Rapid Spectrophotometric Determination of Silicon and Manganese in Cast Iron JOlIN R. BOYD, James B . CZom and Sons, Coshocton, Ohio
This work was begun in an effort to streamline the procedure, and thus shorten the time of determining silicon and manganese in cast iron. A procedure was evolved for determining these elements in a single weighed sample by spectrophotometric methods, in about one third the time necessary by gravimetric and volumetric means. Accuracy compares favorably with former methods. The procedure is of value to cast iron foundries which have a high melting rate and where the possible quick change of analysis might cause heavy losses if allowed to continue long. I t should be of value wherever the rapid determination of silicon and manganese in cast or pig iron is required. In its present form it has been applied only to nnalloyed cast and pig irons under 3% silicon.
A
XIASGASESE
1; INVE8TIGA4TIOS1% as institnied in 1048 in this labora-
tory, m ith the object of finding suitable spectrophotometric methods n hich could be adapted to silicon, manganese, and phosphorus in cast iron, and which could be integrated with the volumetric carbon and evolution sulfur determinations, so that unnecessary steps could be eliminated, and the whole streamlined into a rapid, acrurate, workable routine. Some of the results of this investigation were:
The persulfate method of developing the permanganic color was selected as the best for the use intended. It consists of using ammonium persulfate with silver nitrate as the catalyst, in an acid solution containing the phosphate ion, and has been covcied rather thoroughly in the literature ( 7 , 9, 10, id, 16, 20).
Uniform drilling of the sample, so that thin, snla11, easily dissolved chips were obtained. A sample 1%eight of 0.2500 gram for all the elements named with the exception of sulfur, which helped eliminate weighing errors and eliminated the need for changing weights. A change to colorimetric methods for silicon, manganese, and phosphorus. A single sample for silicon and manganese. A graduated flask of horosilicate glass used directly for the solution and oxidation of the sample; then without cooling, it was diluted immediately to the mark. A rapid rombination filtering-pipetting arrangement, whereby only the amounts needed for the completion of a determination were filtered free of graphite and measured and the washing of filter paper was eliminated. Solid reagents substituted for solutions of solids further to speed up the work (16), wherever this was possible. This has been done successfully by Ridsdale in England, and has been used extensively there for a number of years.
The calibration curves were developed by running a number of Bureau of Standards cast iron samples and plotting the curves on semilogarithmic graph paper. Inspection of curves made by plotting wave length against per cent transmittance showed a minimum for manganese a t 535 mp, and that for silicon wa8 in the ultraviolet region [EIiII gave 380 mw ( 6 ) ,which was beyond the range of the equipment a t hand]. It was found that a good degree of accuracy could be secured by using a wave length of 405 mp. Frequent rheckings of wave-length calibration were established, as mere daily checks against Bureau of Standards samples. This care in silicon determination is necessary because the wave length of 405 mp does not orrur a t a minimum on the transmittancewave length curve and the transmittance value at the region around 405 mp is changing much more rapidlv for a given change of wave length than mould occur in the immC diate region of the minimum.
EXPERIMENTAL
The procedures described in this paper are an integral part of the whole method, adapted from the results above. Perchloric acid has been purposely eliminated from the methods, to avoid the danger of fire and explosion in the presence of organic combustible material The time for a combined determination of silicon and manganese is about 1.5 minutes. Onc carbon., two evolution sulfurs, and the combination silicon-manganese determinations can be aEcomplished by one operator in about 20 minutes, exclusive of JT eighing time.
API’kRATUS AND EQUIP,ME\IT
D
SILICON
hlany color-absorption methods for determining silicon in various types of steels and other silicon- or silica-bearing substances have appeared in the literature. The procedures are based mainly on converting the silicon to the yellow silicomolybdate c o m p l ~ sand measuring its intensity in a solution of definite acidity ( 6 , 11, 15-19, d l ) , or reducing the yellow heteropoly complex to the molybdenutn blue color and then determining its intensity (6-5, 8, 16). It was decided that developing of the yellow complex and then measuring i t offered the greatest possibilities for speed, n.ith sufficient accuracy for the purposes for which it was desired.
