SILICA REFRACTORIES Spectrographic Analysis Using a Controlled Multisource Power Unit A. J. HERDLE AND H. J. WOLTHORN Ohio Works Chemical Laboratory, Curnegie-Illinois Steel Corporation, Youngstown, Ohio In a search for a more reliable and less time-consuming method for the analysis of silica refractories containing approximately 959'0 silica, several methods were studied which made use of the spectrograph, using different techniques for preparing the sample for excitation. A method was developed which utilized a sample of the pulverized refractory directly for the determination of calcium, magnesium, titanium, aluminum, iron, sodium, and potassium, and determined silicon by difference. Excitation conditions are described and data obtained in determining the precision of the method are presented.
I
is recorded on Eastnian spectrum analysis S o . 1 film. Spcctral line densities are determined by rcading their transmittances on a projection type densitometer employing a photocell and galvanometer.
S COKNECTIOS with an investigation of the roof life of
open hearth furnaces, it became necessary to make fairly complete analyses of a rather large number of silica bricks. Because wet chemistry analyses consume a large number of man-hours, and past experiences indicated that even trained chemists frequently disagreed among tlirinselves on the results. spectrographic methods of making I he analyses were invrstigated. A number of methods have beeii described for the quantitativc spectrographic analysis of powders and ceramic materials, including silica. Most of them make use of a buffer material which furnishes the internal standard linrs> and include carbon to render the misturc conducting, as n-ell as to obtain smoother operation of the source. It' is also claimed that the addition of carbon causes elements of ~ i d e l ydifferent volatility to eva.porate simultaneously (5).
PROCEDURE
The dried saniple i7hicli has been pulverized to pass through a 100-mesh sieve is used directly. The lower positive electrode consists of a center-post crater type carbon made from 0.25-inch spectrographic carbons (Figure 1). The saniple is forced into tho crater by gently pressing the crater into the saniple and twisting slightly, repeating if necessary until the crater is level full. The upper negative electrode is hemispherically tipped and also made from 0.25-inch spectrographic carbons. An analytical gap of 3 mm. is used. I t is then sparked with a high capacity ovcrdarnped condenser discharge. Two sets of excitation condi one for calcium oxide, alumina, the other for the alkalies sodiu prespark is used for either. I n exposure with 40 mfd. does no sit!-. Because at thc end of this time, most of the material is blown out of the crater, a longer exposurc has no effcct. Tliis is ovcr-
Fitz and llurray(1) analyzed small samples of powders liy niising 1 nip. of the unknown with 20 mg. of barium nitrate and ammonium nitrate as a buffer, making a pellet 2 nim. in diameter which they burned complctely in a carbon holder n-ith a direct s requiring about 2 minutes. (6) made control analyses of powdered ceramic niaterials by mising with a suitable flux, making 1.25-em. (0.5-inch) briquets of the niisture, anti sparking with a critically damped discharge from a controlled Multisource power unit. They observed that different working curves mere necessary for samples having different average particle size and even for samples having the same average particle size but received from different suppliers. Helz anti Scribner ( 2 ) described a similar method for the anal)-sis of portland cement, but used a high capacity overdamped type of discharge from the Multisource unit. Smith and Hoagbin ( 4 ) also analyzed ceramic materials spectrographically by mixing 2 parts of the 100-mesh sample n-ith 1 qart of a suitable internal standard and carbon powder as a carrier. This mixture they placed in a hollow carbon crater and burned for 30 seconds v,-itha 10-ampere direct current arc.
L 4
t
2" 32
The method herein described for silica refractories having a silica content of approximately 95% has the advantage of permitting the direct use of a sample pulverized to pass a 100-mesh sieve; it thus materially increases reproducibility and makes possible a saving of time othern-ise devoted to preparing the mixture and making pellets. This method consists of using, as the lower electrode, a center-post crater type carbon, containing the sample, and a pointed upper electrode. Thus the carbon center post furnishes the necessary conductivity, and the major constituent, silica, furnishes the internal standard lines.
I
5
1
rcc
Lower
Figure 1.
Table I.
