shg Phosp' N. A. TAI.VITIE
U. S. Public Health Serv,ice, Cincinnati, Ohio Present standards for the evaluation of the silicosis hazard involved in hreathing dusty air call for determinations of the free silica content of dusts and dust-producing materials. A method i s described for aooomplishing these determinations when silicate minerals are present. The silicate minerals are dissolved by heating the sample of dust with phosphoric acid. After dilution of the phosphoric acid with water, the residue of free silica i s separated by filtration and weighed. A heating period of 12 to 20 minutes suffices to dissolve most of the silicate minerals commonly present in industrial dusts. The method is adaptable to the rapid determination of quartz in a wide variety of rooks, ores, clays, and industrial dusts, and requires only the simplest of laboratory equipment for its exeoution.
with a high d egree of success to dust samples from a variety of sources. APPARATUS AND REAGENTS
Figure 1 illustratetes the apparatus now in use for the determination, The container for the treatment of the sample i s a 250ml. borosilicate glass, Phillips, conical beaker with spout (Corning Catalog No. 1080). Although a considerable amount of glass dissolves from the beakers during tho course of the treatment, this in no way interferes with the analysis and is offset by the advantage of conducting the dilution of the treated sample in the Same container. Too rapid a dehydration of the phosphoric acid and loss of sample by spattering are prevented by covering the beaker mith a small funnel, the stem of m.hihich is sharply bent to make contact with the side of the beaker. The stem, in addition, is drawn out into 8 point or beveled to facilitate return of condensed and spattered liquid down the side of the beakcr.
As source of heat, a 550-watt., 115-volt, Type RH, Precision heater (Precision Scientific Co.) is used. The rheostat of the heater is adjusted to apply 75 volts across the heating clement. The beaker i s Dlsced directly an the element of the heater without
E .
VALUATION of the silicosis hazard involved in dusty operations calls for determinations of the freesilica content of dusts and dusbproducing materials. Chemical methods having general application t o these determinations are based on the use of reagents which decompose and dissolve all of the constituents of the dust except free silica. which is then filtered, ignited, and weighed. The necessity of distinguishing between free and combined silica resolves the problem into the selection of a reagent which attacks silicate minerals strongly but leaves the free silica. unaltered. The most promising reagent for the decomposition of silicates appears to he phosphoric acid as used by Hirsch and Dawihl ( 8 ) for the direct determination of quartz in clays. Phosphoric acid has the fortunate property of farming watereoluble complexes with both silicic acid and metallio oxides. Upon heating R silicate mineral with 85% phosphoric acid, vigorous boiling takes place accompanied by an increase in temperature. The boiling subsides as the acid approaches 100% concentration a t about 220" C. With further heating between 220" and 250' C. the orthophosphoric acid, by lass of water, convert8 to pyrophosphoric acid and simultaneously dissolves the d i c a t e mineral. The clear, viscous solution obtained remains stahle upon dilution with water, permitting a simple separation from quartz by filtration through paper. Close control of the heating period is needed to minimize solution of quartz and to avoid preoipitation of the dissolved silica from too great a dehydration of the phosphoric acid, Steger (6),in applying this method t o the analysis of industrial dusts, conducts the treatment in a 30-ml. platinum crucible covered with a bored watch glass through which extends a thermometer. The sieved sample (0.5 gram) is weighed into the crucible and mixed with 25 ml. of phosphoric acid which has been previously concentrated by heating slowly to 250 'C. in a platinum dish and cooled. The mixture is heated rapidly t o 250" C. and held a t this temperature for 5 to 15minutes, after whioh the solution is oooled to 60" C . , diluted to 200 mi. with water, and filtered. In the procedure described herein, the author arrived a t similar oonclusions on the conditions for the use of phosphoric acid. For the past 2 years this procedure has been applied routinely
Figure 1. Apparatus for Heating Samp with Phosphoric Acid
623
ANALYTICAL CHEMISTRY
624
