Project 7 Guide - This is the Chem 152 lab 7 lab. PDF

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This is the Chem 152 lab 7 lab....

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Chemical Thinking

Project 7

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Reaction Characterization II: Exploring Drug Degradation Rates Have you noticed that packaged drugs have an expiration date? Have you wondered how such dates are determined? No doubt you have some idea the rate of drug degradation is involved. Project 7 explores the degradation kinetics of two drugs to characterize their rate of decomposition in aqueous alkaline solutions. In particular, you will determine the reaction order and rate constant for the degradation under fixed conditions. With the order and rate constant known you can then infer the “shelf life” of the drug in an alkaline solution under those conditions. Then by varying the drug concentration and alkalinity, you can determine the effect these changes have on the shelf life and recommend the best conditions to prolong the shelf life of each drug.

P7-1 Background Pyoctanin (pi-ok'-tan-in) is a topical anti-microbial agent and Phenolax (fen-Oh-laks) is the brand name for a specific set of phenolphthalein oral formulations used as a laxative. While no longer marketed in the U.S. for human use, Pyoctanin and Phenolax still find use in other countries and have veterinarian applications even in the U.S. When combined with solutions of sodium hydroxide (NaOH), Pyoctanin and Phenolax degrade. The degradation of Pyoctanin or Phenolax (denoted as X) by NaOH follows the rate law: 𝑅𝐴𝑇𝐸 = −𝑘[𝑋]𝑛 [𝑁𝑎𝑂𝐻]𝑚 . In this equation, k is the rate constant for the overall reaction, n the rate order with respect to the Pyoctanin or Phenolax molar concentration [X], and m the rate order with respect to the molar concentration of sodium hydroxide [NaOH ]. Sodium hydroxide is colorless, but Pyoctanin and Phenolax have intense, distinct colors with characteristic λmax values (590 nm for Pyoctanin and 550 nm for Phenolax). Hence, measuring Pyoctanin or Phenolax at low concentrations in the presence of high NaOH concentrations via spectrophotometry is possible, and we can follow the Pyoctanin or Phenolax degradation directly. (ε = 8.65 x 104 cm-1M-1 at 590 nm for Pyoctanin, and ε = 5.02 X 104 cm-1M-1 at 550 nm for Phenolax.) Now, if the NaOH concentration remains in vast excess over the Pyoctanin or Phenolax concentration, which we can symbolize as [NaOH] >> [X], the proportional decline in [X] will be far greater than the corresponding decline in [ NaOH]. Hence, with [NaOH] >> [X], for all practical purposes [NaOH] may be considered constant and thus combined with k to give a new constant, 𝑘𝑜𝑏𝑠 = 𝑘[𝑁𝑎𝑂𝐻]𝑚 . Substitution of this into the rate law given above, results in 𝑅𝐴𝑇𝐸 = −𝑘𝑜𝑏𝑠 [𝑋]𝑛 . This expression suggests that by maintaining [NaOH] constant, a condition met when [NaOH] >> [X], the rate order n and observed rate constant kobs can be determined by measuring the Pyoctanin or Phenolax absorbance as a function of time, then transforming this data into concentration as a function of time, and graphically analyzing the data. For practical reasons it is desirable to know the shelf life of a drug. Shelf life can be defined as the length of time for a product’s potency to decrease from its original value to some percentage of this original value. For most products, especially drugs, this is the t90, or time at which the product retains 90% of its original potency (or the time at which 10% of the drug has decomposed and 90% of the active drug still remains). In other words, at t = t90, [X] must be 90% that of the initial molar concentration, or [X]t90 = 0.9[X]. This suggests if we know the rate order n of the degradation reaction, we can take the integrated rate law corresponding to the order, substitute in [X]t90 = 0.9[X] when t = t90, and solve for t90 to derive an equation for shelf life (t90) in terms of kobs (Module 4, Unit 5 of the Chemical Thinking text presents an analogous development for t1/2).

Chemical Thinking

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Project 7

P7-2 Your Challenge

DEFINING THE EXPERIMENTAL METHODOLOGY

KINETIC CHARACTERIZATION

Design appropriate experiments that can be applied to either Pyoctanin or Phenolax to determine the rate order n with respect to the drug molar concentration [X], the observed rate constant kobs, and the shelf-life t90, for a NaOH-mediated degradation of the drug at room temperature where the condition [NaOH] >> [X] exists.

With your lab partner, build and implement a plan to characterize kinetically Pyoctanin and Phenolax.

Preparatory Work  Summarize in your own writing the core goal(s) of P7 S1.  View the video (click title to launch)  Generating Kinetic Data Technical Guide, and carefully read the infographic (on D2L)  Graphical Analysis of Kinetic Data Infographic, then write an experimental plan to collect the data necessary to determine n, kobs and t90, at room temperature where the condition [NaOH] >> [X] exists, that can be applied to Pyoctanin and Phenolax.

 Carry out appropriate experiments to determine n, kobs and t90 for the room temperature NaOH-mediated degradation of Pyoctanin and Phenolax, where the condition [NaOH] >> [X] exists. You will want to complete several runs so you can select the trial that best represents the kinetic behavior.  Discuss the similarities and differences in the Pyoctanin and Phenolax results with your labmates.

EXPLORATION

Project 7 Session 2

Project 7 Session 1

You will have two lab sessions to complete this project. You are expected to meet the challenge by carrying out experiments that address:

With your lab partner, build and implement a plan to explore: 1) Different concentrations of Pyoctanin and NaOH on the Pyoctanin degradation rate. 2) Different concentrations of Phenolax and NaOH on the Phenolax degradation rate. This exploration will require running multiple reactions.

Preparatory Work Complete the first page of the Project 7 Technical Communication.

You will be responsible for managing your lab time. Keep in mind there must be time to share your experiences/findings with the class in the first session. For session two (P7 S2), in addition to time for sharing and discussing the results with classmates, you will need time to individually write a Project 7 Technical Communication summarizing the major findings and their implications. Each group will have access to the following in lab:   

Spectrophotometer (attached to the island) 1.57 X 10-3 M Phenolax solution (in one of the hoods) 4.08 X 10-5 M Pyoctanin solution (in one of the hoods)

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1.0 M NaOH solution (in one of the hoods) 0.15 M NaOH solution (in one of the hoods) 100 - 1000 µL Micropipette + 1000 µL pipette tips 20 - 200 µL Micropipette + 200 µL pipette tips Plastic transfer pipettes (on the reagent bench)

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Kimwipes™ (dispenser attached to the islands) Cuvettes (locker item)

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20 mL Vials with caps (locker item) 1 dram Vials with caps (locker item) Waste beaker (locker item) Contents of your shared locker (see the locker inventory sheet and photo on D2L)...