ANALYTICAL EDITION
I N D U STRIAL and ENGINEERING C H E MI STRY Harrison E. Howe, Editor
Sulfur in Plain and Alloy Steels u'
A Critical Study of the Combustion Method C. H. HALE, JR., United States Steel Corporation, Kearny, N. J., AND W. F. MUEHLBERG, American Steel and Wire Company, Cleveland, Ohio
T
HE methods employed heretofore for determining sulfur in steels have not been entirely satisfactory with respect to speed and accuracy, particularly when applied to alloy steels. The methods which involve weighing as barium sulfate are tedious and much too slow to serve as control of a steel-making operation ; the various modifications of the long-used evolution method are somewhat lacking in accuracy, especially when applied to highly alloyed steels, and leave much to be desired in point of speed. Seeking a better method, the authors decided to investigate the determination of sulfur by combustion of the steel in oxygen as holding the best promise of meeting the requirements, but found, although a number of papers on this general procedure had been published, that considerable additional study with careful attention to details of procedure was required before they could regard the method as reasonably satisfactory. A routine method for the simultaneous determination of sulfur and carbon in steel by combustion in oxygen was reported by Holthaus @), following work published by Schmitz (IO) and Vita (14). Since that time several other papers (1, %'6, , 11-13, 15) have appeared dealing mainly with details of the final absorption of the sulfur oxides formed in the combustion, and of the method of titrating or otherwise estimating the amount of sulfur absorbed. These papers, however, consist largely of accounts of special procedures, and of tables of comparative results by combustion and gravimetric methods; they do not give a thorough discussion of precise conditions which the authors' experience leads them t o believe are necessary for best recovery of the sulfur. Further, since most of the results reported appear to be based on some empirical titration factor, obtained from Ptandard steels, the necessary conditions cannot be inferred from the published details of procedure. Indeed, the apparent use of an empirical factor leads one to believe that the method as it has been used yields only some reasonably constant percentage of the sulfur present but not all of it. The authors' investigation indicates that not only is this true, except perhaps for certain special and rather impractical conditions, but that the conditions requisite to a consistent high percentage recovery must themselves be precisely defined and carefully adhered to. Nor is this surprising, for in a 1.6-gram sample of a steel containing 0.05 per cent of sulfur there is only 0.8 mg. of sulfur. The precautions necessary for an accurate determination of this small amount may be inferred from the fact that the retention anywhere within the combustion tube or in the path of the gas t o the absorbing liquid, of even 0.1 mg.
of sulfur dioxide, corresponds to an error of 0.003 per cent on the sulfur or 6 per cent of the sulfur present. The first essential is the complete combustion and liberation of the sulfur. A high temperature is necessary, ranging from a minimum of 1200' to 1425' C., depending upon the nature of the steel. The authors have found that a steel which gives up all its sulfur in 10 or 12 minutes when burned at 1200' C. in oxygen liberates only some 7 5 per cent a t 1100' C. The temperature must be higher for the more highly alloyed steels; it may be reduced somewhat by the use of metallic tin as a flux. The temperature suitable for various types of steels is listed in Table I; at these temperatures, which are recommended as the result of a considerable number of experiments and include a factor of safety, practically all the sulfur from the steel sample is evolved within a period of 10 or 12 minutes. Combustion technic in this country, as applied to the determination of carbon in steel, makes use of a refractory bedding, usually alumina, to protect the boat from the molten oxides formed by the combustion. This practice does not permit of complete liberation of the sulfur, which may be held chemically by traces of alkali or mechanically in gas pockets in the melt. All bedding materials which the authors have tried hold back sulfur. Alumina is particularly bad in this respect; its ability to fix sulfur compounds was noted by Isham and Aumer (4)in 1908 and by Rooney (9) in 1934, and this probably explains the very long heating period found necessary by Kar ( 5 ) . Since a bedding to protect the boat would be desirable if it could be used, the authors have tried a number of materials including alundum, chrome ore, chromic oxide, ferric oxide, ferrous oxide, magnesite, stannic oxide, c. P. silica, Ottawa sand, various mesh sizes of broken boat