ABSTRACT
The azide ion is a strong physical quencher of singlet molecular oxygen (1O2) and is frequently employed to show involvement of 1O2 in oxidation processes. Rate constants (k(q)) for the quenching of 1O2 by azide are routinely used as standards to calculate k(q) values for quenching by other substrates. We have measured k(q) for azide in solvent mixtures containing deuterium oxide (D2O), acetonitrile (MeCN), 1,4-dioxane, ethanol (EtOH), propylene carbonate (PC), or ethylene carbonate (EC), mixtures commonly used for many experimental studies. The rate constants were calculated directly from 1O2 phosphorescence lifetimes observed after laser pulse excitation of rose bengal (RB), used to generate 1O2. In aqueous mixtures with MeCN and carbonates, the rate constant increased nonlinearly with increasing volume of organic solvent in the mixtures. k(q) was 4.78 x 10(8) M(-1) s(-1) in D2O and increased to 26.7 x 10(8) and 27.7 x 10(8) M(-1) s(-1) in 96% MeCN and 97.7% EC/PC, respectively. However, in EtOH/D2O mixtures, k(q) decreased with increasing alcohol concentration. This shows that a higher solvent polarity increases the quenching efficiency, which is unexpectedly decreased by the proticity of aqueous and alcohol solvent mixtures. The rate constant values increased with increasing temperature, yielding a quenching activation energy of 11.3 kJ mol(-1) in D2O. Our results show that rate constants in most solvent mixtures cannot be derived reliably from k(q) values measured in pure solvents by using a simple additivity rule. We have measured the rate constants with high accuracy, and they may serve as a reliable reference to calculate unknown k(q) values.
ABSTRACT
To investigate the validity of the credentialing examination for entry-level practitioners in environmental health, 15 work measures, simulating or assessing important components of job practice, were developed. These work measures, along with the written examination, were administered to a sample of 128 entry-level practitioners drawn from 10 test sites throughout the country. Composite work measures were constructed in which individual work measures were weighted according to their judged importance. The total composite work measure, based on all 15 individual measures, correlated .53 with the total written examination. Correlations between composite work measures and the written test scores were generally in the .3 to .5 range and significant at the .01 level. Seventy-three percent of the sample were consistently classified on both the written examination and the total work measure composite. That is, 73 percent of the sample passed both the examination and the work measures or failed both the examination and the work measures. In comparison to similar studies, the magnitude of the coefficients reported here is acceptable to high. Results show that the examination is a valid, but not perfect, predictor of on-the-job skill in environmental health, and it should help to screen the public from the results of incompetent practice.