Figure 1. Rapid Filter Assembly D. Fritted disk R. Rubber tubing S. Suction tube
805
A Coleman hfodel 14 spectrophotometer using 19-mm. round cuvettes was used in these determinations. The rapid graphite filtering assembly consisted of a 200-ml. tall-form beaker, a Huchner funnel, or an immersion tube x i t h a coarse size fritted disk, to the stem of n hich was permanently attached a 2.5-inch length of small rubber tubing (Figure I). a 20-ml. pipet for silicon and a 10-ml. pipet for manganese, and a rubber tube connected to a suction pump hanging down a t the proper height above the work table. (-4coarse size of fritted disk is first conditioned by actually filtering some graphite from a portion that is left from an analysis, then is cleaned by back-flushing and is dried in an oven.) The pipets each had a 0.75-inch length of smallbore rubber tubing slipped over the suction ends in such a manner that the rubber extended past the ends of the glass of the
806
ANALYTICAL CHEMISTRY
p e t s , so that they formed a quick connection and seal when eld against the end of the suction tube above the table.
DISCUSSION
REAGENTS USED
7% sulfuric acid solution, 70 ml. of sulfuric acid (specific gravity 1.84) to 930 ml. of distilled water. 25% sulfuric acid solution, 250 ml. of sulfuric acid to 750 ml. of distilled water. Potassium nitrate, 1-gram size compressed solid reagent (16). Silver nitrate, 0.012-gram size compressed solid reagent (16). Ammonium persulfate, 0.75-gram compressed solid reagent f 16). \-
?;mmonium molybdate, 0.50-gram compressed solid reagent
(16).
Phosphoric acid mix, 845 ml. of distilled water, 33 ml. of concentrated sulfuric acid, 42 ml. of concentrated phosphoric acid, and 80 ml. of concentrated nitric acid. PREPARATION
A short time before the sample is obtained the following preparations are made:
Place the Buchner funnel or immersion tube, with coarse-size fritted disk and rubber tube connection, upside down in a 200-ml. tall-form beaker. This will receive the dissolved and diluted Aample from the 100-ml. graduated flask. Measure out accurately from a buret 15 ml. of the “phosphoric acid mix” into each of two 50-ml. borosilicate glass-stoppered Erlenmeyer flasks. In one place 0.012 gram of silver nitrate and 0.75 gram of ammonium persulfate. Stopper and let sit for the manganese aliquots. Into each of two 200-ml. Erlenmeyer flasks measure accurately with graduated flask, buret, or pipet 50 ml. of distilled water. These will receive the silicon portions. To one flask add 0.50 gram of ammonium molybdate. Begin to heat some of the 7% (by volume) sulfuric acid slowly, BO that it will have attained a temperature of about 80” C. by the time it is ready to be used to dissolve the sample.
I t is essential that the determination be carried through with the same timing from,the moment the graduated flask is removed from the hot plate until the portions have been pipetted into their respective flasks. The distilled water used for dilution and solutions added should be kept within * 5 ’ C. of the temperatures that these were, when the calibration graphs were constructed. These precautions add to the accuracy of the measurements of the solutions and of the diluting water, inasmuch a8 they are made under nearly identical conditions of temperature. If more than one sample is being run a t one time, they should be carried through the above steps individually from after oxidation until the aliquots are pipetted into their respective flasks. The method works best when room temperature is maintained below 30” C. Silicons should also be carried through singly and in the same manner from the time the 25% sulfuric acid is added until the final reading is obtained for each sample. The reason for this is that some fading takes place gradually after the 25% acid is added (Table I). If a routine is worked out and adhered to, no difficulty is experienced because of this slight fading tendency. KOappreciable fading was experienced with the manganese color.
Tahle 1.
Typical Fading Time of Silicomolybdate Complex Time, hlin. Silicon, %
COMBINED PROCEDURE FOR SILICON AND MANGANESE
Weigh out a 0.2500-gram sample of small thin drillings, which are representative of the sample, into a 100-ml. borosilicate glass graduated flask. Add 30 ml..of the hot 7% sulfuric acid and place on a hot plate with medium heat. When the major reaction has about ceased, remove from the hot plate and add 1.00 gram of potassium nitrate. When oxidation is complete, as evidenced by a “clearing” of the solution, set the flask on the hot plate again, (The flask must be moved away from the heat when the oxidation is taking place; otherwise the vigorous reaction will cause the li uid to foam out of a graduated flask.) Boil the solution gentlylor about 2 minutes. Care must be taken that a minimum amount of liquid is lost by evaporation; however, the long neck of the graduated flask condenses a portion of the vapor and thus minimizes this loss. Major liquid loss could render Borne of the silicon insoluble as silica. Without cooling, make up to 100 ml. with distilled water, and pour into the above-mentioned tall-form beaker, using a standard draining time. Mix with the Buchner funnel or immersion tube and then set the tip of a 10-ml. pipet lightly into the end of the rubber connection from the filter and press the other end of the pipet firmly against the suction tube hanging from above (Figure 1). The pipet will fill rapidly with filtered solution. Quickly disconnect from the suction and retain the liquid in the pipet by means of a finger over the top, in the normal manner. Adjust the liquid to the 10-ml. mark and transfer to one of the prepared 50-ml. flasks. In like manner transfer 10 ml. more to the other 50-ml. flask. Place the one containing the persulfate and silver nitrate on the hot plate and bring to a boil. Meanwhile. in a similar manner. using a 20-ml. pipet, transfer 20-ml. portions to the two prepared 200-ml. flasks, shake, and let stand for 3 minutes for the color to develop fully. In the meantime, the manganese on the hot plate should have boiled. Let it boil exactly 1minute, remove from the heat, and cool rapidly in circulating cold water. Transfer the cooled manganese sample to cuvettes. Add 25 ml. of 25% (by volume) sulfuric acid to the uncolored silicon solution, swirl carefully, and transfer to a cuvette. Then after the 3 minutes are up add 25 ml. of 25% sulfuric acid to the yellow silicon coniplex, swirl gently, and quickly transfer to a cuvette. Determine the silicon a t once, using the uncolored portion to set the instrument a t 100% transmittance. Use a wave length of 405 mp. Similarly determine manganese at 535 mp.
