Clinical Research Associates
Clinical Science Department
Clinical Research Associates
The decomposition rates of bleaching gels were determined using microcalorimetric methods developed by Dr. Lee Hansen, Department of Chemistry & Biochemistry, Brigham Young University, Provo, Utah (Hansen, L.D. (1996) Instrument selection for calorimetric drug stability studies, Pharmaceutical Technology 20, 64-74).
The fundamental equation for chemical calorimetry is:
heat rate = dQ/dt = deltaH(dn/dt)
where: Q is the heat
t is the time
deltaH is the enthalpy change
dn/dt is the reaction rate
Therefore, if deltaH is known, then the reaction rate can be determined by measuring the heat rate.
It is assumed that the only reaction taking place is the breakdown of peroxide into water & oxygen free radical: H2O2 » H2O + O
For this reaction the enthalpy change is 94.9 µJ / nanomole H2O2 (Wagman, et al, 1982).
The heat rate was measured using multicell, differential, heat-conduction, temperature scanning calorimeters (Model 7707 by Hart Scientific & Model 4200 by Calorimetry Sciences Corporation).
Approximately 100 mg of freshly prepared bleaching gel was weighed into a glass vial, which was sealed into a 1 mL hastelloy calorimeter ampoule. The ampoule was placed into the calorimeter along with an empty reference ampoule. The temperature was set to 25° C & held for one hour, then successively at 35, 45, 55, 65, 75, 85, & 95° C.
Decomposition rates (reaction rates) were calculated in milligrams of H2O2 decomposed per hour per gram of prepared bleach gel, using the following equation:
| Rate = |
(heat rate µJ/s)x(34.0 nanogram/nanomole)x(3600
s/hr)x (1x 10-6 mg/nanogram)
(sample weight g)x(94.9 µJ/nanomole
H2O2)
|
Click here to view a diagram of the calorimeter cell and ampoule.
CRA Home | What is CRA? | The CRA Newsletter | Update Courses | Contact CRA
All Content
Copyright 
Clinical Research Associates.