permit a high degree of flexibility in the control of experimental conditions and are very selective for vanadium. Reagents (2, 14, 16, 18, 20) requiring adjustment of the pH within a narrow range or which are used without buffer (20)are not so convenient, and in this respect the use of a strongly acidic medium with p-TTHA and PTHA is a distinct advantage. Both p-TTHA and PTHA equal their analog PBHA (9, 10) in the selectivity and simplicity of vanadium determination but excel it in sensitivity. The method described here should prove valuable in the direct determination of vanadium in low grade ores, alloys, petroleum products, and biological materials. ACKNOWLEDGMENT
The authors are indebted to the Director, National Chemical Laboratory,Poona, for permitting S. G. Tandon
t o work as guest worker. They also thank V. S. Pansare and colleagues for the microanalyses of compounds. LITERATURE CITED
(1) Bhaduri, A. S., Ray, P., Sci. and Culture (Calcutta)18, 97 (1952). (2) Das Gupta, A. K., Singh, M. M., J . Sn’. Znd. Research (India) l l B . 268 (1952). (3) Dyrssen, David, Acta. Chem. Scand. 10,353 (1956). (4) Feigl, F., Anal. Chim. Acta 2, 397 (1948). (5) Hillebrand, W. F., Lundell, G. E. F., Bright, H. A., Hoffman, J. I., “Applied Inorganic Analysis,” 2nd ed., p. 458, Wiley, New York, 1953. (6) Jones, L. W., Hurd, C. D., J . Am. Chem. SOC.43,2422 (1921). (7) . . McCabe, C. R., Chem. Enp. - 13,. 243 (1911). ( 8 ) Meloan, C. E., Holkeboer, P., Brandt, W. W., ANAL.CHEM.32,791 (1960). (9) Priyadarahini, Usha, Tandon, S. G., Ibid., 33,435 (1961). (10) Ryan, D. E., Analyst 85, 569 (1960). (11) Sandell, E. B., “Colorimetric Deter-
mination of Trscea of Metah,” pp. 83, 97, 925, 3rd ed., Interscience, New York, 1959. (12) Schaal, R. B., Znd. Eng. Chem. 13, 698 (1921). (13) Shome, S. C., ANAL.CHEW23, 1186 (1951). (14) Staten, F. W., Huffman, E. W. D., Zbid., 31,2003 (1959). (15) Talvitie, N. A., Zbid., 25, 6‘04(1953). (16) Tandon, S. G., Bhattacharyya, S. C., Zbid., 32,194 (1960). (17) Vosburgh, W. C., Cooper, G. R., J . Am. Chem. SOC.73,437 (1941). (18) Wallace, G. W., Melloh, M. G., ANAL.CHEM.32,204 (1960). (19) Weat, P. W., J . Chem. Educ. 18, 528 (1941). (20) Wise, W. M., Brandt, W. W., ANAL. CHEM.27, 1392 (1955). (21) Wright, E. R., Mellon, M. G Ixb. ENQ.CHEM..ANAL. ED. 9. 25? (22) (1037). Yale, H. L., Chem. Revs. 33, 209 (1943).
RECEIVED for review November 16, 1960. Accepted March 22, 1961. Contribution Number 426 from the National Chemical Laboratory.
Gravimetric Determination of Silicon in the Presence of Boron LASZLO C. PASZTOR Graham Research Laboratory, lones & Laughlin Steel Corp.,
b A new method is described for the gravimetric determination of silicon in which the interference of boron is eliminated. The sample is dissolved in acid, and the silicic acid is dehydrated in the presence of glycerol and precipitated with gelatin. The method is rapid and accurate. It was applied successfully to the determination of silicon in iron, steel, slag, and ore samples, including samples with various boron concentrations.
