# Silica, Crystalline, by VIS (7601)

​SILICA, CRYSTALLINE, by VIS SiO2
MW: 60.08
CAS: 14808-60-7 (quartz) 14464-46-1 (cristobalite) 15468-32-3 (tridymite)
METHOD: 7601, Issue 3
RTECS: VV7330000 (quartz) VV7325000 (cristobalite) VV7335000 (tridymite)
EVALUATION: PARTIAL
quartz (respirable) 10 mg/m3 /(%SiO2 +2); cristobalite and tridymite (respirable) ½ the above NIOSH: 0.05 mg/m3 ; carcinogens ACGIH: quartz (respirable) 0.1 mg/m3 cristobalite (respirable) 0.05 mg/m3 tridymite (respirable) 0.05 mg/m3
OSHA :
PROPERTIES:
SAMPLING SAMPLER:
CYCLONE + FILTER (10-mm cyclone, nylon or HigginsDewell (HD) + 0.8-:m MCE or 5-:m PVC)
7601
Issue 1: 15 February 1984 Issue 3: 15 March 2003 solid; d 2.65 g/cm3 ; crystalline transformations: quartz to tridymite @ 867 °C; tridymite to cristobalite @ 1470 °C; "-quartz to ß-quartz @ 573 °C
MEASUREMENT TECHNIQUE:
VISIBLE ABSORPTION SPECTROPHOTOMETRY
ANALYTE:
Silicon complexes-silicomolybdate (420 nm) and molybdenum blue (820 nm)
DIGESTION:
Phosphoric acid, to remove interfering silicon compounds
FLOW RATE:
HD cyclone: 2.2 L/min; nylon cyclone: 1.7 L/min
VOL-MIN: -MAX:
400 L 800 L
SHIPMENT:
Routine
FILTRATION:
To collect undigested material
SAMPLE STABILITY:
Stable
DISSOLUTION:
Crystalline silica in HF
BLANKS:
2 to 10 field blanks per set CALIBRATION:
NIST SRM 1878a quartz, NIST 1879a cristobalite, USGS 210-75-0043 dissolved in HF
RANGE:
Silicomolybdate: 0.1 to 2.5 mg SiO2 ; molybdenum blue: 0.02 to 0.15 mg SiO2
ACCURACY RANGE STUDIED:
Not studied
BIAS:
None known
OVERALL PRECISION (Ö rT ):
Not determined
ACCURACY:
Not determined
ESTIMATED LOD: 10 µg SiO2 PRECISION ( þ r ):
0.09 [1]
APPLICABILITY: Determination of crystalline silica in respirable or total dust, settled dust and in biological samples [1,2]. The three crystalline polymorphs cannot be distinguished by this method. The working range is 0.04 to 5 mg/m3 for a 500-L air sample. Alternative XRD (e.g. NMAM 7500) and IR (e.g. NMAM 7602 and 7603) methods have better laboratory-to-laboratory agreement than visible absorption methods and therefore this method is recommended for research use only [3]. See also discussion of crystalline silica in Chapter R of this volume [4]. INTERFERENCES: Any silicon-containing compound which is soluble in HF will give a positive interference. See APPENDIX A. OTHER METHODS: This is P&CAM 106 in a revised format [1]. X-ray diffraction (XRD; Method 7500) can distinguish the three main silica polymorphs and does not detect amorphous silica. Silicates interfere with XRD but are removed by phosphoric acid cleanup. Infrared spectrometry (IR; Methods 7602 and 7603) can determine the three polymorphs, although if two or more are present, a less sensitive peak must be used; however, amorphous silica and silicates, in large amounts, interfere with IR.
