April, 1928
INDUSTRIAL AND ENGINEERING CHEMISTRY
In the tests described above, which were carried out with benzene, a previous calibration of the instrument against carbon monoxide was used to advantage. The ratio of the heat of combustion of carbon monoxidebenzene vapor is 3034:32978, or 0.0920:l. The percentages of carbon monoxide indicated by the instrument must, therefore, be multiplied by 0.0920 to obtain the percentage of benzene. It had been previously found that 1 per cent carbon monoxide corresponds to 3060 microvolts; 0.1 per cent benzene vapor therefore corresponds to 3326 microvolts in the record (Figure 2). Notes o n Method
In all tests an arbitrary, though constant, surface must be exposed to the benzene, which can be most easily accomplished by using a standard size of hose for the test and
43 1
immersing a definite length in the liquid. Also, a constant flow of air must be run through the hose to receive comparable and reproducible results. The 100 per cent duration of resistance cannot, of course, be changed by varying the two standards just mentioned but only by changing the apparent rate of breaking down after the hose once starts to permit the penetration of benzene. If this method is used for developing resistant rubber compounds, then the wall thickness of the test sample will also have to be kept constant. This, however, is irrelevant if, as in this case, the method is used to pick the best hose from a number offered. The influence of the temperature upon the rate of penetration was not investigated, although it might well be of importance. The tests were made a t a temperature of 20’ * 3 ” C., and were found to be sufficiently reproducible.
Graphical Methods in Lacquer Technology’ H. E. Hofmann and E. W. Reid MELLONINSTITUTE OF INDUSTRIAL RESEARCH, UNIVERSITY OB PITTSBURGH, PITTSBURGH, PA.
Smeral common methods of graphical representatiof t of experimental data are discussed and their applicability to certain poblems of the lacquer chemist i s noted. Two typical alignment charts are given, showing methods of calculating viscosity and spec$% gravity of mixtures. HE lacquer industry, in the modern meaning of the term, has come into existence during the present era of technical investigation and control in the manufacturing industries. For this reason it is not remarkable that it has been conducted from the beginning on a more scientific basis than many of the older industries. Nitrocellulose lacquers are capable of accurate scientific formulation and control; the seemingly countless number of “exceptions” to what we think are general laws and the seeming impossibility of predicting the result of combining certain ingredients in certain proportions are merely the results of our too inadequate knowledge of the fundamental nature and behavior of these ingredients, coupled with the possibility that there may be many more variables in the procedure than we recognize. Many investigators are already a t work on some of the more basic problems of the industry. A few of these are rather simple, while others present difficulties which may appear almost insurmountable. In any event, the problems generally lend themselves readily to mathematical or graphical treatment. It is the authors’ purpose in this paper to illustrate some of the more common methods of graphical representation which may be applied to the problems of lacquer technology, and to point out their peculiarities and methods of analyzing them.
boiling-point curve, of a lacquer solvent falls in this class. The two variables are “per cent distilled” and “boiling temperature,” the latter depending on the former for any given sample of material. Figure 1 shows a few typical curves. Curve A is one given by a pure compound; note the constancy of the boiling point. Of course, in such a case the curve should be a straight line parallel to the base line; but in the determination of the data from which such a curve is plotted, the initial rise in temperature is due to the fact that it takes time for the thermometer to become heated to the temperature of the vapor (and during this time liquid is distilling), and the rise in the temperature of the vapor near the end of the distillation is due to unavoidable superheating. Such a curve never quite reaches the 100 per cent line, since it takes considerable vapor to fill the interior of the flask, and this vapor does not distil over, but condenses and remains in the flask. Curve B is typical of a liquid containing a percentage of another compound (usually of the same homologous series or, in the case of an ester, the corresponding alcohol). If the boiling points of the two compounds are nTidely different, a rather abrupt rise in the temperature takes place when the second component begins to come over; while if their boiling points are only a few degrees apart they distil together, and there is only a gradual rise in the temperature. Curve C is the type obtained when distilling a mixture Rectangular Coordinates of a large number of homologs, such as is encountered in Systems