Ind. Health, 1964, 2, 181.
DETERMINATION OF FREE SILICA BY
PHOSPHORIC ACID METHOD (I)
Akira HAMADA
National Institute of Industrial Health, Kizuki-Sumiyoshi, Kawasaki
(Received May 11, 1964)
The phosphoric acid method for the determination of free silica mixed in silicates
was improved in its reproducibility by applying the copper mantle as a heat radiator.
The superiority of this method to the electric heater method was proved by the observa-
tion of the dissolution velocities of several kinds of quartz samples using the two
methods.
The dissolution velocities were studied on quartz powders which were produced by
three different methods by observing their dissolution in hot phosphoric acid using
copper mantle and it was cleared that the surface of quartz powder was denaturalized
by the grinding of quartz and this part was dissolved in phosphoric acid with far higher
velocity than the original quartz. The appearances of these quartz were observed
before and after the treatment with phosphoric acid under the optical microscope, and
the denaturalized surface was demonstrated clearly by the phase contrast microscope
with polarizer.
The obtained value of non-soluble part in phosphoric acid had been taken as free
silica but the observations in this experiment showed that the part of quartz denatura-
lized during the grinding was soluble and this part was accounted as non-free silica.
The phosphoric acid method 1,2,3) is a simple and useful method for the deter-
mination of free silica mixed in silicates. The advantage of this method is that
it can be practised without any special apparatus and techniques. The principle
of this method is to obtain the difference of the weight before and after the treat-
ment with hot phosphoric acid, because the solubility of free silica differs from
that of silicates in hot phosphoric acid.
This method, however, contains some factors which are liable to affect the
reproducibility ; that is, the particle size of the samples, the method of pretreat-
ment of the samples, the method of removing the gel precipitate in the acid which
disturbs the following processes, the problem of the silicates which dissolve in
phosphoric acid at different velocities and especially the problem of the steady heating technique.
Talvitie 1) developed the precision heater to improve reproducibility. Schmidt 2)
used the heater using copper mantle which is easier to be constructed than the
heater which Talvitie used.
181
A. HAMADA
The loss of quartz by dissolution in hot phosphoric acid is generally a few
per cent of the whole quartz in the case of large particles but it reaches as far
as more than ten per cent in the case of fine powder.1,4, 5) The estimation of the
amount of loss is difficult in the latter case.
In the present paper, the effect of the heating method on the dissolution
velocity was studied using the electric heater or the copper mantle with electric
heater. The effect of the preparing methods of quartz powder on the reproduci-
bility of the determination of free silica was also observed. Besides, the solubilities
of some kinds of quartz powders in phosphoric acid were measured. The appear-
ances of quartz powders before and after the treatment by phosphoric acid were
studied under the obtical microscopy 6, 7).
EXPERIMENTALS AND RESULTS
Materials and Apparatus
Electric heater : 600W electric heater for the home use was covered with plates
and fibers of asbestos. The temperature was regulated with the slide regulator
and was indicated by the thermometer.
Copper mantle : Copper column of 100 mm in diameter and 120mm in height hav-
ing a heating hole of 40 mm in both of diameter and depth, was placed on the
electric heater. The temperature was regulated in the same manner as the
electric heater.
Pulverizing tools : For the mechanical grinding, agate pestle was driven mechani-
cally at the rotation speed of 120 rpm in the agate mortar of 150 mm in diameter.
For handgrinding, agate mortar of 90 mm in diameter and agate pestle were used.
For crushing, lumps of quartz were treated at the speed of 75 blows per minute
in a stamp mill with iron mortar of 110 mm in diameter and 5.8 kg pestle.
Sieve : Brass sieves of 40 and 200 mesh were used.
Microscopy : Negative-medium-contrast phase microscope was used with the light
source of 6 volts and 5 amperes.
Light polarizer : Polarizing plate of 30 mm in diameter was used in front of the
mirror.
Refractometer : Abbe's refractometer was used to measure the refractive index of
liquids in the immersion method. The standard liquid was the mixture of the
ƒ¿-mono-chloronaphthalene and terpene oil, this immersion liquid had the refractive
index of 1.548, which was the same with the mean value of ƒÃ and ƒÖ of quartz.
