ACID - BASE TITRATION CURVES

Transcription

1 Experiment ACID - BASE TITRATION CURVES The CCLI Inititive Computers in Chemistry Lbortory Instruction The objectives of this experiment re to... LEARNING OBJECTIVES understnd the titrtion curves for the following solutions wek cid: cetic cid, CH 3 COOH. strong cid: hydrochloric cid, HCl. n cidic commercil clenser. bsic commercil clenser. use the titrtion curves to clculte the percent of the ctive ingredients in the commercil clensers. determine the K of wek cid. BACKGROUND A plot of the ph of solution ginst the volume of "titrnt" dded is clled titrtion curve. The ph cn be mesured directly with ph meter while titrnt is dded from buret. For the cids used in this experiment the titrnt will lwys be 0.1 M solution of the strong bse NOH. From the form of the titrtion curve it cn be determined whether the solution consists of strong or wek cid. Furthermore, if it is wek cid, the equilibrium constnt for its dissocition cn be clculted. Strong cids For strong cid HA, the equilibrium constnt K for the process HA H2O H3O A (1) is so lrge tht it is completely dissocited into H3O nd A t usul concentrtions, nd hence the H3O concentrtion simply equls the cid concentrtion which remins unrected by the NOH. This is true until -6 the titrtion hs reduced the concentrtion of HA to less thn 10 M. At this point the dissocition of wter ccording to the eqution H2O H2O H3O OH (2) -14 K w = [H3O ] [OH ] = 10 (3) 1

2 begins to govern the H3O concentrtion. At the exct equivlence point (where the moles of bse dded equl the initil moles of cid present), the H3O concentrtion comes entirely from this source nd is -7 therefore equl to 10 nd the ph is seven. Wek cids -2-7 For wek cid the equilibrium constnt for rection (1) is smll (between 10 nd 10 ), so tht the concentrtion of [H O ] nd hence the ph is governed by K. 3 K = [H3O ] [A ] / [HA] (4) The titrtion curve tht you obtin should be considerbly different from tht obtined when the sme concentrtion of strong cid is titrted. Obviously the [H3O ] concentrtion will be lower (nd the ph therefore higher) throughout the titrtion becuse not ll of the cid is dissocited. You should lso find tht the shpe of the curve is somewht different. The reson for this will become cler if the following three points long the titrtion curve re considered in some detil. () Zero titrnt. Since the dominnt source of [H3O ] is the cid dissocition described by eqution (1), [H3O ] = [A ] from the stoichiometry of the eqution. Therefore eqution (4) reduces to 2 K [HA] = [H3O ] (5) If we further ssume tht [HA] is given by the cid nominl molrity of the solution, then the [H3O ] concentrtion nd hence the ph cn be clculted if K is known. K is most conveniently obtined from the following point on the titrtion curve. (b) Hlfwy to the equivlence point. At this point hlf of the cid hs been titrted, nd hence [A ] = [HA]. Therefore, eqution (4) reduces to the simple expression K = [H3O] or pk = ph (6) This is rther remrkble point on titrtion curves since the ph is determined by the vlue of pk ( log K ) nd is independent of the initil cid concentrtion nd ny subsequent dilution. To determine the pk of wek cid, we need to locte this hlf-equivlent point ccurtely, nd this cn be done by first finding the following point on the titrtion curve. (c) The equivlence point. When the monoprotic cid hs been completely titrted by ddition of n equl number of moles of bse, we hve reched the equivlence point. On the titrtion curve this will pper s the point of mximum slope. If the dt re obtined by dding equl mounts of titrnt throughout, the equivlence point will probbly be very poorly locted becuse there will be sudden jump between two points. We therefore ttempt to dd smll mounts of titrnt in the vicinity of the equivlence point nd thus ccurtely determine the point of inflection. Once this hs been determined, the hlf wy point cn be clculted nd pk determined s discussed in prt (b). Let us now consider the clcultion of the exct ph t the equivlence point. The solution contins n mount of A equl to the originl cid present, but in volume which is lrger by n mount depending on how much titrnt ws dded. The concentrtion of A is then esily clculted by considering the dilution fctor. To clculte the ph t this point, we consider the following equilibrium in which A is sid to be hydrolyzed: A H2O <==> HA OH (7) 2