NIOSH Manual of Analytical Methods (NMAM), Fourth Edition ​SILICA, CR YST ALLINE : MET HO D 760 1, Issue 3, dated 15 M arch 200 3 - Page 2 o f 6 REAGENTS:
EQUIPMENT:
1. Silica Standards. a. Quartz* (SRMs 1878a, 2950, 2951, 2958) and Cristobalite* (SRMs 1879a, 2960, 2957), available from Standard Reference Materials Program, RM. 204, Bldg. 202, National Institute of Standards and Technology (NIST), Gaithersburg, MD 20899; ww w.nist.gov. b. Tridymite* (210-75-0043), available from U.S. Geological Survey, Box 25046, MS 973, Denver, CO 80225. 2. Hydrofluoric acid, 48% (HF ).* 3. O rthopho sp ho ric ac id , 8 5% H 3PO 4.* 4. Silica-free water. All water used must be deionized and stored in polyethylene containers. 5. Hydrochloric acid (H Cl),* 1:10 v/v conc. HCl in deionized water. 6. Sulfuric acid, 10 N.* Cautiously add 555 mL con c. H 2SO 4 to about 1.3 L of deionized water. Cool. Dilute to 2 L. 7. Nitric acid, co nc.* 8. Perchloric acid, conc*. Use only if samples have been collected on PVC filters. 9. Bo ric acid solution , 5% . 200 g boric acid crystals in 4 L w arm deion ized water. C ool. Filter with vacuum through a 0.45-:m mem brane filter. Store in a polyethylene container. 10. Molybdate reagent. Dissolve 50 g amm onium molybdate tetrahydrate in about 400 mL deionized water. Ad d 50 m L conc. s ulfuric acid. Cool. Dilute to 500 mL. Store in dark. 11. Reducing solution. Dissolve 9 g sodium bisulfite in 80 mL deionized water. In 10 mL deionized water, dissolve 0.7 g anhydrous sodium sulfite and 0.15 g 1-am ino-2-naphth ol4-sulfonic acid, in that order. Com bine these solutions and dilute to 100 mL with deionized wa ter. Sta ble about one m onth if sto red in refrigerator. 12. Silica stock standard, 0.5 m g/m L. Dissolve 250 m g qu artz in 10 m L 48 % HF . Dilute to 500 mL with silica-free water. Store in a polyeth ylene contain er. Sta ble indefinitely.
1. Sam pler: a. Filter: Mixed ce llulose ester (MC E) filter, 37-mm , 0.8-:m, or polyvinyl chloride (PVC) filter, 37-mm , 5.0-:m pore size, supported with backup pad in a two-piece, 37-m m cassette filter ho lder (preferably, conductive) held together by tape or cellulose shrink band. NOTE: If PVC filters are u sed , a perchloric acid fume hood, lowtemperature radio-frequency plasma asher or muffle furnace will be needed to ash them (Appen dix B). b. Cyclone: 10-m m nylon or Higgins-Dew ell (HD ). c. Sam pling head holder: Holder must keep the cassette, cyclone and coupler together rigidly so that air enters only at the cyclone inlet. 2. Area Air Sampler: PVC m embrane filter, 37mm , 5-µm pore size in a two-piece filter cassette. Sample closed face at 3 L/min. 3. Sam pling pumps: HD cyclone, 2.2 L/min; nylon cyclone , 1.7 L/m in; and area sam pler, 3 L/min. 4. Precision heater, 550-watt, 115-V, Type RH; 750-wa tt variable transform er with built-in voltmeter; variable speed clinical rotator, about 30 to 80 rpm range (can be achieved with a variable transform er). 5. Stopwatch or laboratory timer. 6. Phillips beakers, borosilicate glass, 250-mL; short-stemmed glass funnels with bent stems (must be done by laboratory); crucible tongs with tygon or rubber-padded tips; polyethylene stirring rods; polyethylene 50mm discs. 7. Vacuum filtration assembly: MCE m embrane filters, 0.45-:m pore size, 47-m m diame ter; filter funnel, including mem brane mount and flask asse m bly, 47-m m . 8. Buret, polypropylene, 10-mL; polyethylene reagent bottles for water, boric acid, and standard solutions. 9. Constant temperature bath, 40 °C. 10. Spectrophotometer, at 420 to 820 nm and 1cm cuvettes. 11. Hotplate, 150 °C surface temperature. 12. Graduated cylinder or graduated pipet, 10and 25-mL. 13. Polyethylene graduated cylinder or pipets, 25- and 5-mL.
NIOSH Manual of Analytical Methods (NMAM), Fourth Edition ​SILICA, CR YST ALLINE : MET HO D 760 1, Issue 3, dated 15 M arch 200 3 - Page 3 o f 6 SPECIAL PRECAUTIONS: Avoid inhaling silica dust [5]. Use pers ona l protec tive equipm ent to prevent contact of acids with the skin. HF dissolves glass. Use plastic labware for all solutions containing HF. Concentrated acids are corrosive.
