Determination of Surface Areas Pigments, Carbon Blacks, Cement, and Miscellaneous Finely Divided or Porous Materials P. H. EMMETT
D
AND
THOMAS DE WITT’, Johns Hopkins University, Baltimore, Md.
URISG the last few years a method for measuring sur-
The absolute surface area values obtained obviously depend upon the accuracy with which the point corresponding to a monomolecular adsorbed layer can be picked. Originally this point (dcsignated as point B ) was selected empirically as the lower pressure extremity of the long linear portion of the experimental adsorption isotherm.
face areas of iron synthetic ammonia catalysts by means of low-temperature adsorption has been developed ( I , 6, 7 ) and extended in its application to other metallic catalysts and to a variety of nonmetallic adsorbents including soils (8), powdered bacteria (IS), silica gel, chromium oxide gel, potassium chloride, anhydrous and hydrated copper sulfate, Darco GI Darco B, and activated charcoal (2). Since the method is comparatively simple and rapid in operation and appeared from previous work to be rather widely applicable, it seemed worth while to attempt to utiliseit in measuring the surface area of a number of industrially important finely divided materials. The present paper contains a summary of the results obtained on a series of carbon blacks, zinc oxide, pigments, titanium dioxide, barium sulfate, zirconium silicate, graphite, cement, lithopone, paper, and cuprene. Although a detailed recapitulation of this method for measuring surface areas need not be included here, a few theoretical and experimental aspects of its nature are briefly summarized. The surface areas are obtained by selecting on each experimental adsorption isotherm of some gas such as nitrogen the point corresponding to a monolayer. A multiplication of the number of molecules required to form the single layer by the average area occupied by each adsorbed molecule yields a numerical value for the absolute area of a given weight of the absorbent. 1
As an illustration, point B on the isotherm for the adsorption of nitrogen on “arrow black” has been indicated in Figure 1. The experimental evidence that led to the selection of B on the iron catalysts was threefold. A series of isotherm determinations for different gases on a single catalyst showed better agreement among the values for the catalyst area if B was considered as the
volume of gas in a monolayer than if an of the other likely points on the isotherms wcre chosen (6). &condly, the heats of adsorption calculated by the Clapeyron equations from a series of isosteres showed that the heat of adsorption at B was intermediate between probable valucs for first-lrtycr and secondlayer heats of adsorption. Thus a selection of a volume 25 per cent greater than that at B yielded a heat of adsorption close to that of liquefaction of the gas, whereas a vQlume 25 per cent smaller yielded a heat of adsorption at least 50 per cent greater than the heat of liquefaction and clearly characteristic of adsorption in the first layer. Finally, the chemisorption of a layer of carbon monoxide over the entire surface of the pure iron catalyst required a volume of carbon monoxide approximately equal t o the volume of physically adsorbed gas at B on the low-temperature isothcrms (6). These three separate types of evidence convinced the authors that the volume of gas corresponding to B on the isotherms was close to that required for a monolayer. An excellent confirmation of the selection of B as the monolaver and a valuable new working tool in using this method * of surface area measurements are contained in a recent paper (3) on the theory of multilayer adsorption such as is apparently being obtained in isotherms of the type shown in Figure 1. It was shown that the isotherms could be plotted in such a manner as to yield a straight line whose slope is closely the reciprocal of the volume of gas required to form a monolayer.
Preaent address, Virginia Polytechnic Institute. Blacksburg, Va.
The complete equation is
P= 1 -+ V(P0
,
ob
id0
I I
I
200
300
400 PRESSURE
!
