ANALYTICAL
VOLUME4 NUMBER2
EDITION
Industrial Chemistry AND ENGINEERING
APRIL15, 1932
PUBLISHED B Y T H E AMERICAN CHEMICALSOCIETY HARRISON E. HOWE,EDITOR
Measurement of Average Particle Size of Fine igments S. D. GEHMANAND T. C. MORRIS,T h e Goodyear Tire & R u b b e r Company, -Akron, Ohio light an estimated average parH E reenforcing properA method of obtaining excellently dispersed ticle size of gas black as 0.05ties of pigments used in suspensions of rubber pigments of accurate 0 . 0 6 ~ . G r e n q u i s t (12) estirubber compounding and concentration is described in which the pigment mated gas black as ranging t h e m i l l i n g characteristics of is milled into rubber and the stock then dissolved from 0.015 to 0 . 2 0 0 ~ . these pigments are so intimately Parkinson (17) makes no estiin a solvent. connected with their degree of mate of the particle size of gas fineness that considerable interThe average particle sizes os carbon blacks black, but quotes the particle est is attached to securing measmeasured by the Zsigmondy count method were size of lampblack as about 0.3urements of their average parfound to range f r o m 0 . 0 6 1 f~o r rubber gas black 0 . 6 , ~w~h i c h a g r e e s well ticle size. Much work has been to 2 . 2 2 ~for the coarsest one measured. The with Green’s estimate, 0.3-0.4~. done on the measurement of the Moore (15)gives for the particle zinc oxide pigments had average particle sizes particle size of microscopic and size of Thermatomic black, l.Op, ultra-microscopic material, and f r o m 0.076~to 0 . 5 7 ~ . Measurements on several and for P-33, 0 . 2 3 ~ . a number of methods have been other pigments of interest are included. BeMost of the results which have devised which, when properly cause of the high visibility in the ultra-microbeen published on the average used, yield satisfactory results. scope, this method gives smaller d u e s f o r average particle size of zinc oxide pigSvedberg (24) gives an excellent ments have been obtained by particle size than the photomicrographic methods. discussion of these methods. Green’s (11) photomicrographic Similarly, Green (11) gives a The results have been used to calibrate a method or modifications thereof. very complete discussion of the microturbidimeter of the extinction type for use In the papers of Green (10, 11), theory of average particle size in measuring average particle size. Haslam and Hall (IC),and Stutz and the significance of average Curves are included showing how the turbidities and Pfund ($3) can be found a diameters, and Wells ($7) a relarge number of measurements of suspensions of zinc oxide and carbon black sume of the field of turbidity for every type of zinc oxide. measurements. vary with the average particle size, concentration, The values quoted in Table 111, The particle size of rubber and wave length of light used. s u p p l i e d by the New Jersey pigments ranges from the defiZinc Co., were obtained bv the nitely microscopic well into the colloidal region, Two of the most important are carbon black method of Stutz and Pfund (as). It is the purpose of this paper to set forth an accurate and zinc oxide, and it is these pigments which have been investigated most thoroughly, although a few others of in- and efficient method for measuring particle size. Since the authors are directly concerned with the particle size of terest have been studied. Considerable work has been done on the determination rubber pigments as dispersed in rubber, and not as dispersed of the particle size of carbon black, but the literature con- in an aqueous medium or rubbed out on a slide, the turbidity tains varying figures. The value given by Spear ( d l ) , 0.1- of xylene-rubber-pigment cements prepared by milling the 0 . 2 ~up to about 0 . 6 ~has ~ been widely quoted. Green pigment into the rubber and swelling in xylene has been used (9) gives the size of gas black as about 0 . 1 5 ~ . Peterfi, as the basis of this method. The turbidimeter used in this for A. Wegelin (26), determined by the Zsigmondy count work was the microturbidimeter used previously by Gehmethod the average particle size of three German carbon man and Ward ( 7 ) . Since the turbidity of suspensions is affected by a number blacks as 0.124, 0.106, and 0.083~. More recently (for F. Hartner, IS) he gives 0.050~for the average diameter of variables not readily calculable, a turbidimeter to be used of gas black. J. J. Barnard, in a communication to Wiegand for particle-size measurements must be calibrated with pigments whose particle size has been determined by an inde(H),reports by ultra-filtration and counting in ultra-violet 157
ANALYTICAL EDITION
158
Vol. 4. No. 2
pendent method. For calibration of the turbidimeter the count method with the cardioid ultra-microscope of Siedentopf (18), manufactured by Zeiss, has been used. Average particle sizes as determined by the count method together with turbidimetric measurements have been obtained on a series of carbon blacks, a series of zinc oxides, and a few other pigments of interest in the rubber industry. The carbon blacks used varied in particle size from 0.025 to 2 . 2 2 ~and the zinc oxides from 0.076 to 0.566~.
