five drops of the appropriate indicators to use for each pH level were added to each of the buffer solutions.Indicator: THYMOL BLUE (1.2 2.8)pH 0.2M Na2H!O4 (mL) 0.1M Citric Acid (mL)2.2 0.20 9.802.4 0.62 9.382.6 1.06 8.912.8 1.58 8.42Indicator: BROMOPHENOL BLUE(3.0 4.6)pH 0.2M Na2H!O4 (mL) 0.1M Citric Acid (mL)3.0 2.05 7.953.2 2.47 7.533.4 2.85 7.153.6 3.22 6.783.8 3.55 6.454.0 3.25 6.154.2 4.14 5.864.4 4.41 5.594.6 4.67 5.33Indicator: CHLOROPHENOL RED (4.8 6.4)pH 0.2M Na2H!O4 (mL) 0.1M Citric Acid (mL)4.8 4.93 5.075.0 5.15 4.855.2 5.20 4.805.4 5.58 4.425.6 5.80 4.205.8 6.05 3.956.0 6.31 3.696.2 6.61 3.396.4 6.92 3.08Indicator: BROMOTHYMOL BLUE (6.0 7.6)pH 0.2M Na2H!O4 (mL) 0.1M Citric Acid (mL)6.0 6.31 3.696.2 6.61 3.396.4 6.92 3.086.6 7.34 2.666.8 7.72 2.287.0 8.24 1.767.2 8.69 1.317.4 9.08 0.927.6 9.37 0.63Indicator: PHENOL RED (6.8 8.0)pH 0.2M Na2H!O4 (mL) 0.1M Citric Acid (mL)6.8 7.72 2.287.0 8.24 1.767.2 8.69 1.317.4 9.08 0.927.6 9.37 0.637.8 9.57 0.438.0 9.72 0.28!ART :Colorimetric Determination of pHThe pH of the following solutions were approximated using pH papers, then applied them with the appropriate indicator/s.The pH of each solutions were then confirmed by comparing their colors to standards (from part A) applied with the same indicator.Results$oIution Observed pHA 0.01M HOAc 5B 1mL 0.1M HOAc + 1mL 0.1M NaOAc + 8mL H204.8C 1mL 0.1M HOAc + 0.1mL 0.1M NaOAc + 8.9mL H2O3.2D 0.1mL 0.1M HOAc + 1mL 0.1M NaOAc + 8.9mL H2O6.8Discussion n the preparation of the buffer solutions in part A, it is important to use the appropriate indicators for each buffer solutions because in colorimetric determination of pH, the indicators in buffered solutions are most effective when it is within the specific pH ranges mentioned in the table. t is at these pH ranges that the indicators show a significant change in color. For test tubes , C and D, using the Henderson-Hasselbach equation, it can be inferred that as the ratio of the molarity OAc-(from NaOAc) to that of HOAc increases, the pH also increases thus making the solution less acidic. pH = pKa + log[OAc-][HOAc] ecause it has the highest ratio of OAc-to HOAc, the solution in test tube D is expected to be the least acidic while the solution in test tube C as the most acidic. Common ion effect can also account for these. ecause of the presence of the common ion, OAc-, there will be a suppression in the ionization of the acid thus decreasing hydrogen ion concentration and increasing the pH. Therefore, our observations are correct!$oIution Observed pHA 0.01M HOAc 5B 1mL 0.1M HOAc + 1mL 0.1M NaOAc + 8mL H204.8C 1mL 0.1M HOAc + 0.1mL 0.1M NaOAc+ 8.9mL H2O3.2D 0.1mL 0.1M HOAc + 1mL 0.1M NaOAc+ 8.9mL H2O6.8UDE QUESTONS AND ANSWERS1.) Calculate the ionization constant of acetic acid using colorimetric data.pH of 0.01 M HOAc 5pH = - log [H3O+] 5 = - log [H3O+][H3O+]=10-5[H3O+] of 0.01 M HOAc

[H3O+] of 0.01 M HOAc

To get Ka:[H3O+] of 0.01 M HOAc

To get Ka:HOAc H++ OAc-nitial 0.01 0 0Change - 1.00x10-5+ 1.00x10-5+ 1.00x10-5Equilibrium 9.99x10-31.00x10-51.00x10-5[H3O+] of 0.01 M HOAc

