Determination of Silica and R,O, in Portland Cement LYLE R. DAWSON'
AND RALPH V. ANDES Universal Atlas Cement Co., Gary, Ind.
T
HE determination of silica in cement and cement clinker usually ( I ) involves treatment of the ignited silica x i t h
sulfuric and hydrofluoric acids, followed by heating to drive off the silicon as silicon tetrafluoride and ignition of the residue to constant weight. The loss of weight effected by this treatment is considered to be total silica. The R2O3.2H20 precipitate is placed in the crucible, dried, ignited, and weighed to obtain the total R203which includes titanium and vanadium oxides as well as the oxides of iron and aluminum. I n this method the silica which passes through the filter and is finally precipitated with the R203.rHz0is not determined, but is reDorted as alumina. I n order to correct for t,his silica Some laboratories make a practice of fusing the ignited R203 with potassium pyrosulfate, dissolving the melt in dilute sulfuric acid, evaporating to lorn volume, and fuming to dehydrate silica. The dehydrated silica is filtered off and ignited, and the amount determined by hydrofluorization. This process is time-consuming and necessit'ates several operations, thereby increasing the possibility of operative errors. Table I presents data from a series of analyses of portland cements in which the silica remaining with the iron and aluminum oxides was determined as described above. Dehydration of silica was accomplished by double evaporations with hydrochloric acid ( I ) , and by the perchloric acid (4, 6) and ammonium chloride (3) methods. sample for analysis, t,he per cent of using a silica found by the usual method ( 1 ) would be too low by twotenths times the weight in milligrams of that remaining in the G O a ; the value for &03would be correspondingly too high.
1
1.8
1.6
Perchloria Acid si@t In si02 ~ 2 0 3 Mg. Mo. 1.1 1.5
2
0.5
1.4
0.5
1.6
3
1.3
1.9
0 6
1.6
4
0.8
1.3
1.6
0.6
5
2.2 2.3 1.3 1.5
1.7 0.9 0.9
1.7 1.8 2.6 2.2
1.0 1.0
1.0 1.0
0.5 0.6
NO.
Double Dehydration RtOs in $io, in SiOn ~ 2 0 2
MO.
6 7
Me.
1.8
R in 203
~ m m o n i u mChloride R?Oa In sjol si02 MQ. 'UQ. 0.9 7 1.3
$58
1.2 1.6 0.5 1.3
0.5 0.6 1 0 0.4 1.6 1.2 1.3 1.3 1.0 0.8
1.2 1.4 0 6 1.2 1.9 1.9 2.0 1.9 1.7 1.8
1.8 1.3
0,i 0.5
2.1 2.1
11.
TRE.lTJfEKT O F
RzOs WITH HYDROFLC-ORIC-SULFTJRIC
ACID SOLUTION 1
' ~ $ ~ e ~ ' , , ~ ~ ~ ~ ~and b l e 11.6470 lveipht after treatment with acids 11.6469
2
3
4
>
12.1517
12.0800
12.0491
12.0437
12.1517
12.0799
12.0492
12.0438
Combination Method for Silica and RzO, For rapid results of a fair degree of accuracy the filter papers containing the silica and Rz03. xH20 precipitates may be placed in a tared plat.num crucible, dried, ignited at 11500 to 12000 c.,
and weighed. The combined oxides are then treated with 6 drops of sulfuric acid (1 to 1) and 7 or 8 ml. of hydrofluoric acid. The solution is evaporated below boiling and the crucible and its contents are ignited to constant weight a t 1150" to 1200" C. The loss in weight represents total silica and the residue is total Rz03.