-
Longitudinal Section of Electrodes [Tppcr
Excitation Conditions Alkalies
Output potential, yolts C8 acitance mfd. Indtctance, 'mh. Rsiistance, o t m s Phme angle, Charge us. discharge O Exposure period, sec: Filter
EQUIPMENT
All the equipment used is commercially available. Excitation is obtained from a controlled Multisource power unit. The spectrograph is a standard 1.5-meter type with a 24,000 line per inch grating have a uniform dispersion of 7 h;. per mm. The spectrum
705
940 60
480 25
70 180 20 None
Others 940 40 480
26 70 180 30 Screen
ANALYTICAL CHEMISTRY
706 come by increasing the capacitance to 60 mfd. and exciting two samples for 20 seconds each, the spectra from the two separate sets of electrodes being superimposed for each determination. The film is developed in Eastman D-19 developer for 3 minutes a t 70" F., hardened in a 5% solution of Eastman Kodak liquid hardener for 15 seconds, fixed in Eastman concentrated x-ray fixer for 1 nlinute, washed 1 minute, and dried for 1.5 minutes on a film dryer. PHOTOMETRY
Analytical line pairs are listed in Table 11. In general, transmittance readings should be between 10 and 90%. The average of duplicate determinations is reported. A control standard is run with each set of samples. Intensity ratios of selected analytical lines compared to a silicon line as internal standard are determined and the various concentrations are read from analytical curves calibrated to read directly as the oxides of the elements determined.
Table 11. Analytical Line Pairs Wave Length, Element Line,
Wave Length, Indexa Range Silicon Line, (Oxide (Oxide), Element A. A. Curves) % 2.17 1.00to4.00 2532.4 Ca 2532.4 1.14 0.30 to 1.75 A1 2532.4 0.45 0.20 t o 1.00 Fe 0.05to 0.30 2532.4 0.13 Mg 2532.4 0.12 0.05 t o 0.20 Ti 0.005 0.01 to 0.50 3991.6 K 0.03t00.35 0.06 3991.6 Na a Defined as concentration a t which intensity ratio of analytical line pair is unity.
I n attempting to apply this method to other types of refractories having higher ferric oxide content, some interference of Fe 2532.5 with Si 2532.4 was noted, especially above about 2% ferric oxide. Most of the above ranges could undoubtedly be extended either way if desired. ACCURACY AND PRECISION
The accuracy of any spectrographic result depends altogether on the accuracy of the working curves used, provided that the sample being analyzed is similar in composition to the samples used in constructing the working curves. The accuracy of the working curves depends on the accuracy of the values used as standards in establishing the working curves, as determined by wet chemical methods, Assuming that dependable standards have been used to construct the working curves, and that the curves are being used in the analysis of materials whose composition is essentially similar to that of material used in establishing the curves, the essential point to consider is the precision or reproducibility of the method. Comparison of the results obtained on a number of samples with that obtained by wet chemical methods is not alcc-ays a fair criterion, for not only are the accuracy and precision of the spectrographic method involved, but also those of the wet chemical methods.
Table 111. Composition of Samples SiO,,
KO.
1 2 3 4
% 93.94 95.64
... ...
FelOl,
%
0.66
0.55 0.43
...
AlzOa, % 1.96 1.02 0.43
...
TiOz,
CaO,
MgO,
NaaO, KzO,
%
%
0.16 0.09 0.07
2.29 2.10 1.58 3.58
0.21 0.17
0.06 0.04 0.10
0.29 0.18 0.05
%
I...
%
%
... ...
...
...
In this work, the following standards wereused to establish working curves: (1) National Bureau of Standards silica brick, No. 102; (2) a sample of silica brick prepared by a Carnegie-Illinois Steel Corporation laboratory and analysed cooperatively by eight laboratories; and (3) from the analyses of several samples of
Table IV.
