Chem(bio) Week 2: Determining the Equilibrium Constant of Bromothymol Blue. Keywords: Equilibrium Constant, ph, indicator, spectroscopy

Transcription

1 Ojectives: Keywords: Equilirium Constant, ph, indicator, spectroscopy Prepare all solutions for measurement of the equilirium constant for romothymol Make a series of spectroscopic measurements on the solutions and from this data, determine the equilirium constant for romothymol. Bromothymol is a chemical indicator used to detect weak acids and ases. It works as an indicator y displaying a change in color from the protonated form (HBB) to the non-protonated form (BB - ). This is illustrated elow in the ph dependent equilirium etween the two forms. HO OH O OH Br O Br Br + H 2 O Br + H O + S O O SO - Figure One: Equilirium of romothymol ( and forms) Oviously, this equilirium is ph dependent and under acidic conditions, the romothymol will e protonated and as a result, the solution will e. Under asic conditions, the deprotonated form results in a solution. But these are the two extreme cases and it is found that etween ph 6 to 8, the solution appears, a color resulting from the presence of oth protonated and deprotonated forms of romothymol. Towards ph 6, the solution is a more ish while towards ph 8, the solution is more of a luish. Hence the color of the solution can e used as a visual guide to the relative amounts of protonated and deprotonated forms of romothymol. The asorance spectra of each of these are thus shown elow: Bromothymol Blue- 1

2 Figure Two: sorance Spectra of Bromothymol Blue ( and forms) The form, BB -, has an asorance maximum at aout 616. The form, HBB, has its maximum asorance at 42. In this experiment, we will measure the asorance of the form at 45, where the asorance is still strong and the asorance of the BB - is minimal (as can e seen in the spectra in Figure Two at 42, BB - also asors). In this experiment, we will consider the equilirium etween the two forms HBB () H 2 O H O + + BB - () (1) The equilirium expression, K c, for the HBB/BB - equilirium (Eq. 1) is: [ H O ][ BB [ HBB ] ] K c (2) The value of K c should e independent of all factors except a change in temperature. t high ph, the concentration of the form, [BB - ], is large and [HBB] is small, and at low ph, [HBB] is large and [BB - ] is small. In this experiment, you will e working with a solution that is in color, so that neither [HBB] nor [BB - ] is much larger than the other. Bromothymol Blue- 2

3 Both the and forms of romothymol have Beer s law expressions that relate asorance () to concentration ([ ], in molarity) at their respective wavelengths: For the form, HBB: [ ] HBB and for the form, BB - : [ ] BB () (4) where ε is the molar asorptivity (L mol -1 cm -1 ) and is the pathlength in cm. Solving Equations and 4 for [HBB] and [BB - ], respectively, gives: 45 [ HBB] (5) 616 [ BB ] (6) These two equalities can e sustituted into the K c expression (Eq. 2) and the s cancel: K c [ H O ] H O [ ] (7) Note that it is not necessary to know the concentrations of [HBB] or [BB - ] in the solution, only the ratio 616 / 45. There are five values from Eq. 7 that you must otain to determine K c for romothymol. These are summarized in Tale 1. You will determine three of these values for a solution that is in color, meaning that oth HBB and BB - are present in reasonale quantities (not almost zero as would e the case in either the or solutions at high and low ph values, respectively). Bromothymol Blue-

4 Tale 1: Quantities measured for the determination of K c Quantity [H O ] How we measure it: Use a ph meter to determine the ph of the solution Determine the asorance of the solution at 616 Discussed elow 45 Determine the asorance of the solution at Discussed elow The K c expression given aove (Eq. 7) is now re-written with laels indicating that you will determine asorance values at two wavelengths for the form of your romothymol solution: K c [ H O ] (8) The value for 45 will e determined when the solution is completely (low ph; ~100% HBB and negligile BB - ) and the value for 616 will e determined when the solution is completely (high ph; ~100% BB - ). This is ecause we need to relate the ε values to concentration via the Beer s law equation ( = ε x x M) and unless the equilirium is shifted almost 100% one way or the other, we will not know the concentration exactly. The experiment is designed so that we will prepare three solutions, each with the identical concentration of romothymol. We will measure out equivalent amounts of romothymol solution into three eakers. Into one eaker you will add an exact volume of an acid, HCl(aq); into another, you will add the same volume of a ase, NaOH(aq); into the third solution, you will add the same volume of a uffer solution with a ph of approximately 6.5. The first solution will e, the second solution will e and the third solution will e. ll three solutions will therefore have the same total concentration of romothymol [either as HBB ( solution), BB - ( solution), or a comination of HBB and BB - ( solution)]. Bromothymol Blue- 4