SAMPLING: 1. Calibrate each personal sampling pump with a representative sampler in line. 2. Sa m ple at 2.2 ± 5% L/min with HD cyclone and 1.7 ± 5% L/min with nylon cyclone for a total sam ple size of 400 to 800 L. Do no t exceed 2 m g dust loading on the filter. NOTE 1: Do not allow the sampler assembly to be inverted at any time wh en u sing a cyclone. Turning the cyclone to anything other than a horizontal orientation may deposit oversized material from the cyclone body onto the filter. NOT E 2: A single sampler/flow rate should be used for a given application. Sampling for both crystalline silica and coal mine dust should be done in accordance with the ISO/CEN/ACGIH/ASTM respirable aerosol sampling con vention. Flow rates of 1.7 L/min for the Dorr-Oliver nylon cyclone and 2.2 L/min for the Higgins-Dewell cyclone have been found optimal for this purpose. O uts ide of c oal m ine dust sampling, the regulatory agencies currently use these flo w rate s with th e D orr-O liver cyclone in the United States and the Higgins-Dew ell sampler in the United Kingdom. Though the sampling recomm endations presented in a NIOSH Criteria Document have been formally accepted by MSHA for coal mine dust sampling, the Dorr-Oliver cyclon e at 2.0 L/m in with 1.38 conversion fa cto r is currently used in the United States for the purpose of matching an earlier sampling convention [6]. In any cas e, a single sampler/flow rate should be used in any given application so as to eliminate bias introduced by differences between sampler types and sampler conventions [5,7]
SAMPLE PREPARATION:
3.
4.
5.
6.
7. 8. 9.
NOTE: Successful use of this m etho d de pen ds o n clos e attention to details and uniform ity in sam ple handling procedures. Place a we ighed sa m ple co ntaining n o m ore th an 2 .5 m g SiO 2, or a m em brane filter sam ple, in a 250-m L Phillips beaker. Add 3 to 4 mL conc. HNO 3 to ash m em brane filters. Heat on a hotplate to absence of brown fumes. Add an additional 2 mL conc. HNO 3 and heat to dryness. Repeat this step until any remaining residue is white. NOTE: As h PVC filters using 2 m L conc. H ClO 4. Heat slowly until the filter shrinks, turns brown and is dissolved. A dd additional portions of H ClO 4 if necessary. If a perchloric acid hood is not available, see APP EN DIX B for alternate ashing procedures for PVC filters. Pretreatment proc edu res to rem ove variou s co ntam inants from bulk sam ples are o utlined in referenc e [7]. Add 25 m L 85 % H3PO 4 to the beaker. Cover the beaker with a bent-stem funnel. Make sure that the tip of the funnel stem touches the wall of the flask. Start reagent blanks at this time. NOTE: The bent-stem funnel is used to prevent too rapid dehydration of the sample and to prevent spattering. Place the 550-wa tt he ate r on the rotato r in a fume hood. Preheat the heater and adjust to maintain a tem perature of 240°C. Heat each sample and blank for exactly 8 min, swirling it by the action of the variable speed rotator. NOTE: The solution in the flask should reach 240°C during this procedure. Rem ove the beaker from the heater with padded tongs and swirl it for 1 min. Allow the be aker to cool. Ad d about 125 m L warm (60 to 70°C) de ionized water and swirl to m ix com pletely. Filte r the sam ple with suctio n through a 47-m m filter. W ash thoroughly with 1:10 H Cl. Place the 47-m m filter in the bottom of a 1 50-m L po lyethylene bea ker. Add 0.5 mL 48% HF to the filter surface. Float a thin polyethylene disc of about 50-m m diameter over the 47-mm filter and cover the beaker. Allow to stand for 30 min.
NIOSH Manual of Analytical Methods (NMAM), Fourth Edition ​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 ​SILICA, CR YST ALLINE : MET HO D 760 1, Issue 3, dated 15 M arch 200 3 - Page 5 o f 6 EVALUATION OF METHOD: Th is m etho d is ba sed on the we ll-characte rized T alvitie m etho d [1,8,9,10].