Analysis of Cooperative S a m p l e
Fe?Oa,
-11203,
CaO,
NgO,
TiOg, KazO,
KzO,
Si02 (Diff.),
0.58 0.51 0.58 0.54 0.62 0.57
% 1.02 1.02 1.04 1.01 1.03 1.05
% 2.07 2.04 2.34 2.06 1.92 2.11
% 0.17
% 0.09 0.08
% 0.22 0.19 0.23 0.16 0.24 0.18
% 95.80 95.95 95.48 95.93 95.88 95.79
gj
No. 1 2 3 4 5 6
0.16
0.18 0.17 0.18 0.16
0.10
0.09 0.09 0.09
% 0.05 0.05 0.05 0.04 0.04 0.05
silica brick, certain values were selected which corresponded to values needed to construct tvorking curves to cover a definite range. These determinations %-ere carefully checked, so as to arrive a t the most accurate values obtainable. The composition of these samples is tabulated in Table 111. In order to establish the precision of the method, 29 analyses B-ere made of the above-mentioned Carnegie-Illinois cooperative sample, determining seven elements on each. Because it was the intention to report silica by difference when this method was used, calculations were made for silica on all the analyses. A few results selected a t random from these analyses are shown in Table IV. From the complete data there have been calculated the standard deviation and per cent standard deviation for the eight elements determined. In doing so, the A.S.T.M. manual on presentation of data was followed. This method uses the formula s =
e
where d is difference between individual results and the
average, and n is the total number of determinations. If n - 1 is used in this formula as recommended by Snedecor (6) and others, the value for s will be slightly greater. Results are summarized in Table V.
Table V. Element Si02 CaO AlzOs Fez08 MgO Ti02 NaaO
Kz0
Average,
% 95.80 2.07 1.03 0.58 0.17 0.09 0.0;
0.21
S t a n d a r d Deviation Standard Deviation,
%
0.15 0.12 0.035 0.035 0.007 0.005 0.006 0.03
%
Standard Deviation 0.16 5.8 3.4 6.0 4.1 5.5 12.0 14.3
No. of Determinationa 27 29 29 29 29 29 27 27
DISCUSSION
It is possible to analyze spectrographically a powdered sample of ceramic or refractory material without the addition of graphite or a buffer when it contains 95% or more of a major constituent, by volatilizing the sample from a center-post crater type carbon. For elements giving faint lines, this operation may be repeated, and the images superimposed and integrated on the film or plate. Although precision is not so high as in the case of metals, it is very satisfactory for the routine analysis of this type o f material. Some error is undoubtedly introduced into the method by using an internal standard line so widely separated from certain of the analytical lines, notably the aluminum 3961.5 line. However, the number of silicon lines available is limited and the aluminum line used gave better precision than any of the others that might have been chosen. During the exposure, the refractory material is blown out of the crater into the gap, but this is an advantage rather than a disadvantage as fresh material is drawn up into the gap continuously. There is only a small amount left in the crater a t the end of the exposure. The presence of the center post tends to slow up the rate a t which the material leaves the crater as well as to center the discharge, thus materially increasing the reproducibility. A few variations have been tried in an effort to improve repro-
707
V O L U M E 2 1 , NO. 6, J U N E 1 9 4 9 ducibility, with varying success. X 2-mm. analytical gap was tried with the 40-mfd. exposure, but showed very little improvement. Pulverizing the sample to pass through a 200-mesh sieve improves the reproducibility somewhat, but it is questionable whether the improvement is sufficient to justify this additional expenditure of time. LITERATURE CITED
(1) Fitz, E. J., and Murray, TIr. M., ISD. ESG. CHEM..ANAL.ED.,17, 145-7 (1945).
and Scribnei B. F., J Research A'atl. Bur. Stand(2) Helz, A . K., ards, 38, 439-47 (1947). (3) Jaycox, E. K., Soc. A p p l i e d Spectroscopy Bull., 3, No. 3, 1-10 (April 1948). and (4) Smith, R. W., (1946).
Hoagbin, J. E., J . Am. Ceram. Soc., 29, 222-8
"Statistical Methods Applied to Experiments in ( 5 ) Snedecor, G. W., Agriculture and Biology," p. 36, Ames, Iowa, Iowa State College Press, 1946. (6) Zander, J. M., and Terry, J. H.. J . Am. Ceram. SOC.,30, 366-70 11947). RECEIYED October 21, 1948.