To a first approximation this is the intensity of the forward scattered light a t depth d. When the intensity of the direct beam equals the intensity of the scattered light-i. e., at the extinction depth d,-we have
THEORYOF MICROTURBIDIMETER The microturbidimeter is an extinction type of turbidimeter. When the light from the filament enters a suspension of a white pigment, it undergoes a loss of intensity because of light scattering by the disperse phase. The process of
So A is the reciprocal of the extinction depth. Introducing this value of A in Equation 3, we get the intensity of the forward scattered light at depth d to be
E
LIGHT \NTLNSITIEI CALCULATED FOR TMF. MICROTURSIDIMETER
60
3,Pso ,a3
1 A = d,
I , = I o -d
e-kd
(4)
de
c e
I, = I, Iae-kd. = AIod,e-kd.
40
\
U-DIRECTI
I
I , has a maximum for a value of d which can be obtained by differentiating Equation 4 and equating to zero. The result which comes out of this procedure is that the scattered light has its maximum intensity for a value of d equal to l/k. Figure 1 is a plot of Equations 1 and 4 for arbitrarily assumed values of k and d,, and helps to visualize how the intensity of the scattered light and of the direct beam vary so as to achieve equality at one point-i. e., where the curves intersect. k was taken equal to 0.005 and de to 500p in order to plot the curves.
I
I
BEAM
\ - -0
ZOO 400 600 800 DEPTH O? SUSPENSION (/A)
IO00 CURVES COR CARBON BLACK S U SPEN6\ONS !$BY WEIGHT OF 0LACH
FIGURE1 I
extinction consists in having the diffuse field due to the scattered light of the same intensity as the direct beam. The way in which the scattered intensity and direct intensity vary to produce this result is explained as follows: Let I O = intensity of incident beam before any scattering I , = intensity of incident beam after penetrating a depth x I , = intensity of forward scattered light at 2 Also let dI, be the light scattered in the forward direction from an element of depth dx which we think of as having unit cross section. The intensity dI. will be diminished exponentially as it proceeds in the direction of x , adcording to Lambert-Beer's well-known law (95). The intensity of the direct beam a t z will be, in accordance with the same law I, = Ioe-b (1)
k is a constant for any one suspension, concentration, and wave length, The increment of scattered light, dl,, will be proportional to and to dx. Hence dI, = AIoe-b dx
(2)
A is a constant. By the time dl. penetrates to a depth d , its intensity will be dI,e-k(d-z),assuming monochromatic 1;ght. This will be its contribution to the intensity of the scattered light at d. The total intensity of the scattered light will be the integral of all the contributions from the small elements between z = 0 and x = d , hence
From Equation 2
(3)
I850
c. I 2 0 0 IO50
E
900 150 0 6 0 0
$
450
I; ~~
1: 150 l 2 AVERAGE PARTICLE S I Z E
(p)
a
FIGURE2 Equations 4 and 1 express the fact that the extinction depth is independent of the filament intensity. This is the case for white pigments and for latex. Since it is not strictly true for carbon black suspensions (see Figure 5 ) , the equations would have to be modified for the cases where light absorption is important. CALCULATION OF PARTICLE SIZE FROM ADHESION-TENSION MEASUREMENTS Superspectra gas black is so fine that the ultra-microscope was not suitable for its measurement, as the results obtained indicated that it was but slightly smaller than Micronex, whereas other properties indicated that it was much smaller. An estimate of the average particle size for a series of blacks, one of which is similar to Superspectra, can be made from data which have been published by Bartell and Smith (3) on the pore radii of carbon black diaphragms used in adhesion-tension measurements. The method used in making this estimate is as follows: Let ~t = number of particles per unit volume R = radius of pores x = number of pores per unit surface T = radius of particle d = diameter of particle v = volume in unit volume unoccupied by particles
April 15, 1932
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
159