5 The asorance at 45 of the solution is due entirely to the form (HBB); while the asorance at 616 is due entirely to the form (BB - ). We can write a Beer s law expression for the asorance of each solution at each wavelength: 45 [ HBB ( )] (9) 616 [ BB ( )] (10) Since we have designed the experiment such that all HBB and BB - originates from the same source, the concentration of romothymol in the solution equals the concentration of romothymol in the solution: [ ( )] [ BB ( )] HBB (11) which, upon sustitution of Eq. 9 and 10 into Eq. 11, gives: (12) The values cancel and Eq. 12 is rearranged: (1) The laels and on and denote that these are the asorance values of the and solutions, at their respective wavelengths. The aove ratio of ε values can e sustituted into the K c expression (Eq. 8): K c [ H O ] [ H O ] (14) From Eq. 14, you should see how we can determine K c from only asorance values and the [H O + ], otained from a ph measurement on the solution. Bromothymol Blue- 5

6 Experimental Prepare a uffer solution y dissolving approximately 0.4 g of sodium phosphate monoasic, NaH 2 PO 4 and 0.8 g of sodium phosphate diasic, Na 2 HPO 4 in approximately 50 ml water. Stir to completely dissolve oth salts. (Note: a uffer is a solution which can maintain a constant ph unless significant amounts of acid or ase are added) dd 20 drops of romothymol indicator to the uffer solution and stir to mix the indicator uniformly through the solution. The solution should appear in color. If the solution is, you can skip this step. If the solution is, add 1 M HCl(aq), a drop at a time, with thorough mixing, until you otain a shade of. If the solution is, add 1 M NaOH (aq), a drop at a time, again with thorough mixing, until you otain a shade of. Determine the ph of uffer solution. the uffer solution using a calirated ph meter. Record the ph and temperature of this solution. Use a volumetric pipet to transfer ml of the, uffered solution to each of three clean and dry 100 ml eakers. Using a 2.00 ml volumetric pipet, transfer 2.00 ml of 1.0 M HCl into one of the three eakers. The solution should turn. This solution will e referred to as Yellow. Clean and rinse the 2.00 ml volumetric pipet and then use it to transfer 2.00 ml of 1.0 M NaOH into one of other eakers. The solution should turn. This solution will e referred to as Blue. Clean and rinse the 2.00 ml volumetric pipet and then use it to transfer 2.00 ml of distilled water into the remaining eaker. The solution should remain. This solution will e referred to as Green. Note: Even though you prepared your solutions in eakers rather than volumetric flasks, they all contain the same final volume of solution (12.00 ml) and the same numer of total moles of romothymol (delivered with the ml pipet). This gives an equivalent total concentration of romothymol in each eaker. On the Ocean Optics spectrometer, measure the spectra for the, and solutions and save copies of the spectra you will replot these in Excel and use them to extract the data needed to determine the equilirium constant. Safety / Waste Disposal The materials used in this reaction should e collected in the waste container in the hood at the end of the experiment. Bromothymol Blue- 6

7 Data nalysis You should now have all of the measurements you need to calculate the equilirium constant for romothymol. Equations 1, 2 and 14 are rewritten elow. HBB() + H 2 O H 2 O + + BB - () K c [H O ][BB ()] [HBB()] K c [H O ] You could simply plug values from the tale into the K c expression, however a more accurate result is otained y sutracting the small asorance of HBB at λ = 616 (where BB - asors) from asorance of the solution, and similarly sutracting the small asorance of BB - at λ = 45 (where HBB asors) from the asorance value of the solution. One can see these small asorances in Figure 2. t 616, the asorance of the solution is small and could proaly e ignored. However, when a measurement is made at 45, one can see that the solution also has an asorance. Because Beer s law is additive, total = n, we can modify the original equilirium expression (Eq. 14): K c [H O ] y sutracting the small asorance of at λ = 45, and sutracting the small asorance of at λ = 616 from the solution (where oth and are present) asorance data: [ H O ]( ) ( K c (15) ( ) ( 616 ) Perform the calculation of K c in your laoratory noteook and report your pk c value (rememer pk c = -logk c ) to two decimal places. ) Bromothymol Blue- 7

8 Grading of Experiment I m not really wanting to throw a full lown report at you for this as this is very much a collect and analyze data style experiment and so as a result, 50 points will e awarded for handing in an overlay spectrum with your and and spectra plotted on a single graph and with the calculations to get K shown elow. I ll also have a post-la quiz for this one will open at end of la and you will have a few days, I ll have exact timing in la. Wiki Component Theoretically, the equilirium constant determined y every group should e the same. Right? Well let s find out how much variation there is? There will e a tale on the wiki for you to enter your data on (there will e a slight temperature variation, something you will learn more aout later in the semester so you should include the temperature in your data. Reference: The Equilirium Constant for Bromothymol Blue: General Chemistry Laoratory Experiment Using Spectroscopy. Elseth Klotz, Roert Doyle, Erin Gross, and Bruce Mattson, Journal of Chemical Education (5), Bromothymol Blue- 8