material, mullite, titania, manganese dioxide, kaolin, clay, and zirconia sand. Zirconia sand is one of the least objectionable, but because of gas pocket formation cannot be used except on a very small sample (such as 0.3 gram) and even then requires 1300' C. for a plain carbon steel which could be run successfully at 1200 ' C. without bedding. If no bedding is used, the boat must be discarded after a single combustion; this, however, can be done at comparatively small expense. The boat must not absorb sulfur, must be sufficiently inert to the molten oxides to protect the combustion tube, and must withstand the high temperature of Combustion. After all the boats ordinarily used heretofore and all obtainable on the market had been tried, unglazed porcelain boats fulfilling all these requirements were 31 7
INDUSTRIAL AND EYGINEERING CHEMISTRY
318
glass fri
FIGURE1. DETAILOF ABSORPTION VESSEL
finally obtained through the cooperation of the McDanel Refractory Porcelain Company. Some writers have recommended the use of a plug a t the exit end of the combustion tube to hold back the finely divided iron oxide produced by the rapid combustion of the steel. The authors have been unable to find any plug which does not retain some of the sulfur oxides, and consequently use none. In any case they believe that a plug is unnecessary, for repeated trials have shown that the fume, even when it passes through the absorbing solution, does not affect the precision or reproducibility of the titration, provided that the oxygen flow is maintained a t the maximum rate permissible through the absorption vessels and the conditions of combustion are the optimum for the evolution of the sulfur. The fume consists not only of iron oxide but also of other oxides, and varies both in amount and composition with the type and kind of steel. The glass frit in the absorption vessel may in time become clogged by the fume; if so, it is readily cleaned by rinsing in warm dilute hydrochloric acid and subsequent repeated washings with water. To ensure the quantitative removal of sulfur from the combustion tube, there should be no cool exit end a t which sulfur trioxide might be adsorbed. The authors maintain this end a t about 200' C. by keeping it close to the heating zone of the furnace. To withstand this temperature and avoid the use of rubber, which readily absorbs sulfur dioxide, the tube is connected to the absorption vessel by a ground porcelain-to-glass joint. This joint, which can be made by a good glass-blower, serves its purpose very well; it is gas-tight a t the pressure used, yet easily disconnected, an arrangement of springs ensuring against accidental disconnection. Absorption of the sulfur oxides a t the speed of oxygen flow needed for rapid combustion has been attempted by various devices ranging from straight tubes to a special spray apparatus (8). Sulfur dioxide is absorbed with no particular difficulty; sulfur trioxide is present in smaller amounts but tends to form a mist which may pass through the usual absorption devices. A glass frit effects the absorption of both gases satisfactorily and is convenient to use. (The authors use a Jena frit, porosity G1, nominal pore diameter 100 to 120 microns.) The absorbing solution is a dilute neutral
VOL. 8, NO. 5
hydrogen peroxide. Estimation of the sulfuric acid formed in this solution offers no difficulty; care must be taken, but by a proper choice of indicator, volume, and lighting, the end point can be noted to an accuracy of 0.05 ml. of 0.01 N sodium hydroxide, corresponding to 0.0005 per cent of sulfur on a 1.6-gram sample of steel. The facts summarized above were discovered experimentally by a process of successive elimination of the several sources of error, a procedure which proved to be unavoidable and required a great number of analyses of standard samples of steels of every type obtainable. Even with all these precautions the results tend to be low, though consistent with one another. By washing out the combustion tube after a considerable number of samples have been burned, the authors found that some sulfur is retained in it, though none seems to be retained by the boat as used. The amount retained in the tube seems insufficient to account for the difference between the authors' results and the accepted values. I n any case, the results on ordinary steels of low and medium sulfur content are sufficiently accurate for practical purposes, and on any steel are consistent even though they may be low by about 8 per cent of the accepted value on steels of the highest sulfur content. I n view of its consistency, the method may safely be used on any steel by use of a factor established by comparison with a series of analyses of a standard similar in type to the sample; and it is especially suitable for highalloy steels (generally of low sulfur content) which resist solution in acid.