Xylene cements were then made up of the entire series The work of Smith, Foote, and Busang ($0) on the packing of spherical lead shot under various conditions showed of stocks, 1per cent carbon black, 4 per cent rubber by weight, that a mean exists between cubic and hexagonal packing or 0.482 per cent carbon black by volume. The weighing depending upon the method of packing, high-pressure pack- was done on an analytical balance. Cements of different ing approaching the hexagonal. They have worked out v concentrations were prepared similarly. The cements were for cubic packing as 0.4764, and for hexagonal packing allowed to stand with occasional shaking until the disper0.2595. For both types of packing, since there will be fi sion appeared to be complete by microscopic examination. The dispersions of zinc oxide were prepared in essentially particles along an edge of a unit cube, x = nP/a. Using these facts, we have for the two types of packing the same manner by milling equal weights of zinc oxide and pale crepe rubber until microscopic examination by the the following equations: squeeze-out method showed the dispersions to be satisfacCUBICPACKINQ HEXAGONAL PACKING tory. Cements were then prepared of the desired concenn = -1 trations by dissolving weighed amounts of stock in weighed 8r quantities of xylene. All of the cements were examined x=- 1 microscopically to be sure the dispersions were complete. 4r2
v = nR% v =
0.4764 =
1 TR’ 4rB
v
=
TR2
4r2
0.2595 =
d = 2.568R
EFFECT OF COLOR FILTERS ON EXTINCTION DEPTHS OF
r 2 aR2 4r
-33
SUSPENSIONS
9 nR2 4ra
I
d = 3.907R
Table I gives the values of pore radii reported in a paper by Bartell and Smith (3) on the adhesion tension of a series of blacks, together with the values computed from them of the particle diameters on the assumption of hexagonal packing and on the assumption of cubic packing. It is interesting to note that the value for rubber gas black is of the same order as that secured by the ultra-microscopic count method. On the assumption that Superspectra was about the same as the specimen designated “color gas black,” the value of 0 . 0 2 5 ~has been introduced into Table .I1 as being the best estimate available for the particle size of this black. TABLEI. PARTICLE SIZE FROM SAMPLE: A
B
C D E
F
G
0
4
;L
RECIPROCAL OF
6
8
IO
% BV VOLUME OF BLACK
FIGURE 3
To secure very concentrated suspensions, evaporation of the more dilute cements was resorted to. A few cements in carbon disulfide and gasoline were prepared in the same manner as in xylene, the concentrations being calculated on a volume per cent basis to make them comparable with the xylene cements. PORERADII MEASUREMENTS LIMITSOF PARTICLE SIZE Hexagonal Cubic packing packing
SOURCE
PORE:RADIUS Y
U
U
Lampblaok Thermal decomposition Thermal deoomposition Rubber as black Ink ga8 heck Ink gas black Color gas blaok
0.0793 0.0780 0.0220 0.0173 0.0190 0.0183 0.0076
0.810 0.306 0.089 0.008 0.076 0.072 0.030
0.204 0.200 0.058 0.045 0.049 0.047 0.020
PREPARATION OF SUSPENSIONS Aqueous suspensions of carbon black containing saponin and gum arabic as protective colloids milled in a small ball mill proved unreliable, since the turbidimeter readings depended on the time of milling and the amount of protective colloid. On the other hand, it was found that carbon black dispersed in rubber, remilled several times, and then made into a xylene cement, was completely deflocculated, and that readings could be duplicated closely. This technic was therefore adopted for all the pigments studied in the preparation of the suspensions for the turbidimeter. The carbon blacks were selected to cover the entire range from the coarsest to the finest. The carbon black was incorporated on an 18-inch laboratory mill. The rubber was broken down on a warm mill. The carbon black was added until the batch came exactly to the desired weight. It was then cooled, remilled, cooled, and milled with an equal weight of clean pale crepe sheet. This batch was remilled until the dispersion was shown to be complete by microscopic examination using the squeezeout method ( I ) . I n the case of Superspectra it was found necessary to dilute the stock further in order to get a smooth cement free from aggregates.