500 MM.
600
Hg
700
Io
800
OF NITROGEN ON CARBON BLACKS AT LIQUID NITROGEN TEMPBRATU~S FIGURE 1. ADSORPTIOX
1. 3.29 grams of 2. 3.09 grams of 3. 2.95 grams of 4. 2.86 grams of
arrow black Micronex Wyex P-33
5. 0.792 gram of acetylene black 6. 6.50 grams of thermstomic 7. 4.44 grams of Thermax
28
- P)
V*C
where v is the volume of gas adsorbed a t pressure p at a temperature at which the vapor pressure of the liquefied gas is PO; V , is the volume of gas in cc. required t o form a monolayer; C is a constant related exponentially to the difference
January 15, 1941
ANALYTICAL EDITION
29
48 m
0
8 cq
F-
?2 32Y
U
m
w
w
A
J
3 m
v)
3
t6
FIGURE 2. ADSORPTIONDATAFOR NITROGEN
ON
CARBON BLACKACCORDING
TO
EQUATIOY1
1. Wyex 4. Thermax 2. blicronex 5 . Thermatomic 3. Arrow black 6. P-33 Weights of samples same as in Figure 1
between the heat of liquefaction of the adsorbate and its heat of adsorption. It is evident that from the slope and inter~ V , and C can be cept of a plot of p l (PO ~ - p ) against p / p both evaluated. Application of Equation 1 to several hundred isotherms obtained during the last few years shows excellent agreement between the volume of gas adsorbed at B and the volume, V,. The linearity of the isotherms plotted according t o Equation 1 extends up t o a relative pressure, p / p ~ of , about 0.35. This is sufficient for evaluating V,. A typical set of such plots is shown for the carbon black samples in Figure 2 and discussed below. If a material is known to give an S-shaped low-temperature nitrogen adsorption isotherm, only a few adsorption points need to be determined in the relative pressure range 0.05 to 0.3 to fix approximately the slope of plots of Equation 1 and hence to yield values for Vm. In fact, a single adsorption point in the above-mentioned pressure range will, when connected with the origin on such a plot, give a line whose slope will usually differ by less than 5 per cent from that drawn with the help of a number of adsorption points. The second factor upon which the absolute values obtained for the surface areas will depend is the area occupied by each adsorbed molecule. I n a previous publication (6) it has been pointed out t h a t two convenient and reasonable molecular area values are those corresponding to the packing of the molecules in the solidified or liquefied adsorbates. Crosssectional molecular areas calculated from the solidified gases are in general about 20 per cent smaller than those obtained by analogous calculations on the liquefied adsorbates. Absolute values for surface areas are therefore uncertain by at least this amount, though experience has shown that relative areas of materials are reproducible to a few per cent.
Apparatus and Procedure In the present measurements nitrogen was ordinarily used as adsorbate; in a few experiments, butane was used. The cold baths for these two gases were, respectively, liquid nitrogen and ice. The apparatus and procedure have been described in detail (1,6); a standard adsorption apparatus was used, the adsorption bulb being calibrated with pure helium a t the temperature of the cold bath immediately preceding the runs on a given adsorbent. To remove water vapor and other gases, each sample of material was evacuated for an hour a t 100" C. except cuprene and two samples of paper which were pumped out a t room temperature. Temperatures of the liquid nitrogen bath varied but little during a run; nevertheless, they were measured periodically by an oxygen vapor pressure thermometer, the vapor ressure data and those for nitrogen being taken from Dodge and Dunbar for oxygen from Giauque, Johnston, and Kelley (11). The exact temperatures were used in preparing plots according to Equation 1. The "liquid nitrogen" temperatures referred to in the captions of Figures 1 to 6 were always bekeen 77.3" and 79.5" K.; the exact temperature depended, of course, on the oxygen content of the commercial liquid nitrogen. The adsorbate was tank nitrogen dried by passage through a tube of phosphorus pentoxide. As pointed out in a previous paper, a few tenths of a per cent of oxygen in the nitrogen will not interfere with the measurements on any material other than those capable of combining with or chemisorbing oxygen a t the temperature of the runs. The butane, a commercial sample from the Ohio Chemical Co., was liquefied, freed from noncondensable gases by evacuation, and partially vaporized. The middle portion of liquid from this vaporization was chosen; its vapor pressure at 0" C. was 777 mm., indicating that the isobutane content must have been small.
B),
Results CARBON BLACKS.A series of seven carbon blacks used in the present experiments included well-known commercial carbons: Micronex, P-33, arrow black, Wyex, Thermax, thermatomic carbon, and acetylene black. The first six were kindly furnished by the Rubber Division of the National Bureau of Standards and the acetylene black, by Shawinigan Chemicals Limited.