Platinum crucible : 10 cc in the capacity was used for the hydrofluoric acid
treatment.
Quartz : Massive blocks from Ishikawayama were used.
Reagents : Phosphoric acid, hydrofluoric acid, hydrocloric acid, boric aicd and
sulphuric acid of guaranteed grade were used. Filter paper with no ashes was used
182
DETERMINATION OF FREE SILICA
without any further treatment.
Method and Results
The following three experiments were carried out ; 1) the comparison of the
two heating methods, 2) the measurment of the dissolution velocities of the quartz
particles produced by the three different methods and 3) recovering efficiency of quartz in the known mixed samples of quartz and feldspar by phosphoric acidmethod.
1. Comparison of the electric heater method and the copper mantle heat radiator
method as a heating apparatus.
To compare the reproducibility of these two methods, quartz in phosphoric
acid was heated by the electric heater and by the copper mantle heat radiator
(shown in Fig. 1) placed on the electric heater, and the dissolution velocity of
quartz by each method was determined. Experimental procedure was as follows.Crushed quartz powder (100mg ; smaller than 200 mesh)
•© Add phosphoric acid (6cc)
Heating by electric heater (200•Ž, 290•Ž, 300•Ž and 410•Ž ; f 10•Ž)
by copper mantle on electric heater (280•Ž ; •}2•Ž)
Washing of the residue
Weighing of the dried residue (P mg)
Fig. 1. Copper mantle.
183
A. HAMADA
Then, the dissolved amount of the sample was expressed as percentage by the
following equation,
W(%)= (100-P)
The results are shown in Fig. 2, which indicates that the dissolution of quartz
progressed linearly with heating time in the copper mantle heat radiator but it
did not linearly in the method of electric heater only.
Beside the inhomogeneity of heating of quartz, the different states of the quartz
surface may be another cause of disturbing the precise analysis of free silica, as
quartz particles show the various surface states according to the producing methods
of the particles. Dissolution velocities of the quartz particles which were from
the same source but pulverized by three different methods were determined by the
phosphoric acid method using the copper mantle.
2. Dissolution velocities of quartz particles prepared by three different methods.
The methods of particle preparation were as follows.
(1) Crushing : quartz blocks of 2 cm size were crushed by a stamp mill, and
the particles of 10-50/ƒÊ size produced in this treatment were used as
experimental specimens.
(2 Braying by hand : quartz particles smaller than 40 mesh were brayed in
a small agate mortar by hand, and two sorts of particles smaller than
200 mesh and 2ƒÊ respectively were used as experimental samples.
(3) Grinding quartz particles smaller than 40 mesh were ground in an agatemortar by the grinding machine and the 2 samples of powder under
200 mesh and 2ƒÊ were prepared.
The whole method was the same as described in the former experiment. The
Fig. 2. Amount of dissolved quartz by heating with two different methods.
184
DETERMINATION OF FREE SILICA
results are shown in Fig. 3. The dissolved amounts of quartz powders smaller
than 200 mesh were as follows : 7.6-8.0% in the case of crushed powder, 17.85-31.65
% in the ground powder. Generally, quartz dissolved rapidly until about ten minutes, and then slowly in all methods. The amounts of dissolved quartz were
Fig. 3. Weight loss of various quartz samples by heating.
Fig. 4. Micrograms of quartz particles in the 1.548 liquid as observed with
a 20 •~ (0.40 N.A.) negative-medium-contrast phase objective and a 15 •~
eyepiece.
185
Left : Before treatment with phosphoric acid.
Right : After treatment with phosphoric acid.
A. HAMADA
differerent according the preparing method of particles and the mechanicaly ground
quartz showed a highest value of the dissolved quartz, and the crushed the lowest.
As this difference was assumed to be produced by the surf ace characters of quartz
particles prepared by the three methods, the particles were examined optically.