cushioned with rubber tubing are used for handling the hot beakers. Continuous mechanical swirling of the beaker with the Yankee Rotator (Clay-.-Adams Co.) has been found desirable when analyzing large numbers of samples. A retainer for the beaker may be constructed of 0.25-inch Transite with a hole slightly larger than the bottom of the beaker and fitted flush with the heating element. About 5 volts less is required when the retainer is used. The fluoboric acid which is used in the procedure is prepared by pouring, with continuous stirring, 1 pound of 48% hydrofluoric acid into a mixture of 450 ml. of water and 250 grams of boric acid crystals which is contained in a Bakelite or hard rubber vessel set in a pan of ice water. Sufficient heat is evolved by the reaction to dissolve all of the boric acid, part of which should recrystallize upon cooling to room temperature. While glass cannot be used for storing the acid, glass apparatus may be used for filtering and measuring the cool acid. PROCEDURE
Grind the material to pass through a 200-mesh sieve, mix thoroughly, and weigh out 0.5 gram or less on a watch glass and transfer to a 250-ml. Phillips beaker by means of a small camel's hair brush. Add 25 ml. of 85% orthophosphoric acid and cover the beaker with a funnel. Place the beaker directly on the element of a Precision heater M hich has been allowed to preheat for 45 minutes, and begin timing with a stop watch. When boiling subsides, begin swirling the beaker for 3 seconds at 1-minute intervals in order to minimize superheating and to keep the sample distributed throughout the acid. At the end of 12 minutes remove the beaker from the heater and swirl for 1 minute to dissolve any gelatinous silica which may have deposited on the side of the beaker above the acid. Set the beaker on a cool surface and immediately but cautiously remove the funnel in such a way that adhering liquid runs down the side of the beaker. Allow to cool to room temperature and then wash down the sides of the beaker rapidly with 125 ml. of hot (60" to 70" C.) water and swirl vigorously until the sirupy phosphoric acid is completely dissolved. Wash down the upper part of the beaker with 10 ml. of fluoboric acid and swirl to mix. Then wash down the upper part of the beaker with 25 ml. of water and allow t o stand for 1 hour. Filter through a retentive paper into R hich has been introduced a small amount of a suspension of paper pulp. Transfer the residue quantitatively to the filter with water and wash thoroughly first with cold and then with hot 1 to 9 hydrochloric acid, followed by one or two washings with water. Ignite the filter paper in a tared platinum crucible, first a t low heat to char the paper, and finally a t about 950" C. Cool the crucible in a desiccator and weigh. Moisten the residue with 1 to 1 sulfuric acid and add 5 ml. or more of 48% hydrofluoric acid. Heat gently until the quartz appears completely dissolved, then increase the heat to volatilize the acids. Repeat the treatment of the residue with sulfuric and hydrofluoric acids to ensure complete volatilization of the silica, then ignite a t about 950" C. for a few minutes, cool, and weigh. EFFECT ON SILICkTE BIMERALS AND OTHER MATERIALS
A group of representative silicate minerals was subjected to phosphoric acid treatment according to the above procedure to determine the effectiveness of the attack. For comparison the loss in quartz of the same particle size was also determined. The results, reported in Table I, show that although a large number of silicate minerals may be completely dissolved from a mixture without excessive loss of quartz, a feTT minerals are highly resistant to phosphoric acid. The resistant minerals are of uncommon occurrence especially in industrial dusts and, when present, are readily identified by microscopic examination of the residue from the phosphoric acid treatment. A correction is then made by computing the xeight of the silicate mineral from the weight of the residue from the hydrofluoric acid treatment and subtracting this value from the weight of the residue from the phosphoric acid treatment. The results in Table IV obtained with a sulfuric acid method illustrate this method of correction. The solubilities reported in Table I represent rates of solution rather than conditions of saturation and are, therefore, variable with the particle size of the mineral but not with the weight of the mineral present in the sample analyzed. I n a 200-mesh sample containing orthoclase and quartz, 93% of the orthoclase and 1% of the quartz may be expected to dissolve in 12 minutes regard-
Table I. Effect of Phosphoric Acid Procedure on Silicate Minerals and Quartz Ground to Pass 200-Mesh Sieve Minerals Which Dissolved Completely within 12 Minutes Actinolite Almandite Amphibole Chrysolite Dickite Diopside Epidote Grossularite
Halloysite Hornblende Idocrase Kaolinite Labradorite Montmorillonite Muscovite Prochlorite
Sericite Serpentine Spessartite Sphene Wollastonite Zoisite
Per Cent of Resistant Ninerals Dissolved in 12 Minutes and Time Required for Complete Solution
% Anthophyllite Oligoclase Orthoclase Enstatite Pumice Tremolite Talc Perlite Albite
97 95
93 92 87 85
74 74 70
Min. 14 14 14 14 14 14 14 16 16
53 41 38 28 20 14
Andalusite Spodumene Pyrophyllite Sillimanite Kyanite Tourmaline Beryl Topaz
3
3
Min. 18 20 20 20 20 >20 >20 >20
Per Cent Quartz Dissolved a t Various Times Heating time, min. Quartz dissolved, %