containing only two variables, one dependent mixtures of petroleum hydrocarbons. A curve similar to A is also obtained with binary and upon the other, are most effectively represented by the familiar rectangular coordinates. In a large number of in- ternary azeotropic mixtures, and the investigator of an stances it is possible to treat systems containing more than unknown liquid should not base conclusions upon the two variables in this manner, provided all but two of the results of only one test. variable factors are controlled in the experiment. I n such EVAPORATION CURvEs-The rate of evaporation is another cases it is possible to show the effect of a third variable by commonly determined property of a solvent which is readily preparing a series of charts each representing the values of plotted by means of rectangular coordinates. The usual variables I and I1 for a certain definite value of variable 111. method is to plot the amount (or per cent) evaporated BOILINGPOISTCURVES-The so-called boiling range, or against the time, keeping the temperature, etc., constant. (Figure 2) It is recognized that control of all but two 1 Received December 2 7 , 1927
T
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Figure 1-Boiling-Point
Curves
variables in a procedure of this kind is rather difhcult and inv e s t i g a t o r s are not agreed as to what the “etc.” includes. The routine procedure as now used consists of allowing small quantities t o evaporate spontaneously under the same conditions, so that in each test it is necessary to include
Vol. 20, No. 4
cellulose in a- true solvent until precipitation begins really represent only one variable, the volume of non-solvent added. The amounts of nitrocellulose and solvent, as well as the temperature, are kept constant. The nature of the true solvent varies from one test to another, but this is not a functional variation. Such data may be represented graphically as shown in Figure 4. The kind of solvent is named, and the amount of non-solvent added per unit of solvent is nieasured by the length of the line or bar. If such dilution ratios, using the same non-solvent, are obtained with successive members of any homologous series
one or more controls. The following observations may be made from the curves in Figure 2: 1-The straighter lines represent pure compounds, while the curved lines represent mixtures. 2-All the liquids, especially the more volatile ones, evaporate most rapidly at the beginning. This is due, especially in the case of a pure chemical compound, to adiabatic cooling caused by the evaporation. This emphasizes the fact that in such an experiment the temDerature should be that of the liquid itself, and not that of its surroundings. 3-A retardation of evaporation is observed when the cont a i n e r is nearly dry, since there is no longer the same active surface available for evaporation. 4-Curve B indicates that the mixture contains a non-volatile ingredient.
When it is desired to observe the rates of e v a p o r a t i o n of a Figure 2-Evaporation Curves liquid a t d i f f e r e n t temperatures, we may plot rate of evaporation in “per cent of standard sample evaporated per hour” or in “grams per square centimeter per hour,” against temperature as the independent variable. TENSILE-STRENGTH CuRvEs-Passing from the investigation of the solvent to the actual lacquer film,one of the more important and readily determinable factors is the tensile strength of the film. Closely associated with tensile strength is the elongation of the film under various loads. The usual method of representing this kind of data graphically is shown in Figure 3, where the ordinates are loads in kilograms per square centimeter and the abscissas are elongations in per cent. The end of each line indicates the point a t which the film b r o k e i n t h e s t a n d a r d method of test. The curves extending almost parallel to the “load” axis with little elongation, indicate strong films which are apt to be brittle; while curves showing g r e a t elongation a t a small load indicate a soft, distensible film. Of course, in obtaining the data for such curves it is necesFigure &Tensile Strength sarv to hold constant the variCurves abfe factors such as temperature, humidity, thickness of film, and rate of applying the load. DILUTION h T I o s - T h e values of dilution ratios obtained when a standard non-solvent is added to a solution of nitro-
ETHEROF ETHYLENE GLYCOL Figure &Dilution Ratios Total ordinate-dilution ratio with toluene. Unshaded portion only-turpentine
of solvents, the effect of increasing molecular weight of the solvent may be observed-and conversely, for a series of dilution ratios using the same solvent but different nonsolvents. If it is desired to study the effect upon the dilution ratio of two non-solvents-e. g., an alcohol and a hydrocarbonmixed in varying proportions, the system again becomes one of two variables-dilution ratio us. composition of nonsolvent. The method of plotting data of this nature is illustrated in Figure 5. There are numerous other applications of rectangular coordinates in plotting the data obtained in investigating v a r i o u s properties of lacquers and l a c q u e r 3 ingredients, and those m e n t i o n e d are only ’ offered as suggestions of the wide possibilities 6 of graphical representa- E tion in this field. d ,L)