The surface of the ground particles were optically denaturalized and the refractive
index of that part dropped to about 1.544 or smaller. Their appearances studied
by the negative-medium-contrast phase microscope with polarizer and immersion
liquid of suitable refractive index are shown in Fig. 4.
3. Test of the method using the known samples of quartz.
The denaturalized surface of quartz by grinding is easily dissolved in phos-
phoric acid contrary to the original quartz, and this part of quartz is determined as
non-free silica by the phosphoric acid method. Therefore, if we neglect this
denaturalized surface, amounts of quartz determined by the phosphoric acid method
would not coinside with the previously weighed amount of quartz in the artificial
samples of mixed minerals. To investigate this problem the artificial samples of
the following compositions were analysed by the phosphoric acid method.
The mixtures of crushed quartz and crushed feldspar (both were
smaller than 200 mesh), containing quartz of 65, 25, 15, 8, 6, 4 and 2%
of the whole weight.
The quartz in this sample contained non-soluble part of 91.73 % was determined
in the previous experiment. Feldspar was already proved to be dissolved comple-
tely in phosphoric acid.
6m1 of orthophosnhoric acid in a 20-ml beaker
Heating for 6 Minutes by copper mantle (280•Ž; •}2•Ž)
•© Add sample powder (100mg; smaller than 200 mesh)
Heating for 6 minutes by copper mantle (280•Ž ; •}2•Ž),
swirling the beaker for 3 seconds at 1 minute intervals
•© Dissolve the sirapy phosphoric acid, and add 2ml
of fluoboric acid
Standing the beaker for 1 hour
Washing of the residue
Weighing of the dried residue (P1 mg)
•© Add 1 drop of sulfulic acid (1 : 1)
and 2m1 or more of hydrofluoric acid
Volatilizing the acids
Weighing of the residue (P2 mg)
Free silica =P1 - P2 (%)
Fig. 5. Analytical processes.
186
DETERMINATION OF FREE SILICA
These samples were analysed by the processes shown in Fig. 5. The results
obtained coincided with the estimated values as shown in Table 1. The differences
between the results and the estimations were in the range of 0.36-0.66 %.
DISCUSSIONS
Change of the Dissolution Velocity with Time
Fig. 3 shows that the dissolution velocity of the quartz powder is large at the
initial period of dissolution and after a certain period it dropps to a very small
value. The amount dissolved during initial period is about 7-40% of the samples.
And these values are affected by the preparing conditions of the powder. Generally,
the dissolved amount of quartz increases with the grinding time and also increases
inversely with the particle size.
Denaturalization of the Surface of Quartz
Denaturalization of quartz surface was observed under the negative-medium-
contrast phase microscope in the immersion method. If refractive index of the
immersion liquid is 1.548, the original quartz should show pink or pale yellow.
The quartz powder really showed these characteristic colours after the treatment
with phosphoric acid, but blue was observed at the outer layer of the ground
particles. The refractive index of this blue part was about 1.544 or smaller These
denatura-lized parts increased with the grinding time of the particles. When
the outer layer was completely changed by long time grinding, the original colours
of quartz could not be observed. These parts were dissolved by phosphoric
acid with high velocity and counted as the non-free silica contrary to its original
part in the determination of the free silica by phosphoric acid method.
Table 1. Results of the test analyses.
187
A. HAMADA
ACKNOWLEDGEMENT
The author is gratefully obliged to Dr. Hiroyuki Sakabe for his encouragement
to start this work.
REFERENCES
1) Talvitie, N. A. (1951). Anal. Chem., 23, 623.
2) Schmidt, K. G. (1954). Ber. Deut. Keram.Ges., 31, 402.
3) Hamada, A. (1952). Ann. Rept. Silicosis Res. Lab., 34. (in Japanese)
4) Jephcott, C. M. & Wall, H. F. V., (1955). A. M. A. Arch. Ind. Health, 11, 425.
5) Schmidt, K. G. (1960). Staub, 20, 404.
6) Schmidt K. G. (1955). Staub, Heft 41, 436.
7) Hamada, A. (1955). Ann. Rept. Silicosis Res. Lab., 24. (in Japanese)
188