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
30
Vol. 13, No. 1
The adsorption isotherms for nitrogen on these seven carbon blacks are shon-n in Figure 1 and the plots according t o Equation 1, in Figure 2. The values for V , and for the surface areas are summarized in Table I. The areas are calculated by multiplying the value of V , by 4.38 and dividing by the weight of the sample. The value 4.38 for the area covered by 1 cc. of adsorbed nitrogen is calculated from the density of liquid nitrogen (6). GRAPHITE,TITANIUM DIOXIDE, BARIUMSULFATE,A X D ZIRCOXIUM SILICATE.In the course of this work Dr. Boyd of the University of Chicago sent the authors samples of graphite, titanium dioxide, barium sulfate, and zirconium silicate, upon which he had made surface area measurements by studying the adsorption of salicylic acid from solution. One of the materials, barium sulfate, had also been measured by the radioactive indicator method by Professor Kolthoff of the University of Minnesota. The isotherms obtained are shown in Figure 3. The plots by Equation 1 for these and the remainder of the runs are not published because of space limitations; in all cases, however, they were linear throughout the usual range of 0.05 to 0.35 relative pressure and PRESSURE MM. Hg were used to obtain V,, from which the area in each instance was calculated. OF NITROGEN AND BUTANEON FINELY DIVIDEDMATERIALS FIGURE 3. ADSORPTION The results of the authors' measure5. Nz on 14.88 grams of Bas04 (closed circles) and Nz on 1. Na on 17.52 grams of ZrSi01 ments are summarized in Table 11, 6.60 grams of Ti02 (openc ircles). Nitrogen runs 2 Nz on 5 45 grams of gra hite together with the surface area values are a t liquid nitrogen temperatures: those for 3: Butane'on 6.60 gram8 ofTiOz butane, a t 0' C. 4. Butane on 14.88 grams of Bas04 determined bv the other methods mentioned above; and surface area values calculated from plots of the data for butane according to Equation 1. The value of T,i per gram is multiplied by 8.69 to obtain the area in square meters per gram TABLE111. SURFACEAREA MEASUREMENTS ON ZINC OXIDE when butane' is used as adsorbate. PIGMENTS 1 The authors would like t o call attention t o a n error in Table I11 in the paper of Brunauer, Emmett, and Teller (3). The value for Vm in column 4 for butane should be 44.7instead of 58.2 cc. per gram. In columns 5 and 6 the surface area values should be 387 for butane and in column 8 t h e value of Ei EL should be 930 instead of 1930 calories. The error arose from using scale A instead of scale B in Figure 2 of t h a t paper in calculating the value of Vm and the constant C for the butane isotherm.
-
TABLE I. SURFACE AREA ~IEASUREME?;TS ON CARBON BLACK SAMPLES
hlaterial
Keiqht
V,,
Grams
Cc.
Diameter of Average Particle B y ultramicrpB y adscopic Area sorution counta Sq. m./g. Micron Microns 106.7 0.031 0.061 22.12 0.151 0.159 112.7 0.029 ., . 110.2 0.030 .. , 7.69 0.43 ... 6.81 U . 49 1.12 64.5 0.052 0.130
Micronex 3.08 75.2 P-33 2.86 14.42 Arrow black 3.29 84.8 Wyex 2.95 74.2 Thermax 4.44 7.8 Thermatomic carbonb 5,497 8.54 Acetylene black 0.792 11.68 a Gehman and Morris (10). b Names here used were those on samples received by authors. However, thermatomic carbon and Thermax are probably one and the same thing, "thermatomic carbon" being ,used ordinarily to designate a class of carbons formed by thermal decomposition of hydrocarbon gases.
Graphite Ti02 ZrSiO' Bas04 r
Weight Grams 5.447 6.602 17.52 14.86
F-1601 K-1602 G-1603 KH-1604
Weight of Sample Grams 5.492 6.444 7.403 17.167
Vm
cc. 11.89 12.95 6.56 2.58
Average Particle Diameter
Area Sq. m./g. 9.48 8.80 3.88 0.658
Microns 0.115 0.124 0.28 1.65
AIiD TABLEIv. SURF.4CE AREAMEASUREMEXTS O N LITHOPONE CEMEKT
Material
Weight Grams
Point B
Cc.
Vm
Cc.
Area Sq. m./g.
Lithopone 12.587 100 .. 34.8 Before calcining and grinding Calcined but not ground 8.324 2.6 .. 1.37 4.9 3.43 After grinding 6.244 Standard cement 15.456 3.8 3:87 1.08" 0 This is t o be compared with the value 0.1890 sq. meters per gram designated by the Bureau of Standards for use in calibrating Wagner turbidimeter.