COUNTING PROCEDURE With the invention of the slit ultra-microscope by Siedentopf and Zsigmondy ( I @ , measurement of the particle siae of colloids was first made possible. I n the slit ultra-microthe light enters the colloidal suspension a t right angles axis of the microscope, the presence of the colloidal e being indicated by a point of light on a dark backd. Subsequent improvements such as the cardioid a-microscope of Siedentopf ( I @ , the system used in the present investigation, have increased the usefulness of the instrument by increasing the resolving power of the system. The cardioid ultra-microscope differs from the slit ultramicroscope in that the illuminating pencil and diffracting pencil of rays which go to form the image are coaxial; the rays of greater aperture are employed to illuminate the re are made to reach The Zeiss cardioid ultra-microscope consists of a cardioid dark-ground condenser, a fused quartz counting chamber with holder, a 3-mm. glycerol immersion objective having a maximum N.A. of 1.0 with an iris aperture stop, and a 30X positive eyepiece all readily adapted to any microscope equipped with a substage rack. A clock-feed carbon arc with a water-cell heat filter was used for the illuminating system. For estimation of the particle size a restricted volume of known dimensions is necessary. This was obtained by inserting in the ocular an Ehrlich stop giving a field 0.0163 mm. square. The depth of the cell was measured by focusing and was found to be 5 . 0 ~ . I n using the cardioid ultra-microscope, one of the prime
Vol. 4, No. 2
ANALYTICAL EDITION
160
essentials is that the cell be absolutely clean. It was cleaned by boiling in cleaning solution, washing in hot distilled water, drying over a hot plate, and heating for a short time in a Bunsen flame. The system was aligned by an ecbentric objective centering mount and the illumination centered by reflecting the image of the crater back into the arc. A drop of the suspension was placed in the cell which was clamped in the holder. The condenser was connected to the cell with a drop of water, the objective to the cell with a drop of glycerol, and the system adjusted until the images appeared as brightly illuminated, sharply defined spots against a dark background. EXTINCTION DEPTHS FOR SUSPENSIONS OF P-33
5;$'
400
g 300 f g- 200
sw
100 0
I 2 RECIPROCAL OF $BY
0
3 4 VOLUME OF BLACK
FIGURE 4
The suspensions for counting were prepared by dispersing one or two drops of the xylene cement in an appropriate weight of Nujol, the amounts being carefully weighed on an analytical balance. After standing a day or so with occasional shaking, complete dispersion was obtained. These cements are highly useful for counting, since Nujol itself is optically empty and the Brownian motion of the particle is almost completely stopped. Since rubber itself is not optically empty, a blank rubber cement was dispersed in Nujol a t a somewhat higher concentration than that used for counting, the count averaging less than one particle per field, owing to the high dilution. The concentrations of the Nujol cements for counting were adjusted so as to yield as near 20 particles per field as possible. Usually twenty fields were counted and the averages obtained were duplicable. Wt. of xylene cement, 1 % black by wt., gra Av. of 20 fields, part
D Where D
1 0.0001
=-
4% s-
average diameter in p C concentration of pigment, grams per cc. V = volume of field counted p = density of pigment TZ = average count
D
= =
=
0.061~
DISCUSSIONOF MICROSCOPICAL METHODS Results obtained by the count method would be expected to be smaller than those given by the photomicrographic method for the following reasons: I n the ultra-microscope, particles as small as 0.010 to 0.015,~can be rendered visible since the perception of the particles depends rather on the following factors (24) than on the resolving power: (1) specific intensity of light source,
r EFFECT OF FILAMENT INTENSITY ON EXTINCTION D E P r n s OF CARBON BLACK S U S P E N S I O N S
-
/
E 1000 800 600
400 200 0
I I I I 1 2 5 4 S 6 RECIPROCAL OF YOBY VOLUME OF BLACH
I 1
FIGURE 5
Although the photomicrographic method may be an excellent and useful one for coarse pigments, the authors believe that the finer pigments are actually much smaller than this method would indicate, for three reasons: 1. Particles of the finer pigments are near the limit of resolution for visible light, and the full theoretical resolving power of a system is rarely realized. A pigment whose average particle size is at the limit of resolution will have a large number of particles which are too small for resolution. Particles below the limit of resolution will appear enlarged (8). 2. It is impossible to get all the particles in the same focal plane, and since objectives of high N. A. have a very slight focal depth, enlarged images and circles of confusion will result 3. No present method of preparing slides will yield as complete a dis ersion as can be obtained by other means, such as milling t f e pigment into rubber.