S
with conventional mineral acid procedures confirmed reports that boron interferes in the determination of silicon, as reported by Lundell and Hoffman (3). The silicon results, especially in the presence of larger quantities of boron, were unreliable and usually high. Boron is rendered easily insoluble a t the higher temperatures used for the dehydration of silicic acid and coprecipitates. Furthermore, boron forms a volatile compound with fluoride and is covolatilized during the usual HF volatilization of silicon, which eliminates other interferences. Preliminary separation of boron by careful methanol distillation procedures is quite time consuming and inconvenient. However, detailed investigation of the Ha01 and HC1-H2SOd procedures showed that with very careful manipTUDIES
1270
ANALYTICAL CHEMISTRY
900 Agnew Road, Pittsburgh 30, Pa.
ulations the interference of boron can be eliminated without preliminary separation. In several cases, satisfactory results were obtained with the mineral acid methods if the boron compounds were converted to boric acid during the dissolution of the sample, if the sample was not overheated during the dehydrations and the boric acid waa not converted to a relatively insoluble boron compound, or if a long enough digestion period was used after the precipitation of silicon to redissolve such slowly soluble boron compounds. However, such manipulations are time consuming, and results are not easily reproduced because control over different phases of the procedure is difficult. Therefore, this approach was not attractive for routine determinations of silicon in the presence of boron. Several less frequently used gravimetric procedures were also examined critically. These included dehydration in the presence of “&I, glycerol, or xylene, and precipitation of shcon with organic colloids or as silicomolybdic acid with different organic bases. These methods generally offered no advantage, especially in speed and accuracy, over the procedures used regularly. A new method was developed based on the precipitation of silicon with gelatin in an acid solution of the RamDle
which had been dehydrated by boiling with glycerol. EXPERIMENTAL
Reagents. Gelatin Solution. Dissolve 25 grams of gelatin (purified, calf skin) on low heat in 450 ml. of water and dilute to 500 mi. Store in cool place and use within 4 days. Procedure. Dissolve a 5.000-gram sample of iron or steel, or a 0.500gram sample of slag (or iron ore) in a 400-mi. beaker, with 50 ml. of HC1 (1 to 1). On low heat, add 5 ml. of concentrated HNOs dropwise to the solution to oxidize the dissolving sample. After the sample is dissolved, use high heat to boil the solution down to 15 to 20 ml. and then add 15 ml. of glycerol. Add the glycerol carefully at first until the evolution of nitrous gases and HCl gases is completed, and boil the solution down to a low level. To avoid charring, shake the beaker several times when the contents start to boil with fine, small bubbles. Remove the sample from the hotplate and cool slightly. Add 20 ml. of concentrated HCl and boil for 5 minutes. Cool the sample to 90” C. and add dropwise 5 to 6 ml. of hot, 5% gelatin solution, stirring constantly with a magnetic stirrer. Stir the solution for 2 minutes. Let the solution stand for a t least 5 minutes. Add 100 ml. of hot water containing 10 ml. of the 5% gelatin solution and
stir well. After cooling, filter through a double filter paper (S&S 589-1 H inside and 589 blue ribbon outside). Police the beaker and transfer the precipitate to the filter paper, rinsing the beaker several times. Wash the precipitate three times with 1 to 1 HC1 and 6 to 10 times with hot distilled water containing 0.5y0 gelatin. Moderate suction can be used to speed up the filtration. Ignite in a previously weighed platinum crucible a t 1000° C. for 20 minutes. The residue is SiOz. If the residue is not white or is apparently contaminated as a result of the high percentage of interfering elements, volatilize the SiOz with H F in the presence of HzS04.
Table 1.
NBS Sample No.