The above methods were developed for unalloyed gray irons and pig irons, and have not been extensively tested with alloyed irons. I t can be assumed, however, that with the usual corrections they can be applied to these also. Caution should be observed when trying a new iron by this method, to see if alloys present might affect the result. Table I1 illustrates the accuracy of the method with Bureau of Standards cast irons.
Table 11. Check Results with National Bureau of Standards Cast Irons NBS No. 4a iron H 4r
5i 6e 7e 7 iron F: 82 115 122b
Silicon Stand. value 1.37 1.37 1.29 1.29 2.44 2.33 1.89 1.89 2.21 8.09 1.60 0.642
Check 1.34 1.34 1.28 1.30 2.46 2.29 1.90 1.88 2.20 2.08 1.61 0.63
, Manganese Stand. value 1.04 1.04 0.721 0.721 0.696 1.36 0.44 0.44 0.444 0.722 1.01 0.561
.
Check 1 05 1.06 0.72 0.74 0.71 1.35 0.44 0.43 0.42 0.73 0.99 0.56
Limited experience with alloys is noted in Table 11-Xational Bureau of Standards standard cast irons No. 82 (nickel 1.00, chromium, 0.245), and No. 115 (Xi-Resist, nickel 15.89, copper 6.44, chromium 2.17). Good results with silicon and manganese are noted here. However, the Ni-Resist sample could not be dissolved with the 7% sulfuric acid until the potassium nitrate reagent was added, when it dissolved readily. Irons such as 6e,
V O L U M E 2 4 , NO. 5, M A Y 1 9 5 2
807 Above 2.00% another graph must be drawn, using either a smaller sample or smaller aliquots from the dissolved siliron solution already in use.
Table 111. Data for Silicon Curve (0.2500-gram sample a n d 20-ml. aliquots)
NBS
T,
Standard Cast Irone
405
mp
I
Si Value,
ACKNOWLEDGMENT
%
The author wishes to thank his associates in the laboratory, who assisted in checking results during the pears the method was being tested. LITERATURE CITED
* 0.1250 gram of each for a determination.
containing over about 0.50% combined carbon, required extra time and extra potassium nitrate for solution. Irons common in the United States do not usually contain over 0.90% phosphorus, and up to this value the final acidity suitably euppresses the phosphomolybdate colored complex. Manganese up to the values shown in Table I1 does not appear to have appreciable effect on the silicon values. Ferric iron, which has appreciable transmittance in the region of 405 mp, is cancelled out by the use of the solution, which has not been treated with molybdate, to set the transmittance of the instrument to 100%. The relatively dark background of the silicomolybdate complex may be somewhat objectionable, but to attempt to get rid of it is time-consuming and is not necessary for the average cast iron foundry. For eutectivity determinations silicon has only about one third the effect of carbon, as indicated by the formula, E, = % T.C. 0.3 (% Si % P), which is approximate and used extensively in one form or another in the industry (1). For total carbon the accepted accuracy in the foundry industry is 1 0 . 0 3 or 0.04%, depending upon the rarbon content; this would give a considerable leeway for the same effect from silicon, if the fonnula were applied. Actually, the accuracy for silicon is urually regarded as about the same as for carbon. I t is not considered advisable to try to run by this method silirons that are much above 3.00%, as the curve begins to flatten out above 2.00%, so that the smallest differences in transmittance represent too great a difference in silicon.