For pigments for which direct illumination the method of Dunn ( B ) , in which the image on a screen and thereby permits one to focus being measured, seems preferable to the regular graphic procedure.
is suitable, is projected the particle photomicro-
RESULTS I n Tables 11, 111, and IV are given the average particle sil;es, obtained by the count method, of the pigments used in calibrating the turbidimeter. Superspectra, listed in Table
INDUSTRIAL AND ENGINEERING
April 15, 1932
CHEMISTRY
161
a reddish tinge just prior t o extinction which vitiates the comparisons. Figure 2 gives the calibration curve for the measurement of the average particle size of carbon blacks with the microturbidimeter. The minimum extinction depth occurs for an SIZE OF CARBOYBLACKPIGMENTS average particle size of about O.llp, but this depends someTABLE11. PARTICLE OBTAINED BY COUNTMETHOD what on the wave length of light used. It shifts toward a Av. PARTICLE SIZE smaller value of the particle size for shorter wave lengths. PIQMENT P The same phenomenon has been observed by Stutz (22) 0.025 Superspectraa 0.061 for suspensions of zinc oxide. Micronex 0.092 Special sample (rubber gas black) The curves in Figure 2 show that the count method of 0.130 Shewinnegan acetylene black 0.159 P-33 determining average particle size yields results which corre1.12 Thermatomicb 2.22 Velvetexb late well with the turbidity measurements in the sense that a Estimated. they are smooth curves. However, anomalous turbidimeter b Counted in blood count cell.
TI, is a special gas black known as a color black, and Micronex is a regular rubber gas black. P-33, Thermatomic, and Velvetex are all thermal-decomposition blacks supplied t o the rubber trade, and are known as soft blacks.
TABLE111. PARTICLE SIZE OF ZINC OXIDE PIQMENTS OBTAINED BY COUNTMETHOD
w
P
0.11 0.15 0.28 0.32 0.70
0.076 0.099 0.169 0.185 0.566
t;: z ,o
TABLEIV. PARTICLE SIZE OF MISCELLANEOUS REBBER PIGMENTS OBTAINED BY COUNTMETHOD PIGMENT
PARTICLE SIZE
Red iron oxide Titanium oxide Blanc fixe Special blanc fixe
0.139 0.190 0.245 0.160
J !
E X T I N C T I O N D E P T H V§. INDEX OF R E F R A C T l O N OF PlW4ENT l.86%BY WOL. IN XYLENE C E m E N r AVERAOE P A R T I C L E SIZE .19p
1
1200
F
900 9001
f
600
1 \
I
2"oo6
I
, I
300 0
I .5
I
r-
I
2.1 2.4 RLFRACTIVE INDEX OF PIGMENT 1.8
"
F
0 0 0 GREEN P d I BLUE
..
500 400 300
;200 .I .Z .3 .4 .5 A V E R A G E P A R T I C L E §lZE ( p )
.6
FIGURE 7
DISCUSSION OF TURBIDIMETER MEASUREMENTS The microturbidimeter used in this work has previously been described in detail (5, 7 ) . It operates on the extinction principle. An incandescent filament is viewed through a film of the suspension contained between a convex lens and a flat glass plate. The turbidity of the suspension is measured by the depth of it required to produce extinction of the filament, or rather, by the reciprocal of this depth.
0 2
z
1000
_0
All of the zinc oxides, together with the values for the particle size as obtained by the method of Stutz and Pfund (dS), have been supplied by the New Jersey Zinc Co. The pigments appearing in Table IV are regular commercial grades.