Triangular Coordinates
T h e triangular coordinate system lends
to adaptation i n t h e 100 so 80 70CoWwalTi@r* ~%AUO*DL30 22 ID 0 Of lacquer tech- Figure 5-Effect of Mixture of Two nology. These coordiNon-Solvents o n Dilution Ratio nates are used to represent the behavior of a system containing three variables, each varying, but whose sum is constant. The method at once suggests adaptation to the study of solvent compositions-e. E . ~where the solvents are an ester, an alcohol, and a hydrocarbon-and to the investigation of variations &I the solid composition, using the same solvent mixture, since the chief components of the solid part of a lacquer are nitrocellulose, resin, and plasticizer. The constant sum is 100 per cent in each case. OF DlLULVT
43 3
INDUSTRIAL A N D ENGINEERING CHEMILTTRY
April, 1928
BUTYL ACETATE
Figure 6-Form a n d Method of Reading Triangular CoGrdinates
Figure 7-Solubility of Nitrocellulos in a Solvent Mixture
The system is represented by an equilateral triangle, each side of which is divided into equal (usually 100) parts (Figure 6). Lines are drawn parallel to each side, through these equal divisions, dividing the large triangle into a number of smaller ones. Any composition may be represented by a point, the points A , B, and C representing 100 per cent of each of the pure components, A , B, or C, respectively. Any point on the line AC represents a mixture of A and C only, in which the component B is absent-e. g., point D represents 60 per cent A and 40 per cent C. If now we leave the line AC, we introduce B in proportion to the distance from AC. Thus if we add pure B to the mixture represented a t D, the composition will vary along the dotted line shown connecting D with B. Thus, a t point E the composition of the mixture is 30 per cent A , 50 per cent B , and 20 per cent C. (Note thst the ratio of A to C is still 60 t o 40.) ' The concen-
ALCOHOL
intervals and the curve is drawn through them. They are determined in much the same manner as the dilution ratio. The non-solvent is added to a solution of nitrocellulose in the solvent until a slight precipitate is obtained which will not dissolve on stirring or shaking. The position of the curve obtained by connecting these points will vary slightly with the final concentration of nitrocellulose in the solutions used for its determination, but the difference can usually be neglected. Solvent compositions represented by points below the line are therefore not solvents for nitrocellulose. It will be noted that for some mixtures of alcohol and hydrocarbon less ester is required to cause the mixture to dissolve the nitrocellulose than in the case of either the alcohol or hydrocarbon alone. This is the same fact as that shown in Figure 5, only the former shows it in a more striking manner. This same type of chart may also be used to plot the BUTYLACETATE
BUTYLACETATE
&TIL
Figure &-Composition of Solvent Mixture at Various Stages of Evaporation
BENZENE
Figure 1OSolvent Containing Butyl Acetate, Figure 9--Solvent Containing Butyl Acetate, Figure l l 4 o l v e n t Containing Butyl Acetate. Butyl Alcohol, and Toluene Butyl Alcohol, and Benzene Butyl Alcohol, and Xylene Behavior of Definite Combination of Solid Ingredients in Solvents of Varying Composition
tration of any component increases as the point approaches the vertex of the triangle representing that pure component. The mathematical or geometrical reason that these coordinates may be so conveniently used is that the sum of the perpendiculars, from any point in an equilateral triangle to the sides, is constant. SOLUBILITY OF NITROCELLULOSE AND REsIivs-one of the first problems suggested to the lacquer chemist in this connection is the relation of composition to the ability of a solvent mixture to dissolve nitrocellulose. Figure 7 shows the form of the solubility curve of nitrocellulose in mixtures of an ester, an alcohol, and a hydrocarbon (heavy solid line), and mixtures of a glycol ether, alcohol, and hydrocarbon (broken line). Note that less ester is required in connection with the alcohol alone than with the hydrocarbon alone. The points on the line MN and RS are obtained a t definite
solubility of certain resins-such as dammar, shellac, manila, etc.-that are not soluble in all the solvent ingredients. In fact, the field of usefulness of the triangular coordinate system is almost unlimited, and the individual investigator must use his own ingenuity to adapt it to the work in hand. COMPOSITION OF A SOLVEKT MIXTURE AT VARIOUSSTAGE^ OF EVAPORATION (Figure 8)-Let us assume a mixture of benzene, butyl acetate, and fusel oil, of the composition indicated a t the point 0. As evaporation progresses the composition changes as shown, the small numbers indicating the percentage evaporated. The bulk of the benzene first evaporates, then a mixture composed chiefly of butyl acetate, the final composition of the mixture tending to approach 100 per cent fusel oil. As in this example, the behavior on evaporation may be deduced without recourse to such a chart, but in a large number of cases, where the solvents