Apparatus and Reagents The apparatus for determining sulfur, either alone or in conjunction with carbon, closely resembles the usual carbon combustion train. In the sulfur determination, however, few variations in apparatus or procedure are permissible. Certain parts are of special design, others must be made of certain materials only; therefore the whole assembly is described piece by piece and in detail. The apparatus can be purchased complete as a unit or parts may be ordered separately. OXYGENSUPPLY.The oxygen used should be not less than 99.5 per cent pure and should be free from carbon monoxide, carbon dioxide, or any other acidic or carbon-bearing material. If oxygen of this grade cannot be obtained, special provision must be made t o purify it. Reducing valves must be used t o step the cylinder pressure down to not more than 10 cm. of mercury and permit a steady controlled flow of gas through the train. Several different types of valves suitable for this purpose are readily obtainable. It is convenient to insert a bubbler containing sulfuric acid t o indicate the rate of flow. OXYGEN PURIFYINQ TRAIN. With oxygen of the purity specified above, there is no need of the purifying train; nevertheless, as a precaution it is customary to insert a carbon dioxide absorbent, such as soda lime, followed by calcium chloride. It is convenient to have attached to the inlet side of the train a reservoir, acting as a pressure regulator, from which an auxiliary supply of oxygen is drawn during the actual combustion. MERCLTRY TRAPOR MANOMETER. Either may be used; the mercury t.rap is preferred by some, as it serves not only to indicate the pressure roughly, but also to prevent back flow of gas. ELECTRIC FURNACE.For the temperatures needed (1200" to 1425" C.) the authors have used a type of resistance furnace with replaceable silicon carbide rods, which operates on 110 or 220 volts, alternating or direct current, and has proved entirely satisfactory for this work. (The authors have used a high-temperature carbon combustion furnace with replaceable Globar elements, controlled by a carbon rheostat, all furnished by the Burrell Technical Supply Co.) It is about 35 cm. (14 inches) long, 30 em. (12 inches) outside diameter, and takes a tube 5 em. (2 inches) in diameter. A new furnace should be heated slowly to the operating temperature, as very rapid heating may cause the refractory to crack and spa11 from the outer insulating material. The furnace withstands continuous service at 1400" C. indefinitely; the useful life of the heating elements is from 1 t o 3 months. TEMPERATURE CONTROL.Some means of controlling the temperature is desirable, as a temperature above that needed for B
SEPTEMBER 15, 1936
ANALYTICAL EDITION
given steel causes unnecessary wear and tear on the combustion tube and furnace. Control of the heating current is accom lished simply by a suitable rheostat in series with the furnace. kuch a rheostat must be capable of carrying nearly 30 amperes; any type meeting this requirement and affording flexible control will do. Carbon plate resistors accomplish stepless regulation of current; the more common plate type of laboratory rheostat is not as flexible, but, in case no ammeter is available to indicate the current flow, the approximate value may be judged by the position of the contact arm if the voltage may be assumed to remain constant. The temperature is measured by use of a noble metal thermocouple whose hot junction is placed at the middle of the heating chamber as close to the combustion tube as is practicable. It is well to check the temperature of this position against that inside of the tube by sighting an optical pyrometer on a heated boat in the combustion position while noting the temperature indicated by the thermocouple. Automatic control is easily applied and is to be recommended wherever the amount