12 1.0
14 1.4
16 3.3
18 4.2
less of the actual weight of the sample taken for analysis or of the ratio of orthoclase to quartz in the sample. Table I serves as a guide in identifying an incompletely dissolved mineral in a sample of unknown composition as well as a guide in establishing the heating time required for samples in which the components are known. The relative solubilities of the silicate minerals with a 12-minute treatment follow in general those presented by Durkan ( 1 ) in a report of a phosphoric acid method for free silica. Investigations of niaterials other than naturally occurring silicate minerals were made concurrently with analyses of samples in which they occurred and an appropriate pretreatment was adopted when necessary. In general, materials soluble in or decomposed by sulfuric and hydrochloric acids are also soluble in phosphoric acid. Dusts containing appreciable amounts of organic matter cause frothing of the phosphoric acid. These are weighed into a porcelain crucible and the organic matter is destroyed by ignition a t 700" C. The residue is crushed with a stirring rod and transferred to a Phillips beaker for treatment with phosphoric acid. If alkalies or carbonates are also present in the dust, these should be dissolved in dilute hydrochloric acid and the residue filtered, washed with 1 to 9 hydrochloric acid, and then ignited; otherwise some of the quartz may be altered during the ignition. Carbon and graphite do not interfere and need not be removed. Some sulfide minerals are insoluble in phosphoric acid. Although these may be dissolved merely by adding a few drops of nitric acid to the phosphoric acid, they are best removed beforehand by heating a weighed sample in a beaker with 10 ml. of concentrated hydrochloric acid and sufficient nitric acid to dissolve the sulfide. After dilution, the residue is filtered, washed, and ignited. Because silicic acid may be formed by the action of hydrochloric acid on some silicates, the solubility of silicic acid in phosphoric acid was determined. Precipitated silicic acid ignited a t 750 O C. for 1 hour dissolved to the extent of 99.6% in 12 minutes of heating with phosphoric acid. Similarly, silicic acid ignited a t 1000" C. dissolved to the extent of 98.4%. Corundum and silicon carbide, which are frequently present in foundry dusts, are only slightly affected by either phosphoric or hydrofluoric acid. Correction is simply a matter of subtracting the weight of the hydrofluoric acid residue from the weight of the phosphoric acid residue. The results in Table IV obtained with the phosphoric acid method were corrected in this manner. Glass, slag, rock ~vool,porcelain enamels, and similar amor-
V O L U M E 2 3 , NO. 4, A P R I L 1 9 5 1
625
plious nixtc~ridsof indeterminatr compositioir are rcatlily dissolved by phosphoric acid. Ceramic rarv materials are analyzed without much difficulty, but less success may be had with thew materials fired a t a high temperature because of the possible conversion of the silicates to less soluble forms such as mullite and sillimanite, and the conversion of quartz into the more soluble forms, tridymite and cristobalite. This is in agreement x i t h Ricke (d), who found that the phosphoric acid method gives onl:. approximatt, results for quartz in porcelain.
6
4
2
EFFECT ON QUARTZ I-
Control samplrs of pure quartz ground to pass a 200-mesh sieve and analyzed by the described procedure showed losses in the neighborhood of 1%; the losses were sufficiently uniform for a single set of conditions to permit the use of a predetermined value to correct for losses of quartz in all sample8 in which the grain size of the qu:trtz \\'as reduced h y grinding to pass a 200-mesh sieve. The prtrticle size of the quart,z in nian\. materials such as (*laysand air-borne dusts is, of course, much smaller and, in addition, the particle-size distribution may vary widely from sample to sample. I n order tjo determine the applicability of the method to materials of this type, thc rates of solution of several sizr ranges of quartz obtained by air elutriat,ion werc irivestigated. Figure 2 shows that the analysrs of these kir dusts may I)P in i~onsiderahleerror unless a mrans is availahle t o rstimate t h r aniount i)l' quartz lost l)y ~ o l u t i o i i . 'I'ahle 11.
a
k!' a
Y
A VI
I c Iv
a a
Derivation of Correction Factor for Idofis of Quartz in Analysis of Fine Dusts Beaker
dtandnrd Sainple
No.
Quartz Found, % 12 min. 1 i min. TI. Ai. .1 R 1on.o - R ..I
1, 2 3. 4 3. 6
42
Sample 10 t o 2 0 p quartz
5 to lop qi1srtz 0 t o 1011 quartz Kaolin with 0 to ,511
TL/AL
A 1.8 2.6 2.4 1.8 2.6 2.4 1.8 2.6 2.4 1.8 2.6 2.4
B 98.8 98.8 98.8 98.2 97.S 98.1
92.4 92.2 92.2 17.4 17.3 17.5
Present,
%
+
C A ' B - C) C 97.7 99.7 97.9 100.2 97.9 100.1 96.8 9 9 , ~ 96.6 99,7 96.8 99.9 87.0 96.7 86.6 101.2 87.3 99.1 16.5 18.1 16.6 18.4 16.8 18.5 ~
~
99.7
99.7
99.8 18.6 ~~
~
14
I5
16
Figure 2.
Loss of Quartz by Solution in Phosphoric Acid
