Study On The Experimental Conditions Of Determination Of Free Silica In Dust By Pyroic Acid Method

Objective:Quantitative analysis of free silica in dust is the main factors that lead to pneumoconiosis. It is also the important basis to determine occupational exposure limit of dust.Pyrophosphoric acid method is the recommended method of Chinese occupational health standard, Method for determination of dust in the air of workplace Part 4:Content of free silica in dust (GBZ/T 192.4-2007).Compared with IR(infrared spectrometry) and X-ray diffraction method, pyrophosphoric acid method is more convenient and reasonable price. It is widely used in the local dust laboratory. But pyrophosphoric acid method also has the shortcoming such as large measurement error. Different experimental conditions were set to determine the potential dust free silica content optimization conditions, so as to improve the stability and accuracy of measurement of dust free SiO2 content., thus to improve the dust job hazard prevention and control in our country, and contribute to the occupational health supervision level of prevention and treatment of pneumoconiosis.Methods:literature review and questionnaire survey were conducted to collect the existing main problems, including:dissolution time effect on the determination results, different sources filter effect on the determination results, different thickness of filter paper on different dispersion influence the determination results of dust and different high temperature furnace and ashing time effect on the determination results, etc. Before the start of the experiment, blank experiment, precision and accuracy experiments were conducted to ensure the quality of experimental control. Dust samples were selected from the test samples of the Chinese center for disease control and prevention of occupational health and poison control, with the reference range as (4.53±0.94)%、(20.77±3.30)%、(45.32±2.64)%、(32.09±2.03)%、(30.46±1.79)%、 (41.98±1.63)% and(16.42±3.02)%. Dissolution time as 10 min,15 min and 20 min, filter conditions in single, double, three layers of filter paper, and filter paper with pulp group, each group of 9 cases of samples; ashing time as 30 min,45 min,60 min and each group with 6 samples.SPSS17.0 was used for statistical analysis. Measurement data by the normality test are accord with normal distribution or approximate normal distribution, mean comparison among multiple ways by single factor analysis of variance, inspection level of alpha= 0.05.Results:Results show that in addition to the dust sample 4, free SiO2 content of dissolution time not less than 15 minutes are significantly less than 10 min. As the dissolving time increases, the percentage of average free SiO2 is decreasing. Under the condition of the dissolution time of 15 min, different sources of slow quantitative filter paper (A, B, C, D) were used. For number 1,2,3,dust, free SiO2 content differences were not statistically significant within each group (p>0.05), For dust 2, 3,, there was no statistically significant difference compared with the standard reference determination results under the dissolution time of not less than 15 min (P> 0.05)..For the dust samples (4,5,6,7) with high dispersion, single filter present macroscopic observational filtrate turbidity condition, and for dust 4,5,6, tested with singlefilter, double filter, and triple filters, the confidential interval of free SiO2 content deviate from their standard referencerespectively.Only for double filter with paper pulp filtration, the average content of SiO2 ci include standard reference value.ANOVA showed that different ways of filter have filtering effect, and with the method of filter paper and pulp, dust free SiO2 measurement could be more accurate. Free SiO2 content of dust sample 1 and 2 have significant different within each group by high-temperature electric resistance furnace I(p<0.05), and there has no significant difference when tested by high-temperature electric resistance furnace Ⅱ(p>0.05). Free SiO2 content were higher than that of standard reference tested by high-temperature electric resistance furnace I with the ashing time of 30mins (p<0.05)and under the same condition there was no statistically significant (p>0.05) difference by using high-temperature electric resistance furnace II(p>0.05). Using high temperature furnace I, free SiO2 content of dust 1,2 and 3 are 6.39%,25.07% and 6.39% respectivelyunder the ashing time of 30 min, the determination results were higher than standard reference value (p<0.05).Conclusion:Dissolving time should not be less than the standard requirements of 15min., the filter and the Muffle furnace should be considered in quality control, the ashing time should be extended if necessary. Different filter conditions should be considered according to thecontent of free silica and the distribution of particulate in dust sample. For the dust sample which has the higher content of free silica and the distribution of particulate,2 filter papers with paper pulp in themis the better filter condition compared with the traditional way.