Dehydration of Silica
Precipitation of R 2 0 swith Bromothymol Blue as Indicator An aqueous solution of the sodium salt of dibromothymolsulfonphthalein may be used to advantage in precipitating R203. xH,O. T o prepare the indicator solution, 100 mg. of the dry commercial material are ground in an agate mortar with 1.6 ml. of 0.1 N sodium hydroxide and then diluted with water to 100 ml. Six or seven drops of indicator per 100 ml. of solution are sufficient. The precipitation is made by heating the filtrate from the silica, t o which indicator has been added, to about 90" C. and adding concentrated ammonium hydroxide dropwise with constant stirring until the color becomes blue-green. The solution is then boiled until it becomes pure green (may appear yellowgreen in the presence of a large amount of iron), when it is removed from the heat, allowed to stand until the precipitate settles, and atered. Washing and reprecipitation may be carried out as in the usual procedure.
Direct Hydrofluorization of Residual Silica The residual silica in the S O 3 may be hydrofluorized directly with accuracy and relative ease of manipulation. Treatmentof the ignited ~~0~with 6 drops of sulfuric acid (1 to 1) and about 5 ml. of hydrofluoric acid, followed by evaporation below boiling, serves to remove any silica. The crucible is then ignited to constant weight at 1 0 5 0 ~to 1100" C. 1
TABLE
I n either of the methods described any recognized procedure for dehydrating silica may be used. The ammonium chloride method developed by Maczkowske (3) a t the Kational Bureau of Standards, with some slight modifications, was used in this work. I n this laboratory cleaner silica precipitates have been obtained by dehydrating on a n asbestos pad on a hot plate and by washing the silica on the filter three times with hydrochloric acid (1 t o l), alternating with hot water. Hot water is finally used to finish the washing. llaczkowske's original recommendations were to dehydrate on a water bath and to wash the silica on the filter with hot water only.
TABLE I. RESIDEAL RzOsAND RESIDUAL SILICA Sample
The weight of silica removed is subtracted from the total RlO; and added to the weight of silica previously hydrofluorized. Care must be exercised to prevent loss of elements other than silicon as fluorides during the evaporation process. I n the presence of excess sulfuric acid and if the solution is evaporated below boiling, it appears that no such loss of any consequence occurs. To test this point, ignited residues containing only iron and aluminum oxides were treated with the hydrofluoric-sulfuric acid mixture, evaporated to dryness, ignited, and weighed. The results are given in Table 11.
Present address. Louisiana Polyteohnic Institute, Ruston, Ls.
138
ANALYTICAL EDITION
MARCH 15, 1940 TABLE111.
scribed above, with those obtained by double dehydration of METHODSFOR DETERMISATION silica with hydrochloric acid followed by fusion of Rz03with SILICAA N D R203IN PORTLAXD CEMENT pyrosulfate to recover the residual silica; also with values NHdC1, ”4CI, obtained by dehydration with perchloric acid followed by pyResidual SiO: Double Si02 Yolarilised rosulfate fusion of the &03. Dehydration, HCIOI. Volatilized irom
COhlPARIsON O F
OF
Sample
KtS20; Fusion
irom RzOa
20.52
20.48 20.54 8.16 8.06
No.
Deterrnination
1
SiOt
Fusion 20.50
RaOa
8.16
8.20
Bioi
2
3
4
5
22.14
22.12
7.86
7.82
Si02
19.86
19.78
RzOs
8.52
8.84
+
Summary
22.10 22.20 7.72 7.68
Combined Rn0a 20.56 20.68 8.02 8.12 22.20 22.22 7.68 7.58
19.76 19.82 8.68 8.70
20.10 20.10 8.50 8.62
From this work it appears that in the analysis of portland cement and cement clinker, the silica remaining with the ignited Rz03 may, under carefully controlled conditions, be determined by direct hydrofluorization. A rapid method, in which the silica and R2O3.sH20precipitates are combined, has been tested. Fairly accurate values may be obtained, although there is some evidence that the value for silica may tend to be slightly high. This may be due to loss, as fluorides, of a small amourit of titanium or other elements ordinarily counted in the R203. The use of bromothymol blue in precipitating the hydrous oxides of iron and aluminum has been tested and found to give excellent results.