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434 ESTER
BUTYL ACETATE
Vol. 20, No. 4 CELLOSOLVE
h
TOLUENE
ENATUAED
ALCOVOL
Figure 12-Viscosity a n d Solvent Composition NITRDCELLULOSE
Figure 13-Cost
Data
tend to evaporate at m o r e n e a r l y equal rates, data of this sort are valuable aids to solvent formulation. BEHAVIOROF A C E R T A I NCOMBINATION OF SOLID ING R E D I E N T S I N SOLVENTS OF VARYINQ COMPOSITION-Let US
assume that we wish to study a mixture of equal parts of I/TsecFigure 14-Lacquer Properties ond nitrocellulose and wax-free b l e a c h e d shellac in varying compositions of a solvent containing butyl acetate, butyl alcohol, and benzene. The details of the method of obtaining the data will not be given, but suffice to say that it is the dry film we observe, and not the lacquer solution. The finished graph, an example of which is shown in Figure 9, then consists of three zones. The compositions represented by points below the line M N will be eliminated from further consideration, since they do not dissolve nitrocellulose. Solvent compositions represented by points in zone B above and to the right of the broken line in the center of the chart, produce films that are heterogeneous-that is, they tend to be turbid or separate into two phases. Finally, the mixtures included in zone A are good solvents for the lacquer in question and produce clear films. Any solvent composition in this zone may be used, and the choice will then depend upon further considerations, such as cost, viscosity, etc. The effect of substituting other hydrocarbons, such as toluene and xylene, for benzene is indicated in Figures 10 and 11. Increasing the boiling point of the hydrocarbon decreases the amount that may be used, since it evaporates more slowly and shellac is insoluble in hydrocarbons or mixtures containing more than a definite proportion of hydrocarbons. Of course, this fact may be deduced from the properties of the substances studied, but the quantitative results may only be gained from a procedure similar to that outlined above. It is especially important to bear in mind that a lacquer solution may be clear and homogeneous in the liquid state, but that this gives no assurance that the same lacquer will deposit a clear transparent film. It is therefore advisable to obtain all data for compatibility charts, such as have been described above, on the dry lacquer film. Such data will also be more useful in connection with practical formulation. VrscosITY-Another property of a lacquer or nitrocellulose solution that varies with different solvent compositions is viscosity. For example, it is well known that a solvent
ester containing 5 to
Figure 13A-Cost
Data
April, 1928
INDUSTRIAL Ah'D ENGINEERING CHEMISTRY
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The shape of the solubility curves and the slope of the price number would have viscosities varying only within rather curves will vary with different solvents and combinations narrow limits. If the data were plotted as suggested, the of solvents. For example, see Figure 13A, in which the curve would appear as in Figure 15, the value of the viscosity components of the solvent mixture are Cellosolve (mono- corresponding to the peak of the curve probably being the ethyl ether of ethylene glycol), denatured alcohol, and toluene. optimum viscosity. This method of analysis will lend The solubility curve (heavy curved line) is lower a t the right itself to the study of numerous other properties of lacquers end than in Figure 13, showing that a larger percentage of and their raw materials. A very useful device for the representation of the compotoluene may be used. The lines of constant price (dotted lines) have a different slope than those in Figure 13, since the sition of a lacquer, and one that is easily read and understood raw materials have different prices. It will be seen from by the uninitiated, is that shown in Figure 16. The circle Figure 13A that the cheapest combination of Cellosolve, represents the whole lacquer, or 100 per cent, and each sector denatured alcohol, and toluene that will dissolve nitrocellulose represents the proportion of each ingredient present in the will have a raw mate- whole. For greater contrast, the sectors may be shaded rial cost of approxi- differently or made up of different colors. mately 55 cents, when the ingredients have Nomographic Charts the following prices: Cellosolve, $1.64 per The chief province of the nomographic or alignment chart gallon: denatured al- is in the solution of equations having many independent cohol, $0.50 per gal- variables, and for simple problems has no advantage over l o n ; a n d t o l u e n e , ordinary arithmetic or logarithms. There