of work to be done justifies the expense. COMBUSTION TUBE. Only an unglazed, vitrified tube of porcelain may be used; other commercial types are either affected by the high temperature or tend to react to some extent with the sulfur dioxide evolved. The tube life should be as long as is usual in the case of carbon determinations. As to size, a tube 2.6 cm. (1.125 inches) inside diameter and 68 or 70 cm. (27 or 28 inches) in length has been found most satisfactory, though the length may be reduced to 62.5 cm. (25 inches) if suitable provision is made for keeping the intake end reasonably cool. The exit end is connected to the absorption vessel by a ground orcelainto-glass joint. By carefully standardizing the taper adgrinding of the two tubes, the parts have been made interchangeable so that a gas-tight joint is secured between any combustion tube connected to any standard absorber. The joint is seated and held firmly in lace by means of s iral springs anchored to the safety shield ofthe furnace and ensing in hooks which slip over lugs on the inlet tube of the absorber. Details of this connection are shown in Figure 1. COMB.USTION BOAT. The boat material must be carefully selected. The shape also is designed to permit easy evolution of the sulfur; the boats are rectangular in plan with length 10 cm. (4inches), width 2 cm. (0.8inch), and depth 1cm. (0.4 inch), outside dimensions. The walls are thin and at right angles to the flat bottom, affording a large bottom surface upon which the sample can be thinly spread in a single layer. ABSORPTION VESSEL. This vessel 20 cm. (8 inches) in height and 3 cm. (12 inches) .n diameter, shown in Figure 1, consists of an inlet tube, connected through the interchangeable ground joint with the combustion tube, sealed to a glass frit (Jena Gl), which breaks the gas stream into a cloud of small bubbles, thus effecting complete absorption of the sulfur dioxide and sulfur trioxide in the solution in the vessel. The inlet tube enters the absorption vessel through a ground-glass joint, so that the gas passing .through this vessel may be forced without loss through the train for carbon absorption. The solution is rendered distinctly acid by absorption of the sulfur gases, and retains none of the carbon dioxide, even that which would normally dissolve being swept out by the stream of carbon dioxide-free oxygen passed after the combustion is complete. REAGENTS.The 0.5 per cent hydrogen peroxide solution must be made by dilution with carbon dioxide-free water. A convenient strength for the sodium hydroxide solution is 0.01 N ; this also must be made up and kept free of carbon dioxide. It is well to have at hand 0.01 N acid for back-titration or for deliberate adjustment of an end point.
Procedure DETERMINATION OF SULFUR ALONE. First make sure that the sulfur absorber is perfectly clean and otherwise in good condition. If the frit or vessel appears to contain much oxide fume, add a little warm 1 to 1 hydrochloric acid through the intake tube, then rinse thoroughly with distilled water until the washings are neutral to the methyl red indicator. Transfer to the absorber 50 ml. of a 0.5 per cent hydrogen peroxide solution (prepared by diluting 17 ml. of 30 per cent hydrogen peroxide to 1000 ml. with carbon dioxide-free water) and add 2 to 3 drops of methyl red indicator, prepared by dissolving 0.1 gram of methyl red in 60 ml. of 85 to 95 per cent alcohol and diluting to 100 ml. with carbon dioxide-free water. If the solution is red in color, as it generally is, owing to a trace of acid in the eroxide, add 0.01 N carbonate-free sodium hydroxide solution &opwise until one drop just dispels the red color; this is taken as the end point. If this point is overtitrated, add one or more drops of 0.01 N carbon dioxide-free acid, and adjust to the end point with the standard aodium hydroxide solution.
