SILICA, CR YST ALLINE : MET HO D 760 1, Issue 3, dated 15 M arch 200 3 - Page 4 o f 6 CALIBRATION AND QUALITY CONTRO L: 10. Prepare a calibration graph for the silico-molybdate range. a. Dilute 1-, 2-, 3-, 4-, 5- and 6-mL aliquots of the silica stock standard (NIS T S RM 187 8a q uartz) to 25 mL with silica-free water in polyethylene beakers. b. Analyze together with samp les and blanks (steps 14 through 16). c. Plo t absorbance vs. m g SiO 2. 11. Prepare a calibration graph in the molybdenum blue range. a. Dilute the working standards in step 10 1:25. b. Analyze the resulting standard solutions (steps 14 through 16 ). c. Plo t absorbance vs. m g SiO 2. 12. Prepare control sam ples contain ing a silicate c om pound (a bout 50 m g M gS iO 4 sam ple) in add ition to the quartz. Recovery of quartz is low and precision is much worse if quartz only is present during the pho sph oric acid treatm ent. NOTE 1: Calibration standards are limited to specified NIST and USG S certified standards of known purity, particle size and sam ple-to-sam ple hom ogeneity. At least 12 materials including 5 µm Min-U-Sil previously used by laboratories throughout the United States and Canada have been evaluated. None have be en fo und to be acceptable alternatives to the certified standards cited within this m ethod [3]. Standard reference materials should be corrected for ph ase purity. NO TE 2: Crystalline silica methods require standards of kno wn purity, sp ec ific pa rtic le s ize distribution, and sam ple-to-sam ple hom ogeneity. E sta blishing traceability of secon dary standards to the specified NIST and USG S primary standards requires the use of measurem ent m eth ods w ith bette r precision and accuracy th an the XR D, IR and visible absorption spectropho tom etry methods comm only used within the industrial hygiene field. In add ition, particle size distribution m easurem ents have co nsiderable error. Therefore the use of secondary calibration standards that are traceable to NIST and USGS certified standards is not appropriate. NOT E 3: S om e training (university or short course) in geology or mineralogy of the analyst or laborato ry m anagem ent is useful to pro vide a back ground in crystal structure and mineral transformation in order to answer the questions that laboratory clients m ay hav e an d to understand any matrix effects.
MEASUREMENT: 13. Add 25 mL silica-free water to samples in polyethylene beakers. 14. Add 50 mL boric acid solution. Stir well. Cover. Heat the solution in a 40 °C waterbath for 10 min. 15. Add 4 mL m olybdate reagent while stirring, staggering the addition at 2-m in intervals between samples. Ex actly 20 min after the addition of the molybdate reagent, add 20 mL 10 N H 2SO 4 and stir thoroughly. 16. Note the color of the solution. b. If a colorles s s olu tion results, allow it to stand for 2 to 5 min and add 1 mL 1-am ino-2-naphth ol-4-sulfonic acid reagent. Mix and read after 20 min at 820 nm against deionized water. This color is stable for several hours. c. Record the absorbances of the sample, A, and reagent blank, B.
CALCULATIONS: 17. Ca lculate the concentration of crystalline silica, C (m g/m 3), in the air volume sam pled, V (L), using the slope, m (:g -1) of the appropriate calibration graph:
NIOSH Manual of Analytical Methods (NMAM), Fourth Edition