are a few cases, $0.40 per gallon. two of which will be discussed here, in which the use of such a PROPERTIE OFS chart is more expeditious than simple calculations. This is L A C Q U E R-The last especiaUy true if problems of the same kind are encountered use of these coordi- repeatedly, as in the actual manufacture of lacquers. As a first example, assume that it is desirable to connates which will be mentioned does not trol the viscosity of a line of lacquers and that the formulas lend itself to quanti- do not always produce lacquers of the same viscosity; or, Figure 16-Composition of Lacquer tative analysis on ac- suppose that we have a thin batch and a thick batch. How count of the lack of suitable and accurate methods of meas- much of each must be mixed in order to obtain a finished uring some of the properties of a lacquer, such as gloss, product of the required viscosity? It is approximately true that when two liquids are mixed adhesion, toughness, etc. This graph has for its variables nitrocellulose, resin, and plasticizer, and is shown in Figure it is not the viscosities that are additive, but rather the 14. On this chart are indicated qualitatively some of the fluidities or reciprocal viscosities. By the use of a chart properties of lacquers of widely different composition; the similar to Figure 173we may calculate at once the proportions general zones of compositions suitable for furniture lacquers, in which to mix two liquids in order to obtain one of a definite automobile lacquers, and the like, are indicated but are not intermediate viscosity. If, for example, two lacquers have definitely outlined, as this would be impossible. It is al- viscosities of 2 and 4 poises, and it is desired to mix them most as futile to ask what is a formula for a good furniture lacquer, as it is to ask what is a good formula for a lacquer. It will be noted from Figure 14 that each ingredient of a lacquer imparts to that lacquer certain specific properties peculiar to that ingredient, and it is almost hopeless to expect to obtain all the desirable properties in one lacquer with the raw materials now in use. I n other words, it is impossible for a point to be in one corner of the triangle and a t the same time in the center or in another corner. The triangular coordinates may also be used to calculate mixtures of three substances, each containing varying percentages of A , B, and C, to obtain a mixture containing a certain desired proportion of A , B , and C. This type of problem, however, seems to have no important place in lacquer manufacture. 0
Miscellaneous Graphical Methods
Several other graphical methods may be adapted to the studies of the lacquer chemist. One of these is the statistical method. Assume, for example, that it is desired to decide upon the proper viscosity for a brushing lacquer. The statistical method of determining this would be to obtain as many samples of brushing lacquers as possible that were in actual satisfactory use, and determine their viscosities. The usual procedure is to plot the viscosities (say to the nearest unit) against the number found having this viscosity. It would undoubtedly be discovered that zt. few would be quite thin find some would be quite thick, and that by far the greatest
A
Figure 17-Nomographic Chart for Use in Mixing Lacquers t o Give Product of Desired Viscosity
to obtain a viscosity of 3 poises, the procedure would be as follows: Place a straightedge connecting the points on the two outside scales A and B , representing the two viscosities 2 and 4. Where this line crosses the line marked 3 poises, read the percentage of substance A in the mixture. A glance at the figure shows that the proper proportions are 33l/3 per cent of the 2-poise and 662/3 per cent of the 4-poise material. 3
Marshall, "Graphieal Methods, etc.," p. 177, McGraw-Hill Book
Co., New York, 1921.
INDUSTRIAL ,AND ENGINEERING CHEMISTRY
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varnish, etc.) to add to a heavy batch of finished product to reduce it to the required weight per gallon, or specific gravity. This problem involves a rather complicated calculation or, if the batch is reduced by guesswork, nonuniform results; however, the use of the chart (which must be prepared for each type of product) greatly simplifies the p r ~ c e d u r e . ~As an illustration, assume that a batch of brushing lacquer has just been finished, which should weigh 9'/2 pounds per gallon. A gallon taken from the mixing tank weighs lo1/* pounds, and the clear lacquer that is to be added weighs 7'/2 pounds per gaIIon (the chart being constructed on this basis). Place a straightedge connecting the two points marked g1/2 (the desired weight) on the two scales. Then find the present weight (lo1/*) on the left-hand scale. Where this horizontal line meets the straightedge, read off (on the horizontal scale) the number of gallons of clear lacquer to be added for each 100 gallons of the heavy lacquer. Knowing the size of the batch, it is a simple matter to calculate the total amount to be added.