1500
CURVES FOR Z n O S U S P E N S I O N S .SI BY VOLUME OF Z n O
-AVERAGI PARTICLE SIZ*?N. J. Zinc Co. values Count method
PIQMENT
I 2 .'I
FIGURE 6
The radius of curvature of the lens used in the extinction cell of the turbidimeter was 8.636 em. Red, green, and blue color filters were used. The red filter was a Wratten filter. The green was a Corning filter known as Sextant Green. For a blue filter, a combination of Corning light theater blue and dark heat-resisting blue was used. Unless the green and blue filters used with the microturbidimeter are very free from red transmission, as is difficult to judge from transmission curves obtained photographically, the filament shows
readings can be obtained by mixing blacks of widely different average particle size, so that the fact that smooth curves were obtained indicates that, in general, the carbon blacks had similar size-distribution curves. Figure 3 shows the straight-line relation existing between the reciprocal of the concentration and the extinction depth for suspensions of P-33, with the different light filters used. I n Figure 4, the line is extended to higher concentrations, the most concentrated cement used being 7.91 per cent by volume of black and having an extinction depth of only 28p. The points in Figure 4 show a slight systematic deviation from a straight line, the curve bending toward the origin. For zinc oxide (see Figure 8), the linear relation holds all the way down to an extinction depth of about 30p, corresponding to a concentration of 12.4 per cent by volume of zinc oxide. This was as high a concentration as could conveniently be obtained. It is interesting to compare these results with those previously reported for latex ( 7 ) . The dependence of turbidity on concentration for high concentrations and the appearance of a maximum such as occurs for latex is not well understood, although such maxima have been reported for other systems (16, 25). Figure 5 shows an effect due to light absorption. For white pigments, such as zinc oxide or barium sulfate, and for latex, the extinction depth is practically independent of the filament intensity. For carbon black, however, there is a small dependence, the extinction depth being greater the brighter the filament. The effect is not extremely large, however, since the differences shown in Figure 5 were produced by a twelve-fold change in filament intensity. Measurements with color filters, for carbon black, have not taken account of differences in intensity of the transmitted light. This may explain why carbon black suspensions show better transmission with no filter than they do with a red filter, whereas the reverse is the case for zinc oxide (see Figures 3 and 8). The turbidity of a suspension is dependent among other things on the difference in the index of refraction of the
ANALYTICAL EDITION
162
two phases, so that in general a change in the index of refraction of the medium can be used to secure information about the index of refraction of the dispersed phase. It was thought that it would be interesting to do this for an absorbing dispersed phase such as carbon black. For this purpose, additional suspensions of carbon black in carbon disulfide and in gasoline cements were prepared. For this range of the index of refraction, 1.489 to 1.617, the extinction depth showed no significant variation. LXTINCTION DEPTHS FOR SUSPENSIONS OF A0 AVP. PARTICLE SIZE .ies p
3aoo
Eg
a00
f
f
;
IO0
G
:: 0
0
.25
.5
X5
1.0
90BY VOLUML FIGURE8
RECIPROCAL OF
OF A0
Figure 6 shows the effect of the index of refraction of the pigment on the extinction depth, giving the curve obtained by plotting the extinction depths for suspensions in xylene cement of barium sulfate, zinc oxide, and titanium oxide against their refractive indices which are respectively 1.64, 2.02, and 2.50. I n Figure 7 we have the calibration curves for the turbidimeter which make possible its use for measuring the average
Vol. 4, No. 2
size of zinc oxide pigments. If there is any question as to which side of the minimum the pigment belongs, it can usually be answered by taking readings with the red and with the blue filters. The ratio of these two readings is different on the two sides of the minimum.