Types of Samples Tested in the Determination of Si
Si, % 1.34 0.028
Type Cast iron Bessemer Basic open hearth Acid open hearth 2.4y0Cr, 0.9% Mo 18% Cr, 9% Ni Ni-Mo steel Cr, Ni, Ti steel Cr, Ni, Nb steel
0.332 0.356
130 B.C.S. 272
Lead-bearing steel
0.237
Mild steel
0.30
174/1 J&L "D"
Basic slag Blast furnace slag
J&L
Boron, Cr, Ni steel High-boron steel High-boron steel
4h
8h 12F 21d 36s lOlc lllb 121a 123s
0.. 244 ~~-
0.589 0.302 0.523 0.46
DISCUSSION AND RESULTS
This procedure was developed from two rapid methods, the gelatin (5) and the glycerol (2) methods, which were not of sufficient accuracy for boroncontaining steels or slags. Weihrich and Winkel were responsible for the final form of the so-called gelatin procedure ( 5 ) , which is based on the coagulating property of gelatin. (Other coagulation procedures using albumin, casein, and agar-agar were also tried, and, while acceptable, were not so good as the gelatin method.) The procedure was tried on various samples (Table I). The results were generally low after the usual sulfuric-hydrofluoric acid treatment (Table 11). Litheanu and his coworkers ( 2 ) reported another rapid method for determining silica (in minerals) in which the silica is rendered insoluble in glycerol. The dissolved sample is evaporated with hydrochloric acid almost to dryness and then heated and boiled with additional glycerol until the temperature of the mixture rises to 150" to 170" C. for approximateIy 5 minutes, while the precipitation of silica occurs. This method was also tried for steels and slags, but the results with one dehydration were low, as shown in Table 11. As a part of the investigation, several modifications were tried to improve these methods. Different acids, such as HC1, H2S0,, "03, CHsCOOH, or mixtures of these acids were tried in various amounts. The effect of temperature and time on the dehydration and precipitation as well as the effect of stirring and filtration techniques were investigated unsuccessfully. Only with special ultrafiltration (1) were better results obtained, but ultrafiltration, using fine filter paper treated with gelatin and formaldehyde, is very time consuming and impractical. Finally, the gehtin precipitation was tried on a glyceroldehydrated, strongly acid solution (a). The use of glycerol during the dehydration seemed promising to avoid the possible interference of boron, since boron forms a stable soluble complex with glycerol. Variation in time and temperature
Table II.
.
36s. lOlc lllb
121a 123a 130 B.C.S. 272
Nb, 0.106; CO, 0.084; V, 0.049 Ti, 0.363 Nb, 0.75; V, 0.037; W, 0.11; Ta, 0.02 W, 0.015; Nb, 0.15; Zr, 0.045; Ta, 0.015
SiOz, % 14.7 33.8
V$OO,1.20
T101, 0.88
si, % 1.13 0.45 0.56
B, 0.13 B, 1.10 B, 1.10
Comparison of Results of Gelatin and of Glycerol Methods with Certified Values
NBS Sample No. 4h 8h 12F 21d ~~-
Interfering Elements, % Ti, 0.024; V, 0.011 V, 0.017
Average Values, yo Si Gelatin Glycerol
NBS Certified Value, % Si 1.34 0.028
0.244 0.332 0.356 0.589 0.302 0.523 0.46 0.237 B.C.S. certified value
1.20
0.021 0.218 0.285 0.34 0.560
0.264
0.482 0.433 0.201
0.293
0.30
Slag samples, % Si02 174/ 1 14.7 J&L "D" 33,84O Average obtained with mineral acid methods.
of the dehydration and precipitation showed that best results were obtained if the HCl-HKOs-glycerol-containing sample solution was heated above 150°C. (decomposition temperature of silicic acid) for 5 minutes,.if it was boiled for another minimum of 5 minutes with additional concentrated HCl, if the precipitation was carried out between 70" and 80" C., if the gelatin solution w a ~added dropwise with constant stirring, and if the solution was diluted only after a minimum of 8 minutes of precipitation time. The procedure can best be used for the range of 2 to 500 mg. of Si with good reproducibility and accuracy. The results obtained with this method are in excellent agreement
1.32
0.025 0.234 0.324 0.345 0.575 0.293 ~~. 0.514 0.452 0.225
14.4 33.6
0.276
14.1 32.7
with those obtained by other gravimetric methods (Table 111). Experiments with boron steels or samples to which boric acid was added showed that with the HSO, and with the HCl-H&O, methods, if 10 minutes of fuming were applied, high results were obtained; with 1to 4 minutes of fuming, low results were obtained. The glycerol-gelatin method gave consistently good results (Table IV). In addition to the ones mentioned previously, the glycerol-gelatin procedure has several other advantages. Hydrolyzing elements contaminate the silica precipitate only when present in large quantities. Nb, Ta, and W, if present, always contaminate the Sios VOL 33, NO. 9, AUGUST 1961
1271
Table 111.
Comparison of Classical Gravimetric Si Determinations with Glycerol-Gelatin Method (Values are given as per cent Si)
Ha04 MethodD 1.33’ 1.36‘ 0. 024b 0.031b 0.238-0.247 0 . 2 4 6 4 .24gb 0.330 0.324-0.33gb 0.350 0.355-0.365‘
NBS Sample No. 4h
8h
12 21d 36s l0lc
...
lllb
0.298 0.297-0.307*
121a
...