319
Next transfer 1.6 grams of the steel sample (0.8 gram, if sulfur is over 0.3 per cent) to a clean new combustion boat, spreading the drillings or chips over the bottom so that they lie as nearly as possible in a single layer. If tin is required as an accelerator, y d 0.15 to 0.2 gram of tin shot (20-mesh) over the sample, istributing it as evenly as possible. Push the boat into the combustion tube for a distance of 5 to 7 om. (2 to 3 inches), and attach the sulfur absorber to the exit end of the tube, making sure that all parts are held firmly together by the rubber bands and the coil springs provided for the purpose. Finally, push the boat into the hottest zone of the tube, insert the stopper, and turn on the oxygen to flow at the rate of about 300 ml. per minute. Continue to admit the oxygen at this rate for 10 minutes. The solution in the sulfur absorber will suddenly turn red in color as the evolved sulfur dioxide reaches it. When sufficient time has elapsed to sweep all the sulfur gases out of the tube, turn off the oxygen and disconnect the absorber; if its temperature has been noticeably raised, place it in running water to cool. Then pull the boat out of the furnace and discard it; in removing the boat do not let it rest in the cold end of the tube as it may crack the tube, but pull it out a t one stroke and without a stop on to the receiving tray held just under and in front of the opening. Using the glass frit with its delivery tube as a lunger, add the 0.01 N sodium hydroxide solution until the r e i color just disappears. Rinse the frit three times through the inlet end of the tube by means of a stream of carbon dioxide-free water from a wash bottle, adding the rinsings to the solution in the absorber, and once more add the standard sodium hydroxide solution to the disappearance of the red color. Finally raise the frit over the surface of the liquid, allow it to drain, and complete the titration if the red color reappears. If the solution is exactly 0.01 N and 1.6 grams of sample were used, the number of milliliters used gives the sulfur in hundredths of one per cent. Otherwise, multiply the number of milliliters used by the sulfur titer of the solution, multiplied by 100, and divide by the weight of sample used. The titrated solution is poured from the absorber and replaced with another 50 ml. of the hydrogen peroxide solution preparatory to another determination. No rinsing of the absorber is necessary except after cleaning with acid.
MIXIMUMCOMBUSTION TEMPERATURE FOR DIFFERENT STEELS. It is desirable to operate at the lowest temperature possible to prolong the useful life of the tube and heating elements. The exact temperature necessary varies somewhat with the kind of steel being analyzed and the time available for continuing the stream of oxygen through the train. If no tin or other accelerator is used, the lowest safe operating temperatures and other operating data for a number of steels are given in Table I. TABLE I MINIMUM COMBUSTION TEMPERATURE Kind of Steel
Wt. of Sample Grama
Plain carbon Low alloy 18-8 2 5 7 chrome Hi&-speed tool
1.6 1.6 1.6 1.6 1.6
Lowest Safe Temperature ' C. F. 1200 2200 1260 2300 1340 2450 1400 2550 1370 2500
Total Time Rate of of Passing
02 Flow
Os
Ml./min. 250 250 250 250 250
Min. 12 12 12 12 12
These temperatures are satisfactory for the sulfur determination without the use of tin, but for complete carbon elimination tin shot should be added to samples of the last four types. This use of flux also permits a somewhat lower combustion temperature for sulfur liberation, thus conserving heating elements and combustion tube. Hence the addition of tin and a temperature of 1300" t o 1350" C. are recommended for steels containing more than 20 per cent of chromium or more than 30 per cent of total alloying elements. At these temperatures combustion of the steel is complete within 3 minutes after pushing the boat into the hot zone and turning on the oxygen. The rest of the time is required to sweep the sulfur dioxide and carbon dioxide completely into the absorption vessel, this depending upon the maximum flow that can be maintained. The maximum flow in turn depends upon the porosity and condition of the bubbler in the sulfur absorber, and on the method used for
INDUSTRIAL AND ENGINEERING CHEMISTRY
320
VOL. 8 , NO. 5
TABLE11. DETERMINATION OF SULFUR ALONE --Certificate No. of analyses
Standard Employed U. S. Steel Corp. Standard K, plain steel Standard L, plain steel Bureau of Standards Standard Sample 35a, plain steelo 16b, plain steel 9a, plain steel 160,plain steel 130,plain steel 20c, plain steel 8d, plain steel 55 ingot iron 50' W 17 56 V 0 756 Cr 3.61 73: Cr 13:93: Ni 0.07i,Si 0.360 101 1&8 steel 105' special high sulfur, not standard
25 25
Value of Standard, Sulfur-
High %
Low
0.121 0.064
0.104 0.056
%
17 15 21 21 14 16 18 14 12 17 20
Value Obtained by Combustion as Described, Sulfur No. of analyses High Low Av.