+
+
(1)American Foundrymen’s Association, Chicago, “Handbook of Cuoola Ooeration.” 1946. (2) Bolts; D. F ,with Aiellon. If. G.. IND.EKG.CREM.,ANAL. ED.. 19,876 (1947). (3) Clausen, D. F.,and Roussopoulas. H. D.. Aitachem ~ V P W S 6,. 41 (1946). (4) Gentry, C. H. R., and Sherrington, I,. 0.. J . SOC.Chem. I n d . , 65,90 (1946). ( 5 ) Haywood, F. W., and Wood, A. A . R., “Mrtallurgical Analysis
by Means of the Spekker Phot,oelectric Absorpt,iometer,” London, Adam Hilger, 1944. (6) Hill, 5’. T.,ANAL.CHEM.,21, 589 (1949). ( 7 ) Hilson, H. D., I N D . END.CHEM.,ANAL. ED.,16, 560 (1944). Little, J., J . SOC.Chem. Ind., 64, 118 (1945). 1 Lord and Demorest, “Metallurgical Analysis.” New Tork, JIoCraw-Hill Book Co., 1924. ’ Lundell, Hoffman, and Bright, “Chemical Analysis of It,on and Steel.” New York. John Wiles & Sons. 1931. Malov, S. I., Yakovlev, P. Ya., and Eliseev. A . A , . Zauodskaya Lab., 5,665 (1936). Marshall, H., Chem. Sews, 83, i6 (1901). Pinsl, H., Arch. Eisenht’iltenw. 8, 97 (1934) Ibid., 9,223 (1935). Ibid., 10,139 (1936). Ridsdale, K. D.,Ridsdale & C’o., Middlesbrough. England Analoid Descriotive Booklet 336 (Januarv 1948). Rozental, D., an; Campbell, H.. I m EN;. CHEM.,ANAI E D , 1
17,222 (1945).
Schwarte, M.C., and Morris, 1,. W., I h i d . , 15,20 (1943). Thayer, L..I.,Ibid., 2, 276 (1930). Walters, H. E., Chem. .Veu.s, 84,239 (1901). Weihrich, R.,and Schwarts. W. Arch. Eisenhlittenw., 14, ,501 (19311. RECEIVED for review March 28, 1951. Sccepted January 2 5 , 1952. Presented a t Pittsburgh conference on Analytical Chemistry and Applied Spectroscopy, Pittsburgh, Pa.,March 7, 1951.
CaIcuIator for SpectrochemicaI AnaIysis Application of Seidel Function of Density in Its Development GEORGE OPLINGER Solvay Process Division, ‘4llied Chemical 61: Dye Corp., Syraciise: N. Y .
T
HE use of calculators in modern spectrochemical analysis has been demonstrated as a practical necessity in regard to both the saving of time and the reduction of error. Of the many types described (2-6, 10-12, 14, 16-20, 83,2 4 ) , no one calculator was found to provide all the features desired to correct for the variables encountered in spectrochemical analysis. This paper describes a calculator that has proved highly satisfactory in reRolving variables introduced by the characteristics of the photographic emulsion, while still permitting adjustments to correct for the recognized shifts of the analytical curve The successful application of any calculator depends fundamentally on reliable calibration of the photographic emulsion, the essential prerequisite for all photographic intensity measurements. A study of the various means of accomplishing a precise calibration, as summarized by Churchill (6) and paralleled in the author’s work, resulted in the choice of the two-line method using pairs of iron lines selected from those classified by Dieke and Crosswhite (7-9) In applying Crosswhite’s ( 7 ) and Dieke’s
(8) data oil the intensity relationships of various lines in the iron spectrum, the Seidel (13) function of density, log ( I o / I - I ) , hereafter termed the Seidel density, was employed in expressing spectral line blackening. This function was first suggested by Baker (1). 10represents the incident light being used to measure the emulsion and I is the light transmitted by the spectral line being measured. This function of density, explored by Srhmidt (%’I), makes possible a much more precise method of establishing the characteristic curve of the photographic emulsion. His experiments showed that the plot on rectangular coordinates of the Seidel density for the stronger line against the Seidel density for the Feaker line was linear for all emulsions tested in the density range 0.1 to 2.0, and beloa 3200 A. It was desirable to use this method in extending the wave-length region to 4600 A As no data were given for any emulsion above 3200 A. and the emulsions tested were not listed by Schmidt, an experimental study of Eastnian Spectrum Analysis S o . 1 and Type 103-0 was undertaken. For the No 1 emulsion, the wavelength re-
