| Abietic-type acids in gum rosin or pine oleoresin are easily oxidized and lead to the decline of the product quality. In response to above problems, the thermal and photo oxidation process of levopimaric acid was studied. And the results provide guidance for the antioxidative research of rosin and oleoresin. The main experiments and results were as follows.(1) The oxidation device for solid levopimaric acid was designed and was detected by the UV spectrophotometry.(2) The quantitative analysis methods of levopimaric acid in the ethanol (liquid state) and on the polyethylene film (solid state) were established according to UV Spectrum Characteristic of levopimaric acid and its oxidation products. The results indicate that the standard curve in the ethanol and on the polyethylene film were A=0.0180C+0.0021(the value of C was5.58～58.50μg·mL-1) and A=0.6730×10n-0.4688(the value of n was1.32×10-6～3.97×10-6mol), individually. The relative standard deviations of precision and stability were in the range of0.14%～0.26%and1.07%～1.38%, respectively.(3) The thermal oxidation kinetics of levopimaric acid was studied. The results showed that it was pseudo first order reaction. When at temperature30℃,35℃,40℃,45℃,50℃, the apparent rate constants were0.0031min-1, 0.0043min-1,0.0065min-1,0.0091min-1,0.0127min-1, respectively. Its reaction activation energy was58.10kJ·mol-1.(4) The photo oxidation kinetics of levopimaric acid was studied under the irradiation of254nm ultraviolet light. The results showed that it was pseudo first order reaction. When under intensity10μW·cm-2,20μW·cm-2,30μW·cm-2,40μW·cm-2,50μW·cm-2, the apparent rate constants were0.0138min-1,0.0178min-1,0.0216min-1,0.0264min-1,0.0308min-1at temperature30℃; were0.0161min-1,0.0201min-1,0.0238min-1,0.0286min-1,0.0330min-1at temperature35℃; were0.0184min-1,0.0227min-1,0.0259min-1,0.0308min-1,0.0350min-1at temperature40℃; were0.0208min-1,0.0248min-1,0.0285min-1,0.0333min-1,0.0372min-1at temperature45℃, respectively. And the relationship between rate constants and light intensity were k=0.0004I+0.0093, k=0.0004I+0.0116, k=0.0004I+0.0142, k=0.0004I+0.0165, accordingly.(5) The photo oxidation kinetics of levopimaric acid was studied under the irradiation of365nm ultraviolet light. The results showed that it was pseudo first order reaction. When under intensity200μW·cm-2,400μW·cm-2,600μW·cm-2,800μW·cm-2,1200μW·cm-2,1600μW·cm-2, the apparent rate constants were0.0041min-1,0.0048min-1,0.0053min-1,0.0058min-1,0.0067min-1,0.0076min-1at temperature30℃; were0.0054min-1,0.0061min-1,0.0067min-1,0.0075min-1,0.0084min-1,0.0094min-1at temperature35℃; were0.0071min-1,0.0081min-1,0.0087min-1,0.0094min-1,0.0105min-1,0.0116min-1at temperature40℃; were0.0095min-1,0.0103min-1,0.0109 min-1,0.0116min-1,0.0127min-1,0.0138min-1at temperature45℃respectively.And the relationship between rate constants and light intensity were k=2×10-6I+0.0038,k=3×10-6I+0.0050,k=3×10-6I+0.0067,k=3×10-6I+0.0090,accordingly.(6)The relationship between activation energy and light intensity was studied. The results showed that at temperature range of30～45℃, the equations were Eα=-7.287lnI+39.12(the value of I was10～50μW·cm-2) and Eα=-6.089lnI+77.45(the value of I was200～1600μW·cm-2),respectively, under the irradiation of254nm and365nm ultraviolet light. |
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