Av. %
%
%
%
0.114 0.059
11 9
0.111 0.059
0.101 0.053
0.106 0.056
0.037 0.031 0.036 0.044 0.023 0.026 0.083 0.017 0.031 0.031 0.013
2
0.044
...
0.044
01043
0 042
0:ois 0.017 0.033
0.044 0.033 0.036 0 042 0,023 0.028 0.077 0.016 0.031 0.027 0.011 0.53
1
1 3
...
:. . .
1
1 12 4 17 2 21
0.027
0.oii
01075 0.015 0.030 0.027 0,010
4
0.54
0.52
f & sulfur About 0.6 a Bureau of Standards does not recomme!nd this steel as a standard for sulfur by evolution method.
I
Results
determining the carbon. I n the gravimetric method for carbon the maximum flow through the bubbler governs; in the volumetric method the maximum flow permissible through the carbon dioxide absorption bulb is the controlling factor.
Some typical results on a variety of steels are presented in Tables I1 and 111, all obtained by following the exact procedure described above, which show that this analytical SIMULTANEOUS DETERMINATION OF SULFURAND CARBON. method is capable of yielding consistent results, either for Since this type of sulfur absorber removes all oxides of sulfur sulfur alone or for sulfur and carbon simultaneously. (as found by direct trial) carbon may be determined simulIt is to be noted that these results are nearly always sometaneously by leading the gases from it through a carbon abwhat lower than the accepted standard values, the differsorption train arranged for either the volumetric or the gravience being greater the higher the sulfur content. A part of metric determination of carbon. For the gravimetric deterthis difference arises from the fact that a trace of the sulfur mination of carbon it is essential that the concentration of gases is retained within the combustion tube. Another poswater vapor in the gases entering the carbon-absorption sible source of slight error is that the combustion of the steel bulb be controlled so that no appreciable amount of water is under the authors' conditions sometimes produces a slight either yielded to, or abstracted from, that bulb. This can be fume, visible in the absorption vessel, the amount varying done by use of a sulfuric acid bubbler, followed by a tower from one steel to another, and that this fume may in effect containing the identical desiccant used in the carbon-absorpfix a trace of the sulfur oxides. The authors have, however, tion bulb. The type of bubbler described by Lundell, Hoffbeen unable to establish this effect, and mention this posman, and Bright (7) is convenient, as it readily permits the sibility merely because the results on one steel, which evolved acid to be changed after each run without disconnection of a larger amount of fume, appeared to be relatively lower, in the tower. If the carbon is to be determined volumetrically comparison to the accepted values, than those on the other and is under 0.60 per cent, use 1.6 grams of sample; if more steels. It is their present belief that these two sources of than 0.60 per cent carbon is expected, use 0.8 gram of sample. error do not suffice to account for the differencesbetween their If the carbon is to be determined gravimetrically, 1.6 grams of results obtained by the combustion method and the accepted sample may be used for all steels containing less than 0.3 values, and that the reason for these differences can be disper cent of sulfur, regardless of the percentage of carbon covered only by further careful investigation both of the compresent. bustion method and of the methods by which the accepted DETERMINATION OF SELENIUMSIMULTANEOUSLY WITH values were obtained. I n the meantime it seemed worth SULFUR AND CARBON.I n the method as described selenium while to publish this study of the method, which in any case is evolved as SeOz. Unfortunately the selenium dioxide is satisfactory for steels of low sulfur content, and particubegins to sublime a t 250" C. and, consequently, condenses in larly for high-alloy steels which are not easily analyaed for the cooler portion of the combustion tube. Research is now sulfur by other methods. being carried on with a view to devising a method which will permit the determination of selenium as well as sulfur and Summary carbon. The work done so far looks very promising and The apparatus and procedure are described in detail for the indicates that a solution of the problem in the near future may determination, within 15 minutes, of sulfur or of sulfur and be anticipated. TABLE 111.