3 It is not difficult to show tht K h for rection (7) is relted to K nd K wby multiplying both numertor nd denomintor of the Kh eqution by [H ], nd rerrnge, s follows: K b = [HA][OH ][H ] = [HA] X [H ][OH ] = K w (8) [A ][H ] [A ][H ] K From the stoichiometry of eqution (7), [HA] = [OH ] nd eqution (8) cn be written 2 K b = [OH ] (9) [A ] -14 When K is obtined from prt (b), K bcn be clculted with eqution (8) (K w= 10 ). Then [OH ] nd poh cn be clculted using eqution (9). The ph t the equivlence point is then given by (14 poh). Acid-Bse titrtion of commercil clensers Potentiometric titrtion cn be pplied to the determintion of cidic nd bsic compounds in household clensers. The clensers generlly contin one or two cids or bses nd their concentrtions cn be determined from the one or two equivlence points on the titrtion curves. The cidic household clensers usully contin hydrochloric cid, phosphoric cid, sodium bisulfte, or hydroxycetic cid, which remove lkline scle deposits nd stins. The bsic clensers usully contin mmonium hydroxide, sodium hydroxide, sodium hypochlorite or sodium crbonte. The weker bses cut grese while the stronger bses nd oxidizing gents dissolve niml mtter such s hir, grese, nd foodstuffs. Lysol (Reckitt Benckiser (UK) Limited) is n cidic clenser, contining hydrochloric cid s n ctive ingredient. The percentge content of HCl cn be clculted from the titrtion dt. Liquid Plumr (The Clorox Compny) is two component bsic clenser contining sodium hydroxide nd sodium hypochlorite (NOCl). Sodium hypochlorite is n oxidizing gent s well s wek bse nd, together with sodium hydroxide, is used s drin clener. The titrtion of bse with strong cid is just the reverse of the titrtion of n cid with strong bse. The student will find it esy to grsp wht hppens during the titrtion of mixture of strong nd wek bses if he/she considers the following two points long the curve. () (b) Zero titrnt. The strong bse will be completely dissocited, giving n equivlent OH concentrtion. The wek bse will be only prtilly dissocited nd mke much smller contribution to the totl [OH ] present. The presence of OH from the strong bse will, in fct, suppress the dissocition of the wek bse, since the equilibrium B H O => BH OH (10) 2 will be shifted to the left. Thus, the first equivlence point gives the volume of the titrnt recting with the strong bse. When strong bse hs been titrted. At this point we simply hve solution of the wek bse tht hs been diluted. The difference between the first nd the second equivlence points gives the volume of strong cid required to rect with the wek bse. SAFETY PRECAUTIONS Sfety goggles must be worn in the lb t ll times. Any skin contcted by chemicls should be wshed immeditely. 3

4 BEFORE PERFORMING THIS EXPERIMENT you will need one of the following MicroLAB progrms. ph.vs.kbd experiment: Sends the totl volume of titrnt (the solution in the buret) dded to the nlyte (the solution in the beker) to the Grph view X Axis, the Digitl Disply view, nd Spredsheet column, nd (2) sends the ph vlue resulting from the ltest ddition of titrnt to the Y Axis of the Grph view, the Digitl Disply view, nd second, corresponding Spredsheet column. This progrm is suitble for mnul titrtion in which you will mnully dd smll volumes of titrnt to your nlyte solution nd input ech buret reding into the computer. ph.vs.time experiment: progrm tht sends time nd ph to the spredsheet file every ½ second. After the titrtion is complete, you cn use the Spredsheet to convert the time into volume. To do this, however, you need constnt flow buret. This cn be ccomplished by first djusting the flow of your buret to deliver 1of solution in bout 8 seconds, then swinging the buret into the titrtion beker. During the titrtion, keep dding the NOH to the buret so tht the buret level is in the sme volume rnge s when you mesured the flow rte. To convert the time into the volume of NOH dded, the time dt in the Spredsheet should be divided by the flow rte (in sec/ml). Although this technique tkes little bit more finesse, the results re normlly very good nd require much less time thn the "mnul" method described in the previous prgrph. You will decide which method of titrting is best for you. Interpreting the ph titrtion curve An exmple of ph titrtion curve obtined using the lb interfce is shown in Figure 1. A strong bse (NOH) ws dded to strong cid (HCl). The equivlence point is the point t which the ph versus volume curve is the steepest. The curve shown in Figure 1 ws obtined with s drop counter titrtion, obtining 356 dt points in less thn five minutes, then the drops converted to volume. If you do mnul titrtion, you will hve fr fewer dt points. The Spredsheet mkes it esy to find the equivlence point of titrtion curve. Since the equivlence point is the steepest point of the curve, tking the derivtive of the curve will mke the equivlence point redily pprent. Click on Plot Derivtive function in the Anlysis option nd select ph vs. Volume. Figure 2 shows n exmple of wht the derivtive looks like when superimposed on the titrtion curve from Figure 1. Preprtion of NOH for the titrtion of the cids EXPERIMENTAL PROCEDURE The sodium hydroxide solution supplied is bout 2.5 M. Using grduted cylinder, mesure 10 ml of 2.5 M NOH into 250volumetric flsk. Mke up to the mrk with de-ionized wter nd mix well. Rinse nd fill the buret with this dilute solution of NOH. Figure 1. Titrtion curve for strong bse versus strong cid. 4