Ascorbic Acid, Dehydroascorbic Acid, and Diketogulonic Acid In Fresh and Processed Foods 3IARY B. MILLS, CHARLOTTE RI. DAMRON, AND JOSEPH H. ROE
School of Medicine, George Washington C'nirersity, Washington, D . C . Foods in various states of preservation were analyzed by the method of Roe, Mills, Damron, and Oesterling for L-ascorbic acid, dehydro-L-ascorbic acid, and diketo-L-gulonic acid. Values obtained are reported. The majority of fresh foods assayed showed less than 5 % of the total vitamin C-like compounds present as the antiscorbutically inactive diketo-
T
HE primary oxidation product of L-ascorbic acid in plant materials is dehydro-Irascorbic acid. The latter is not a stable compound; it undergoes spontaneous conversion to a second product which has been fairly well characterized as 2,3diketo-bgulonic acid, formed by the opening of the lactone ring of dehydroascorbic acid (I, 3). Dehydroascorbic acid has considerable antiscorbutic potency because it is readily reduced to ascorbic acid in the animal body (4,6, IO). The exact value for its antiscorbutic activity has not been satisfactorily determined, but it is probably about 75010 of that of ascorbic acid. Diketogulonic acid, on the other hand, has no demonstrable antiscorbutic activity ( 4 ) . The actual antiscorbutic potency of a foodstuff depends on the amount of both ascorbic acid and dehydroascorbic acid present. The oxidation-reduction methods for the determination of vitamin C are reliable only if all the antiscorbutic material present is ascorbic acid, and other reducing substances are not present in large enough concentrations to interfere. This ideal condition is usually not encountered. The Roe-Kuether ( 7 ) and Roe-Oesterling (9) methods, which use 2,4dinitrophenylhydrazine to couple with the oxidized forms of ascorbic acid, do not differentiate between dehydroascorbic acid and diketogulonic acid, both of which react with the reagent to give an identical derivative. The original dinitrophenylhydrazine methods are accurate for antiscorbutic assay only when ascorbic acid and dehydroascorbic acid occur in the food and diketogulonic acid has not been formed in appreciable amounts. This is some improvement over the oxidation-reduction methods by virtue of increased specificity and the inclusion of the dehydroascorbic acid portion, but these methods lose their usefulness when diketogulonic acid is present in the foodstuff, unless they are used as Guild, Lockhart, and Harris suggest (8) to show what the original ascorbic acid content of the foodstuff was a t the time of harvesting.
gulonic acid. Processed foods contained more of the oxidized forms of ascorbic acid, and dehydrated foods showed the greatest amount of the inactive diketogulonic acid. The rate of change of ascorbic acid into dehydroascorbic acid and of dehydroascorbic acid into diketogulonic acid in orange juice and potato slurry during storage at 2' C. wasfollowed.
The new method developed by Roe, Mills, Damron, and Oesterling (8) permits the simultaneous determination of ascorbic acid, dehydroascorbic acid, and diketogulonic acid in the same tissue filtrate. It is based upon the following principles. Diketogulonic acid is the oxidation product of ascorbic acid in a metaphosphoric acid filtrate that is not reducible by hydrogen sulfide and couples with 2,4dinitrophenylhydrazine. Dehydroascorbic acid is the fraction in the filtrate that is reducible by hydrogen sulfide and couples with the reagent only before reduction. The ascorbic acid in the filtrate does not couple with 2,4dinitrophenylhydrazine under the conditions of the method until after it is oxidized by bromine. The analytical steps involved theoretically confer a high degree of specificity upon the method used. This procedure should show the closest correlation possible up to the present time between the antiscorbutic biological assay and the actual chemical analysis of a foodstuff. There are many reports in the literature of the discrepancies between biological and chemical assays for vitamin C. There are also instances of comparative analyses of stored foodstuffs wherein the indobhenol value decreased and the dinitrophenylhydrazine value remained constant (5) or increased (7) with storage. These results are consistent with the known chemistry of ascorbic acid, dehydroascorbic acid, and diketogulonic acid. Storage with the resultant oxidation of ascorbic acid to dehydroascorbic acid would decrease the indophenol value; however, the biological potency would change but little until diketogulonic acid was formed from the dehydroascorbic acid. As soon as diketogulonic acid was produced by "mutarotation" of the dehydroascorbic acid, the biological assay value would begin to decrease, but the dinitrophenylhydrazine assay value by the original methods (7, 9) would remain the same, or show an increase because of the more rapid rate of reaction of diketogulonic acid with 2,4-dinitrophenylhydrazine in the 3-hour coupling period employed.