8
t I I
13k
i
33
loa
-
30 40 50 60 70 80 SO id0 110 GALLONS LIGHTPRODUCT TO BE ADDED TO 100 GALLONS HEAVY PRODUCT T O OBTAIN DESIRED WEIGHT Figure 18-Alignment Chart for Calculating Quantity of Liquid to Reduce Product to Required Specific Gravity 10
Vol. 20, No. 4
Acknowledgment
20
Figure 18 shows the type of alignment chart used to calculate the quantity of a standard liquid (solvent, clear lacquer,
The authors are indebted to E. A. Stoppel, of Valentine and Company, New York, for valuable suggestions and for permission to reproduce Figures 8 to 11, inclusive. 4 For method of constructing this chart, see Deming, J . Ind. Eng. Chem., 8, 264 (1816).
Titrometric Determination of Calcium and Magnesium Carbonates in Limestone' J. Stanton Pierce and W. C. Setzer, with A. M. Peter2 GEORGE~OWN COLLEGE,GBORGETOWN, KY.
M
AGNESIUM is the most common metal associated the same indicator, determined magnesium, in the presence with calcium in limestone. In some industries, of calcium, by precipitating magnesium hydroxide in about particularly in cement manufacture, it is very im- 70 per cent alcohol solution. Still more recently Schoch,'a portant that the percentages of calcium and magnesium be with the same indicator, determined the magnesium in waters known accurately, and for plant control an easy, practical, containing calcium by titrating the hot solution with limerapid method of analysis of limestone for these elements is water. Kolthoff l 4 determined magnesium, in the presence of calcium, by precipitating the former with a slight excess very much needed. I n recent years g r a ~ i m e t r i c ,col~rimetric,~ ~ centrifugal,6 of alkali, filtering, and titrating the excess alkali. photometric,6 nephelometric,' conductivity,* electr~metric,~ Theoretical Considerations andvolumetric'Omethods have been studied for the determinaI n the method described in this paper, an alcoholic solution tion of calcium or magnesium in the presence of the other. A decade ago Moirl' used thymolphthalein as an indicator of trinitrobenzene is used to tell when the hydroxyl-ion confor the separation of magnesium from calcium alkalimetrically. centration becomes great enough to indicate that all the More recently, Willstatter and Waldschmidt-Leits12 using magnesium has been precipitated. Several indicators, including thymolphthalein, were tried, but trinitrobenzene 1 Received October 6,1927. was chosen because it was the only one which gave a distinct 9 Chief chemist, Kentucky Agricultural Experiment Station, Lexingcolor, at room temperature, in the absence of organic solvent, ton, Ky. 3 Congdon, Eddy, and Milligan, Chcm. News, 128,244 (1924). in saturated calcium hydroxide solution, and none in saturated 4 Gregoire, Carpiaux, Larose, and Sola, Bull. soc. chim. Belg., 32, magnesium hydroxide solution. 123 (1823). The method described herein may be used for the deter6 Arrhenius, J . A m . Chem. Soc., 44, 132 (1922); Arrhenius and Reim, mination of the calcium and magnesium present as carMedd. Vctcuskapsakad. Nobclinsf.,8, No. 14 (1928). 6 Hoskell, Concrete, 97, 101 (1925). bonates in limestone, dolomite, or magnesite, or part of the 7 Krisr, Biochrm. Z.,168,203 (1925); Ibid., 169, 359 (1925). procedure may be used to determine magnesium in the 8 Harned, J . A m . Chem. SOC.,89, 262 (1917). presence of soluble calcium compounds. 0 Wright and Morris, Proc. Iowa Acad. Sci., 81, 290 (1924); Pinkhof, The carbonate is dissolved in a measured portion of standPharm. Werkblad, 66, 794 (1919); Britton, J . Chem. Soc. (London), 197, ard acid, in excess, the carbon dioxide boiled out, and the 2110 (1925). Averitt, J . Ind. Eng. IOBucherer, 2. anal. Chem., 69, 287 (1920); excess acid determined by titrating back with standard Chcm., 14, 1139 (1922); Froboese, 2.anwg. Chcm., 8% 370 (1914). 11 J . Chem. Met. Mining SOC. S. Africa, 17, 129 (1917). 1 2 Brt.. 86, 488 (1923).
13 14
Ind. Eng. Chem., 19, 112 (1927). Rcc. trav. thim., 41, 787 (1922).