Standard Employment
sIMULTANEOUS
Values Obtained by Combustion
---
-Carbon-NO. of rtnaly-
9 0.583 0.569 0.573
14 0.025 0.020 0.023
7Sulfur High Low Av. High Low Av. % % % % % % 5 0.582 0.563 0.~572~0.020 0.020 0.020
13 0.682 0.630 0.656 13 0.064 0.058 0.063 3 , , ., . . . 0,109
12 0.034 0.026 0.031 20 0.015 0.010 0.013 3 ... . , . 0.031
5 0.680 0.665 0.671b 0.031 0.030 0.030 2 0.065 0.060 0.063, 0.012 0.010 0.011 10 0.110 0,102 0.106'~ 0.033 0.026 0.030
% Bureau of Standards. 130. Dlain steel Bureau of Standard 50; *alloy steel, W 17.56, V 0.756, &r 3.61 Bureau of Standards, 101, 18-8 steel Standard B, plain steel Bureau of Standards, 105,special highsulfur, provisional certificate 0 Carbon determined volumerrically b Carbon determined grnwnetricalls.
DETERMINATION O F SULFUR AND CARBON
Certificate Value of Standards Employed -Carbon-SulfurNo. No. of of analyansly58s High Low Av. ses High Low
, ,
,,
%
.
.
...
%
0.193
%
%
Av.
%
About 0.6
ses
1
.,.
...
0.21a
,
..
...
0.52
SEPTEMBER 15,1936
32 1
ANALYTICAL EDITION
carbon simultaneously, in all kinds of steels, by combustion of the steel in oxygen at a high temperature. The results for sulfur are slightly lower than the values commonly accepted as correct, but are consistent and reproducible and satisfactorily accurate for most practical purposes, particularly in high-alloy steels containing little sulfur.
Acknowledgment The authors wish to acknowledge the hela advice. and encouragement of John Johnston a n i to recordtheir appreciation of the cooperation accorded by various members of the United States Steel Corporation Chemists Committee.
Literature Cited (1) Guedras, Aciers spdciaux, 6,76-80(1931). (2) Holthaus, Arch. Eisenhiittenw., 5 9 5 (1931-32).
(3) H o l t h a ~ sStaM , u. Eisen, 44,1514 (1924). (4) Isham and Aumer, J *Am* Chem. SOC., 30,1236-9 (1908). (5) Kar, IND.ENG.CHEM., Anal. Ed., 7,244 (1935). (6) ~ ~Chem.-Ztg,, ~ s7,573-4 ~ (1933). l ~ ~ , (7) Lundell, Hoffman, and Bright, “Chemical Analysis of Iron and Steel,” New York, John Wiley & Sons, 1931. (8) Misson, Chimie & Induatrie, Special No. 27, 326-8 (March, 1932). (9) Rooney, Analyst, 59,278-80 (1934). (10) Schmitz, Stahl u. Eisen, 39,406-12 (1919). (11) Seuthe, Ibid.. 52.445 (1932). (12) Swobodal Chem.9 77,269 (1924). (13) Thanheiser and Dickens, Mitt. Kaiser-Wilhelm Inst. Eisenforsch. DQsseldorf, 15, 255 (1933); Arch. Eisenhattenw., 7, 557-82 (1934). (14) Vita, Stah2 u. Eisen, 40,933 (1920). (15) Zenker, Arch. Eisenhiitfenw., 5,101 (1931-32).
z.
RECEIVED July 23. 1936.
The Dextrose-Levulose Ratio and the Polarizing Constants of Raw Cane Sugars F. W. ZERBAN, New York Sugar Trade Laboratory, Inc., 80 South St., New York, N. Y.