123a 0.233Lb’. 241
HClOd Methoda 1.35-1.36 0.027
“08-HnSOc Methods 1.33
0.026
0.028
0.241-0.249
...
0.239
0.244
0.248
0.245
...
...
0.328
0.332
0.334
0.330
0.355
0.356
0.356
0.358
0.355
...
0.584
0.589
0.591
0.299
0.296
0.302
Strong interference 0.303-0.307
...
0.516
0.523
0.532-0.546 0.5244.528
0.452
0.46
Strong interference 0.243
0.458-0.461
Interference 15.2
0.298-O.301 14.6
0.353-0.357‘ 0.576-0.595 0.582-0.601’ 0.294-0.309 0.511-0.540 0.5144.547b 0.4545 0.233-0.243’
0 . 2 3 G .243‘
b
0.237 Certified value 0.294-0.298 0.30 14.6 14.7 0.235
272 174/1 J$,bD:ag standard
...
...
...
14.6#
...
...
... 33.75c ... Figures representing lowest and highest averages given in NBS reports. Double dehydration with intermediate filtration. Results obtained here. Results obtained here with double dehydration.
Table IV.
No boron added; 1 gram boric acid added before dehydration
HzSO4‘
HC1-HzSOda
Glycerolgelatin
1.34 1.40 1.45 1.39 1.30b 1.31b
1.33 1.49 1.38 1.40 1.2gb 1.2gb
1.34 1.335 1.34 1.33 1.345 1.335
J&L boron steel 1.13% B
1.18 1.17 1.10’ 1.10% B 0.86 0.80 1.10% B 0.79 0.81 With HF volatilization. * Short fuming, 1 to 4 minutes. Boron was separated preliminarily. d Spectrographic results by lithium fluoride method.
precipitate, but less than 0.5% of Mo, Ti, V, and Zr do not interfere appreciably. The total amount of coprecipitating impurities on the samples tested was about 1 to 2% or less of the weight of the Si and therefore the HF volatilization of Si02 can be omitted for many types of samples. The determination takes about 1 hour, which is approximately the same as without preliminary separation of
1272
33.80”
Effect of Boron on Determination of Per Cent Silicon
Procedure
NBS standard 4h
ANALYTICAL CHEMISTRY
Glycerol-Gelatin Method Without HF With HF treatment treatment 1.34 1.336
0.027
130 B.C.S.
a
HCLH~SO~ Certified Methoda Value 1.33 1.34
NBS
Standard Value
1.34
1.13
1.13.
0.45 0.57
0 . 44d
33.84d
0.028-0.029
0.027-0.028
0.296-0.303
0.236
33,92
33.82
reliable results on iron, steel, and slag samples and can be used advantageously also for routine analysis. The recommended Si range is from 2 to 500 mg. of silicon. The accuracy and reproducibility of the procedure is aa good as, or better than, that of the conventional mineral acid gravimetric methods, and the preliminary separation of boron can be omitted. In addition to the applications considered in this paper, this silicon procedure may be applied to a wide range of other related materials.
0.56d
boron required for the perchloric acid method (which presents a possible e x plosion hazard) and less than the time necessary for the conventional gravimetric silicon methods. The procedure was tried with success on different National Bureau of Standards, British Chemical Society standards, and Jones & Laughlin Steel C o p . sampleg. Comparisons with other classical methods show that this method gives
LITERATURE CITED
(1) Davidov, A. L.,Kochugova, E. I., Zavodskaya Lab. 8 , 1308 (1939). (2) Litheanu, C., Rum, G. H., Strusievici, C., A d . rep. populare R a t n e Studii cercetdri chim. 3 , 55-9 (1955). (3) Lundell, G. E. F., Hoffmann, J. J., “Outlines of Metfiods of Chemical Analvsis,” DD. 37,43,45,47, t8, Wiley, _. New-York,-i954.’ ‘ ’ (4)Paaztor, Laszlo C.,Pittsburgh Conference on Analvtical Chemistrv. PaDer No. 31, 1959. (5) Weihrich, R., Winkel, A., “Die Chemische Analyze in der Stahlindustrie,” p. 29, Enke Verlag, Stuttgart, 1954. ’
-
’
I .
RECEIVEDfor review January 4, 1961. Accepted March 31, 1961.