LITERATURE CITED (1) Allen, IND. ENG.CREM.,Anal. Ed., 2, 311 (1930). (2) Barnard, J . Roy. Microscop. Soc., 38,1 (1919). ENO.CHEM.,21, 1102 (1929). (3) Bartell and Smith, IND. (4) Bond, Phil. Mag., 7, 163 (1929). (5) Conklin, J . Optical SOC.Am., 10, 573 (1925). (6) Dunn, IND. ENG.CHEM.,Anal. Ed., 2, 59 (1930). (7) Gehman and Ward, Ibid., 3,300 (1931). (8) Green, J . Franklin Inst., 192, 637 (1921). (9) Green, Chem. Met. Eng., 28,53 (1923). (10) Green, J . Ind. Hug., 7, 155 (1925). (11) Green, J. FranklinInst., 204,713(1927). ENG.CHEM.,21, 667 (1929). (12)Grenquist, IND. (13) Hartner, Rubber Chem. Tech., 3,215 (1930). (14) Haslam and Hall, J . Franklin Inst., 209, 777 (1930). (15) Moore, IND.ENG.CREM.,24, 21 (1932). (16) Ostwald, “Licht und Farbe in Kolloiden,” pp. 21,29,Steinkopf, 1924. (17) Parkinson, Trans. Inst. Rubber Ind., 5, 1263 (1929). (18) Siedentopf, veerhundl. deul. phys. Ges., 12, 1 (1907). (19) Siedentopf and Zsigmondy, Drude’s Ann., 10, 1 (1903). (20) Smith, Foote, and Busang, Phys. Rev., 34, 1271 (1929). (21) Spear, Colloid Symposium Monograph, p. 332 (1923). (22) Stutz, J . FrankZinInst., 210,67(1930). ENG.CREM.,19, 61 (1927). (23) Stutz and Pfund, IND. (24) Svedberg, “Colloid Chemistry,” p. 130,Chemical Catalog, 1928. (25) Toerell, Kolloid-Z., 53, 322 (1930). (26) wegelin, Kautschuk, 3, 196 (1927). (27) Wells, Chm. Rep., 3, 331 (1927). (28) Wiegand, India Rubber World, 75, 81 (1926). RQCBIIVED September 10, 1931. Presented before the Division of Rubber Chemistry at the 82nd Meeting of the American Chemical Society, Buffalo, N. Y . ,Auguat 31 to September 4,1991.
Volumetric Sulfate Determination Rapid Method for Determining Sulfur in Organic Compounds ANDREW CHALMXRS AND GEORGE W. RIGBY,Research Laboratory, Du Pont Rayon Co., Buffalo, N. Y. (2) determination of the inorganic sulfur. Two practicable means of converting organic sulfur into inorganic sulfur are and necessary to follow the course of a reaction by means available: reduction to hydrogen sulfide (6, 19, 21, W), of sulfur determinations. To be of value, these determina- oxidation to sulfate. At present, oxidation by means of tions were of necessity very numerous, and for that reason a sodium peroxide in a Parr sulfur bomb (26) is the most rapid rapid volumetric method has been developed by means of and the most universally applicable method of obtaining the which a complete determination may easily be completed inorganic sulfur. The chief problem, therefore, lies in finding within 30 minutes and as many as six completed within 1 a rapid yet accurate means of determining the sulfate ion in the presence of a large excess of sodium chloride. hour. Determination of the sulfate ion may be based on the Although the principles upon which the method is based are not new, the combination of oxidizing the organic compound insolubility of silver (9), lead (2.6, 26, 38), benzidine (13, 16, in a Parr sulfur bomb followed by a sulfate determination 67, 2& 51), and barium sulfates, the least soluble being barium with standard barium chloride, the excess of which is deter- sulfate. Silver and lead, of course, form insoluble chlorides mined with sodium carbonate solution using phenolphthalein and cannot be used in the present case (58). Benzidine is not as an indicator, may be of value to other organic chemists. applicable in the presence of large quantities of any neutral Three distinct advantages may be claimed for this method over salt, (16). This leaves barium as the best suited for the other proposed volumetric methods: First, a minimum of purpose. Barium chromate ( 1 , 14, 18, 29), phosphate (dW), operations is involved; second, no special reagents or acetate (10, 15), hydroxide (55), and chloride have been used apparatus other than a Parr sulfur bomb are required; and as precipitating agents. The most advantageous appears to third, comparable and quantitative results are obtained in 30 be barium chloride since it forms stable solutions and is available in a very pure state. minutes. Three methods are available for determining sulfate under LITERATURE SURVEY these conditions: (1) photometric (32) determination of The determination of sulfur in organic compounds requires suspended barium sulfate; (2) gravimetric determination (11) at least two steps: (1) conversion to inorganic sulfur, and of barium sulfate; and (3) volumetric determination of excess
D
URING the course of some synthetic organic chemical reactions undertaken by one of the authors, it became