SiOz
Si02
21.06
20.94
R2Oa
8.62
8.72
20.95 21.08 8.66 8.58
21.02 21.06 8.62 8.54
SiOn
20.78 20.74 8.70 8.74
20.80 20.86 8.62 8.58
20.88 20.88 8.64 8.5i
20.90 20.86 8.46 8.61
19.46 19.48 9.56 9.52
19.36 19.32 9.58 9.48
19.64 19.66 9.36 9.34
19.66 19.72 9.36 9.28
21.62 21.82 7.86 7.86
21.42 21.50 7.66 7.68
21.74 21.86 7.76 7.76
21.68 21.72 7.84 7.82
Si02 Rn03
SiOn
7
KZs207
RzOs
Rz0a 6
139
RzOs
This indicator is chosen because its color change is distinct and covers the p H range (6.0 to 7 . 6 ) in which the isoelectric point of hydrous aluminum oxide occurs ( 2 ) . Several tests hare indicated that no manganese is precipitated with the R2O3.2H20by this method. -Table I11 presents a comparison of values for silica and RZ03in commercial cements obtained by the methods de-
Acknowledgment The authors wish to express their appreciation t,o H. L. Tschentke, B. F. Erdahl, and L. (2.Zack for their assistance with certain analyses.
Literature Cited (1) Am. Soc. Testing Materials, Committee G l , Standards on Cement, pp. 18-20 (December, 1938). (2) Kolthoff, I. >I., and Sandell, E. B., “Textbook of Quantitative Inorganic Analysis”, p. 304, New York, Mai:millan Co., 1936. (3) Macnkon-ske, E. E., J . Research Natl. Bur. Standards, 16, 549-52 (1936). (4) hfeier, F. IT.. and Fleischmann, O., 2. anal. Chem., 88, 88 (1932). ( 5 ) TTillard, H. H., and Cake, W. E., J . Am. Chem. Soc., 42, 2208 (1920).
Toximetric Method for Oil-Soluble Wood Preservatives ERNEST E. HUBERT, Western Pine Association, Portland, Ore. The method described in this article is a correlation of the ideas and laboratory technique developed by the following members of the Preservative Standards Advisory Committce of the Yational Door Rlanufacturers Association and was prepared for publication by the chairman: S. 0. Hall, National Door 3Ianufacturers Association; Geo. 31. Hunt, Roy H. Baechler, C. Audrey Richards, and Theo. C. Scheffer, Forest Products Laboratory; Ira Hatfield, AIonsanto Chemical Co. ; Dale Chapman, A. D. Chapman and Co., Inc.; Gardner Garlick and William Bradley, Protection Products Rlanufacturing Co. ; J. B. )Iellecker, Curtis Companies, Inc.; D. K. Ballman, Dow Chemical Co.; F. H. Kaufert, E. I. du Pont de Nemours & Co., Inc.; J. 0. Frank, RIorgan Co.; Henry Schmitz, University of RIinnesota; and Ernest E. Hubert, Western Pine Association, chairman.
D
i G the past few years interest in treating millwork UR1y products with oil-carried toxic chemicals (1-6) has developed steadily, and as a result of the rapid and extensive adoption of this type of wood preservative by millwork fac-
tories there has arisen the need for a laboratory test method whereby the numerous proprietary slash preservatives could be evaluated for their toxic properties. The launching, by the Kational Door hlanufacturers Association in 1938, of the “Seal of Approval” program (4)which has as its objective the standardization of all preservative treatment operations and preservatives in licensed mills, has also been a prime factor in the development of a standard toximetric laboratory testing method. The main purpose of this program is “to provide for the identification of windows, frames, and other architectural wood products which have been preservative-treated in accordance with minimum standards of excellence as a protection to the consumers and distributors of such products”. The program also provides for the use by millwork manufacturers of an identification mark, the “Seal of Approval”, to be branded into the treated products as a means of maintaining the high standards of preservative treatment. The preservative minimum standards of the K. D. h1. A. establish the minimum qualities of the toxic* chemical, such as toxicity and permanence in wood; and of the treating solution, such as concentration of toxicant, flash point, volatility, and leachability. Penetration and absorption of
