Lab 6. Questions to Answer in Your Report.

Please do not write your report as a set of answers to the following questions. Instead, use these questions for guidance of what important points should not be missed in presenting your results and discussion

• Show a couple of kinetics of *Ru(bpy)₃²⁺ luminescence: one with a quencher and one without it. Demonstrate a quality of the exponential fits for the curves. You may use any software package you like to do the fitting. Excel would do just fine, but you need to take care of a background which might be a nonzero and has to be adjusted manualy (excel does not include that parameter into the fitting)
• Show absorption spectra of all your quenchers except for O₂
• Given that the solubility of O₂ in water follows Henry's Law,

[O₂] = K_HP_O2 with K_H = 1.228 x 10^-3 mol O₂/L atm @ 25°C

calculate the molar concentration of dissolved O₂ for each solution. See the table below for K_H at other temperatures.

• Show the graphs of the Stern-Volmer plots for each quencher and calculate k_q for each quencher. Since you measured t directly, it is less confusing to plot 1/t vs. concentration of quencher. Then:
  1/t = 1/t_o + k_q[Q],
  where t_o is the lifetime of deoxygenated solution when evaluatig k_q for oxygen quenching and it becomes the lifetime at ambient airated solution when evaluating quenching by ions (this is appropriate since you do not deoxygenate them).
• Based on k_q for O₂ and the lifetime of *Ru(bpy)₃²⁺ without purging, what do you calculate for oxygen concentration to be in atmosphere at normal conditions?
• Based on all the arguments above, suggest the quenching mechanism for each quencher
• Calculate values of the quenching radius, R_q, from your k_q's using

k_d = 4p N_a DR_q

where N_a is the Avogadro's number 6.022 x 10²³ mol^-1 and D is the sum of diffusion constants calculated using the Stokes' Law expression:

D_i = k_BT/6phR_i

Take R_O2 = 0.18 nm and radii for all ions, including Ru(bpy)₃²⁺, R_ion = 0.50 nm. Viscosity at different temperatures can be found in the table below

• Compare your experimental radius R_q values with theoretical expression for the effective radius l for a charged particle reaction in the limit of zero ionic strength:

where R₁ and R₂ are the ionic radii and

is Onsager radius for water at 25°C and equals 071 nm when Z₁ = Z₂ = 1. Z₁ and Z₂ are the ion charges (with signs), which would make r_c negative for oppositely charged ions. Note that you should also scale r_c appropriately for Z₁ and Z₂ different from one.

• Make up a table :

  	kq (units)	D (units)	Rq (units)	R1+R2 (units)	rc (units)	l (units)
  Fe(CN)63-	value with accuracy		value with accuracy
  Fe(CN)64-
  O2

• Discuss the important sources of errors in this experiment.

T(°C)	103 x KH (mol O2/L atm)1	104 x h (H2O) (kg/m s)2
20	1.356	10.09
21	1.329	9.84
22	1.303	9.61
23	1.277	9.38
24	1.252	9.16
25	1.228	8.95
26	1.205	8.75
27	1.183	8.55
28	1.162	8.36
29	1.141	8.18
30	1.121	8.00

References.

1. Lange's Handbook of Chemistry, 9th Edition, Handbook Publishers Inc., 1956, p. 1093
2. International Critical Tables, Vol. 5, p. 10. Don't be confused by the 10⁴ in the column heading--for example at 25°C the viscosity is 8.95 x 10^-4 which when multiplied by 10⁴ yields the listed value.

Extra Credit (5 points)

High quality data for the quenching of *Ru(bpy)₃²⁺ by ions exhibit a variation in k_q as the concentration of quencher is increased. You may see such a trend in your values for example for k_q decrease in case of Fe(CN)₆^3-. The chief reason for the decrease is that the ionic strength of the solution increases as [Fe (CN)₆^3-] increases. The radius of the ionic atmosphere thus shrinks as quencher concentration is increased, and the oppositely-charged Fe(CN)₆^3- and *Ru(bpy)₃²⁺ ions interact less and less strongly. Despite the high dilution of the *Ru(bpy)₃²⁺ and quencher solutions, this "ionic strength effect" is far from negligible largely because the ions are highly charged. A correction for the increasing ionic strength of the solution may be estimated using the ideas. Such details are also worked out in Atkins pp. 884-885. Calculate the ionic strength I of each solution -- Ru(bpy)₃²⁺ was supplied as the chloride salt and the Fe(CN)₆^3- as the potassium salt. Use Eq (24.68) on p. 884 in Atkins to calculate the ratio k°₂/k₂ where k°₂ is a second order rate constant at zero ionic strength and k₂ is the rate constant at finite ionic strength. Multiply each experimental value of k_q by the appropriate ratio k°₂/k₂ to correct k_q to zero ionic strength. Are the corrected values more constant than the original k_q's?

Last updated 04/01/09.