To get Ka:HOAc H++ OAc-nitial 0.01 0 0Change - 1.00x10-5+ 1.00x10-5+ 1.00x10-5Equilibrium 9.99x10-31.00x10-51.00x10-5Ka = [H+][OAc-] = (1.00x10-5)( 1.00x10-5)= 1.00x10-8[HOAc] 9.99x10-32.) Calculate the pH of three mixtures of HOAc and NaOAc(solutions A,and C) using the Henderson-Hasselbalch equation and compare with the observed pH. (Use the value of ionization constant of HOAc at 25C.) Support your answers with computations. 1 mL 0.1 M HOAc + 1 mL 0.1 M NaOAc+ 8 mL H2OC 1 mL 0.1 M HOAc + 0.1 mL 0.1 M NaOAc+ 8.9 mL H2OD 0.1 mL 0.1 M HOAc + 1 mL 0.1 M NaOAc+ 8.9 mL H2OKa of HOAc at 25C = 1.8x10-5pKa= -log Ka = - log (1.8x10-5) = 4.74pH = pKa + log [conjugate base][acid]The acid is HOAc and its conjugate base is OAc-.For solution : 1 mL 0.1 M HOAc + 1 mL 0.1 M NaOAc+ 8 mL H2OMA1 VA1= MA2 VA2(0.1M) (1mL) = [HOAc] (10mL)[HOAc] = 0.01M = 0.01M[NaOAc] = [OAc-]M1 V1= M2 V2(0.1M) (1mL) = [OAc-](10mL)[OAc-] = 0.01M = 0.01MpH = pKa + log [conjugate base][acid]= 4.74 + log0.01 M0.01 M= 4.74 + 0pH= 4.74 4.74For solution : 1 mL 0.1 M HOAc + 1 mL 0.1 M NaOAc+ 8 mL H2OFor solution C: 1 mL 0.1 M HOAc + 0.1 mL 0.1 M NaOAc + 8.9 mL H2OMA1 VA1= MA2 VA2(0.1M) (1mL) = [HOAc] (10mL)[HOAc] = 0.01M = 0.01M[NaOAc] = [OAc-]M1 V1= M2 V2(0.1M) (0.1mL) = [OAc-](10mL)[OAc-] = 0.001M = 0.001MpH = pKa + log [conjugate base][acid]= 4.74 + log0.001 M0.01 M= 4.74 + (-1)pH= 3.74 3.74For solution C: 1 mL 0.1 M HOAc + 0.1 mL 0.1 M NaOAc + 8.9 mL H2OFor solution D: 0.1 mL 0.1 M HOAc + 1 mL 0.1 M NaOAc + 8.9 mL H2OMA1 VA1= MA2 VA2(0.1M) (0.1mL) = [HOAc] (10mL)[HOAc] = 0.001M = 0.001M[NaOAc] = [OAc-]M1 V1= M2 V2(0.1M) (1mL) = [OAc-](10mL)[OAc-] = 0.01M = 0.01MpH = pKa + log [conjugate base][acid]= 4.74 + log 0.01 M0.001 M= 4.74 + 1pH= 5.74 5.74For solution D: 0.1 mL 0.1 M HOAc + 1 mL 0.1 M NaOAc + 8.9 mL H2OSolutionpH observed calculated0.01 M HOAc 5 ----1 mL 0.1 M HOAc + 1 mL 0.1 M NaOAc + 8 mL H2O4.8 4.741 mL 0.1 M HOAc + 0.1 mL NaOAc + 8.9 mL H2O3.2 3.740.1 mL 0.1 M HOAc + 1 mL 0.1 M NaOAc + 8.9 mL H2O 6.8 5.74A

CDConclusion The pH of a solution can be approximated with the use of colorimetry and pH indicators. ut it is also important to have to have proper knowledge on which indicator to be used on certain pH ranges and their color transitions for a successful colorimetric analysis. t is strongly advised to have accurate measurements for the preparation of buffer solutions to have an efficient standard and also for the solutions that will be used for colorimetric analysis.RecommendationReferences Lemay, H., rown, T., ursten, ., & urdge, J. (2004). Chemistry: The Central Science. New Jersey: !earson Education South Asia !te Ltd. http://www.answers.com/topic/colorimetry#ixzz1MK1Yv3Sl http://www.ph-meter.info/pH-measurements-indicators http://www.chem.purdue.edu/gchelp/howtosolveit/equilibrium/buffers.htm http://www.biochemlab.cn/shiji/peizhi/20993.html http://medical-dictionary.thefreedictionary.com/Henderson-Hasselbalch+equation http://www.inc.bme.hu/en/subjects/genchem/phdet2.pdf