5 Clibrting the ph probe 1. The ph probe is delicte, complicted sensor comprised of n ion-sensitive glss tip, metl electrodes, nd chemicl solutions seled in glss or plstic tube. Be creful not to drop the ph electrode or hit it with the stirring rod. Gently rinse the glss tip with de-ionized wter before moving the electrode from one solution to nother. Once you re finished using the electrode, lwys return it to the storge beker. 2. Click on ph in the Vribles View nd reclibrte your probe with the ph 4, 7 nd 10 buffers supplied. Be sure to rinse the ph probe with distilled wter fter ech buffer nd before you plce it in your nlyte solution. Between titrtions, the probe should be stored in the ph 7 buffer, then rinsed well with distilled wter before inserting into your titrtion beker. Titrtion of the cids 1. All the cids supplied will be bout 0.1 M. Using pipet, mesure 25.00of HCl into 250beker. Add 100of H2O nd five drops phenolphthlein. Be sure to record the concentrtion of HCl, since it will be used to clculte the concentrtion of your NOH solution. Figure 2. ph vs Volume with derivtive superimposed. Note: Do either step 2 or 2b. Do NOT do both! 2. () Using ph titrtion progrm designed for "mnul" titrtion, (ph.vs.kbd experiment), titrte the smple with the NOH solution. Remember to mesure the ph of the solution with respect to the volume of NOH dded. In the beginning, dd NOH in 1increments. As the ph begins to rise significntly, use 0.5increments or less. As you pproch the equivlence point (indicted by more persistent pink color from the phenolphthlein), the smller the increment, the better (~0.05 ml, or one drop). Once the end point hs been reched with the indictor, note the volume of bse dded nd continue the titrtion t the one-drop volume increment for lest n dditionl five drops. Then grdully increse the volume increments (up to mximum of 1.0 ml) until you hve dded pproximtely 10 ml of NOH beyond the end point. For good titrtion curve, you will wnt t lest 30 dt points. A timed titrtion is likely to give you 10 times s mny points! 2b. Using ph titrtion progrm designed for n "utomtic" or "timed" titrtion, (ph.vs.time experiment). Before beginning the titrtion, mke sure you hve djusted the flow rte of the buret to two to four seconds per drop, for exmple between the 2 nd 3 ml mrks, with the flow dropping into wste beker. 2c. Then, with the buret full nd still flowing, plce your beker under the buret nd 5

6 simultneously strt your progrm to begin tking dt. It is now criticl to keep the liquid level of the buret t the sme level s tht which you mesured the flow rte, using the titrnt in the plstic wsh bottle. Depending upon the exct flow rte, the titrtion will tke between four nd seven minutes. Wtch for the color end point nd tke dt for t lest two minutes beyond this point. 3. Using grduted cylinder, mesure 25of 0.1 M cetic cid (CH3COOH) into 250 ml beker. Add 50.0of wter nd 5 drops phenolphthlein. 4. Titrte the cetic cid with strong bse s directed bove for HCL. Titrtion of Lysol Weigh 150beker on the nlyticl blnce. Dispense 1of Lysol into the beker. Cution: hndle Lysol with cre - strongly cidic. Reweigh the beker on nlyticl blnce. Add 60of wter. Repet step 2 bove, the NOH supplied is M Titrtion of Liquid Plumr Weigh 150beker on nlyticl blnce. Dispense 2of Liquid Plumr into the beker. Cution: hndle Liquid Plumr with cre - strongly bsic. Reweigh the beker on nlyticl blnce. Add 60of wter. The HCl solution supplied is M. Rinse nd fill the buret with this solution. Repet step 2 bove. Continue the titrtion to ph ner 2. Your dt should show two inflections. DATA ANALYSIS 1. Using the Spredsheet, plot the ph versus volume dt. Use the Anlysis option to find the derivtive of the curve nd plot the ph on Y1-xis, the volume of NOH dded on the X-xis, nd the derivtive of the ph-volume curve on Y2-xis. Remember, if you did "timed" titrtion, you will hve to use the Anlysis option to convert time into volume using your flow rte. Obtin printout of your plot. DO NOT PRINT DATA TABLES, they re too long. 2. Using your titrtion curves nd derivtive plots, determine the equivlence points for ll the titrtions. Clculte the exct molrity of the NOH solution using the HCl titrtion dt. 3. Determine the hlf-equivlence point for the titrtion of cetic cid nd use it to determine its -5 K vlue. Compre the K with the ccepted vlue for cetic cid, 1.8 x For the Lysol, determine the equivlence point nd clculte the percentge content of HCl. Use the exct concentrtion of the NOH solution clculted in (2) bove. 5. For the Liquid Plumr, determine where the two equivlence points re nd then record the ph nd volume of the HCl t both the first nd second equivlence points. Clculte the percentge content of both the NOH nd NOCl in Liquid Plumr. 6