REFERENCES: [1]
[2] [3] [4]
[5]
[6] [7]
[8] [9] [10]
NIOSH [1977]. Silica, crystalline, Method P & CAM 106. In: Taylor DG, ed., NIOSH M anual of Analytical Methods, 2nd ed., Vol. 1. Cincinnati, OH: U.S. Department of Health, Education, and W elfare, Publ. (NIOSH) 77-157-A. Sweet DV, W olowicz FR, Crable JV [1973]. Spectrophotometric determination of free silica, Am Ind Hyg Assoc J 34:500-506. Eller PM, Feng HA, Song RS, Key-Schwartz RJ, Esche CA, Groff JH [1999]. Proficiency analytical testing (PAT) silica variability, 1990-1998. Am Ind Hyg Assoc J 60: 533-539. Key-Sch wartz RJ, Baron PA, Bartley DL, Rice FL, Schlecht PC [2003]. Chapter R, Determination of airborne crystalline silica. In: NIOSH M anual of Analytical M etho ds, 4 th ed., 3 rd Suppl. Cincinnati, OH: U.S . Departm ent of He alth an d Hum an S ervices, P ublic H ealth Service, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, DHHS (NIOSH) Publication No. 2003-154. NIOSH [1974]. Criteria for a recomm ended standard: Occupational exposure to crystallin e s ilica. Cincinnati, OH: U.S. Department of Health, Education, and Welfare, Health Services and Mental He alth Adm inistration , National Ins titute for O ccu pation al Safety and Health, DHEW (NIOSH) Publication No. 75-120. Inhaled Particles and Vapours [1961]. Pergamon Press, Oxford, U.K. HSE [2000]. General m eth ods for sam pling and gra vim etric analysis of resp irable a nd inh alable dus t: MDHS 14-3. In: Methods for the determination of haza rdou s su bsta nce s. Londo n: Health a nd S afety Executive. Talvitie NA, Hyslop F [1958]. Colorimetric determination of siliceous atmospheric contaminants, Am Ind Hyg Assoc J 19(1):54-58. Talvitie NA [1964]. Determination of free silica: Gravimetric and spe ctrophotometric procedures applicable to airborne and settled dust, Am Ind Hyg Assoc J 25:169-178. Talvitie NA [1951]. Determination of quartz in presence of silicates using phosphoric acid, Anal Chem, 23(4):623-626.
METHOD REVISED BY: Issue 1: Jensen Groff, NIOSH/DART Issue 2: Rosa Key-Schwartz, Ph.D. and Paul Schlecht, NIOSH/DART.
APPE ND IX A: INTERFERENCES S om e silicate s m ay be re sistan t to p hosphoric acid digestio n [8]. An y silicate s not rem oved by the phosphoric acid treatment will give a positive interference. A m orphous silica, if not c om pletely rem oved by phosphoric acid treatm ent, will give a positive interference . Phosphate ion complexes molybdic acid to produce a yellow pho sph om olybda te color which is dissipated by lowering the pH of the solution with 10 N sulfuric acid. Excess ferric ion (more than 1 mg) will consume the reducing agent and inhibit the reduction of silicom olybda te to molybdenum blue. If more than 1 mg ferric ion is present, it should be rem oved by pretreatm ent w ith 10:1 HCl:HNO 3.
NIOSH Manual of Analytical Methods (NMAM), Fourth Edition ​SILICA, CR YST ALLINE : MET HO D 760 1, Issue 3, dated 15 M arch 200 3 - Page 6 o f 6 APPENDIX B: ALTERNATE ASHING PROCEDURES FOR PVC FILTERS If a perchloric acid fume hood is unavailable, ash PVC filters with one of the following methods. 1.
M uffle furnace ashing: W ash the filters with 25% v/v HC l if the samples contain a significant amount of calcite (20% of the total dust loading); otherwise, proceed to next paragraph. Place a 0.5-:m , 47-m m PVC filter in the filtration apparatus. Center the sample filter on top of the first filter. Clamp the filter funnel over the frit so that th e dust deposit is completely exposed. Add 10 mL HC l solution and 5 mL 2-propanol to the filter funnel; allow to stand for 5 min. Apply vacuum to aspirate the acid/alcohol solution from the filter. W ash with three 10-mL portions of deionized water. Release the vacuum. Place the filter sam ples in porcelain crucibles, co ver loo sely and ash in a muffle furnace for 2 h at 600 °C (800 °C if graphite is present). Add several m L of 2 -propan ol to the ash , scra pe the cru cible to loosen all particles and transfer the residue to a 250-mL borosilicate glass Phillips beaker. W ash crucible several times and add wash to the beaker. Heat on a sparkless hotplate to evaporate the 2-propanol. W hen beaker is dry, proceed with step 4 of sample preparation.
2.
Low temperature ashing: Place the filters in 50-mL glass beak ers within the LT A s o that th e sam ple expos ure to the plasm a is o ptim ized. Ash accord ing to m anufa cturer's instructions. After ashing, add 2-propanol to the beaker to transfer the residue to a 250-mL borosilicate Phillips beaker. W ash the beaker with 2-propanol several times, adding the washings to the Phillips beaker. Heat on a sparkless hotp late to remove the 2-propanol. W hen the beaker is dry, proceed to step 4 of sample preparation.
NIOSH Manual of Analytical Methods (NMAM), Fourth Edition