U
NTIL recently the only practical method for the determination of dextrose and levulose in raw sugars was that of Browne ( I ) , based on the difference between sucrose, S, and direct polarization, P, and the total reducing sugars, R. The ratio between S - P and R has been termed the “polarizing constant.” The numerical value of the constant is about 0.3 when the reducing sugars consist of dextrose and levulose in equal proportions, varies inversely with the dextrose-levulose ratio, and becomes negative when this ratio rises above 64 to 36. To calculate the percentages of dextrose and levulose in a raw sugar from S - P and R, Browne makes use of Equations 1 and 2: x + k y = R (1) cz qy s =P (2)
+
+
where x and y are the percentages of dextrose and levulose, respectively, k is the reducing ratio of levulose to dextrose, c the polarizing ratio of dextrose to sucrose at 20” C., c1 that of levulose to sucrose, R the percentage of total reducing sugars, expressed as dextrose, P the direct polarization of the normal weight solution at 20” C., and S the percentage of sucrose found by inversion with invertase. Solving the equations for x and y, CR + S - P y = percentage of levulose = kc - CI x = percentage of dextrose = R - ky The numerical value of c is 52.74 : 66.5 = 0.793, that of el is -92.88 : 66.5 = - 1.397. The latter varies somewhat with concentration and temperature, but in practice only the temperature correction needs to be considered, and it is best to make all polarizations at exactly 20” C. For k an average value of 0.915 may be used if Allihn’s method of reducing sugar determination is employed. Applying this method to the analysis of mixtures of pure sugars, Browne found that it gives very good results when the ratio between sucrose and reducing sugars is low. But when the ratio is high, as in raw sugars, the difference between the sucrose and the direct polarization becomes very small, and any error in the polarimetric readings has a large effect on the percentages of dextrose and levulose found, especially if the errors in the two readings happen to be in the opposite direc-
tion. In one experiment, with a mixture of 98 per cent of sucrose and 1 per cent each of dextrose and levulose, Browne found 0.75 per cent of dextrose and 1.17 per cent of levulosethat is, a percentage ratio of 39.1 between dextrose and total reducing sugars, instead of 50. In 1934 Zerban and Wiley (6)published a new method for the determination of dextrose and levulose in the presence of large amounts of sucrose, extending previous work by Jackson and Mathews (3) to the analysis of raw sugars. The total reducing sugars are determined by the method of Lane and Eynon, and the levulose is determined by selective reduction of a modified Ost’s solution at 55” C. A table of Lane and Eynon factors for 10 ml. of Soxhlet solution and for mixtures of dextrose and levulose in all possible proportions, in the presence of 10 or 25 grams of sucrose, has been given by Zerban and Wiley (4, as well as a method for calculating the percentages of dextrose and of levulose, by successive approximations. The calculation may be shortened materially by the use of Equations 3 and 4: a x + y = R 0.0806 x y = RI
+
(3) (4)
where x and y are the milligrams of dextrose and levulose, respectively, in 100 ml. of solution analyzed. R represents milligrams of total reducing sugars, expressed as levulose, in 100 ml. of solution. It is calculated, as usual, by multiplying the Lane and Eynon factor in Table I by 100, and dividing by the titer found. a is the varying reducing ratio between dextrose and levulose, and is also given in Table I. R1 is the milligrams of apparent levulose in 100 ml. of solution, found by the method of Jackson and Mathews, and corrected for the reducing effect of the sucrose. The factor 0.0806 is the constant reducing ratio of dextrose to levulose in the method of Jackson and Mathews, 12.4 mg. of dextrose having the same reducing effect as 1mg. of levulose. Solution of Equations 3 and 4 gives R -Ri u - 0.0806 y = mg. of levulose in 100 ml. of solution = R - ax z = mg. of dextrose in 100 mi. of solution =
The values of the denominator, cluded in Table I.
Q
- 0.0806, are also in-