ZISC OXIDE. Four samples of standard zinc oxide pigments on which measurements had previously been made by other methods xere kindly furnished by the New Jersey Zinc Company. They are designated as samples F-1601, K-1602, G-1603, and "-1604, and are the same as samples 1, 2, 4, and 5, respectively, of the paper by Ewing (9). The isotherms obtained are shown in Figure 4; the results are tabuTABLE11. SURFACE AREA blEASCREMESTS lated in Table 111. CEMENT AND LITROPONE. One Area Calculated from: ddsorption Adsorption Adsorption Radioactive sample of cement furnished by Vm by Vm by of of of salicylic indicator the National Bureau of StandPiitrogen Butane nitrogen butane acid method ards was measured. It was cc. Cc . Sq. meters per gram standard sample from lot 1140 38.22 30.73 3.96 ... . and is used for calibrating the 14.91 5:bO 9.88 6:iS 5.55 11.05 2.76 1.33 Wagner turbidimeter. The 14.60 4:60 4.30 2:is 1.73 2:2 isotherm is shown in Figure 5. together with those for samples 7
Material
Sample No.
ANALYTICAL EDITION
January 15, 1941
30
a: I+ 20
z
d n w m a
5: n
a
s 10 3
-1
0
>
0 FIGURE 4. ADSORPTION OF NITROGEN ON ZINCOXIDESAMPLES AT LIQUID NITROGESTEMPERATURES 1. 6.44 grams of sample K-1602 2. 5.49 grams of sample F-1601
3. 7.40 grams of sample G-1603 4. 17 2 grams of sample KH-1604
31
carbon samples were prepared prior to 1932 and that those in the present paper mere obtained in 1939, the agreement betmeen their results and the authors' seems remarkably good. The ratio of the average particle size obtained by their method to that obtained from the nitrogen isotherms varies from 1.06 to 2.5 on the four samples studied by both methods. I n general, one would expect the particle size obtained by the lowtemperature adsorption method to be smaller, if anything, than that obtained by the ultramicroscope, since the lower limit of the size that can be detected by the latter method is about 0.01 micron, whereas the adsorption method ought to apply equally well to smaller particles. Recent determinations of the size of llicronex carbon black particles by use of the electron microscope yield values of 28 millimicrons (14). This value is in excellent agreement with the value 31 millimicrons obtained in the present work (Table I) for a different sample of llicronex. I n Table V are listed the results of five methods for calculating the diameters of the particles of zinc oxide in the four samples studied. The values for all but the gas adsorption method were furnished by the Yew Jersey Zinc Company. All their results but the ultramicroscopic have been summarized recently by Ewing (9). The direct microscopic examination yields values for both S,the surface area in square meters per gram, and S,the number of particles per gram of material. Both adsorption methods yield values for S, whereas the ultramicroscopic method
of lithopone selected a t three different points in a manufacturing process. Curve 1 is for theyprecalcination stage, curve 4 for t,hecalcined lithopone, and curve 2 for the calcined sample after the final grinding. The area of the cement sample tabulated in Table I V was calculated from a plot of the data according to Equation 1; the areas of the lithopone samples Fere obtained by selecting B on the adsorption isotherms. Plots by Equation 1 could not be made from the data on the lithopone samples, since, a t the time the runs were made, an accident to the oxygen thermometer prevented measurement of the exact value of the temperature and hence of PO, the vapor pressure of liquid nitrogen. PAPER AND CUPRESE. The isotherms, together with the surface area values for two samples of paper and one of cuprene, are shown in Figure 6. The two samples of paper are of the type used as insulation in telephone cables; they were measured both after being evacuated a t room temperature and at 100" C. The cuwas furnished by Shawinigan Chemicals imited; the paper, by J. B. Whitehead of Johns Hopkins University.
Discussion of Results T h e particle diameters (in microns) calculated from the surface areas obtained for the various samples of carbon are compared in Table I with those published by Gehman and Morris (10) based on ultramicroscopic measurements. A density of 1.80has been taken for the density of carbon in calculating the particle size from the surface areas. I n view of the fact that their
ob
400
'200
300 400 500 PRESSURE M M Hg
600
70b0
FIGURE 5. ADSORPTION O F XITROGE?; ON LITHOPOXE AND O S A STASD.4RD CEMENT SAMPLEAT LIQUIDKITROGEN TEMPERATURES 1. 12.59 grams of lithopone before calcination and grinding 2. 6.24 grams of lithopone-finished product 3. 15.46 grams of a standard cement sample 4. 8.32 gram8 of lithopone after calcination b u t before grinding
Vol. 13, No. 1
INDUSTRIAL AND ENGINEERING CHEMISTRY
32
by the ultramicroscope and by the nitrogen adsorption method are in remarkably close agreement, as can be seen from Table V. The first three oxides listed give especially close agreement, the values for the diameters by the nitrogen method being, on the average, a little smaller than, but less than 20 per cent different from, those obtained with the help of the ultramicroscope. Of course, a close comparison of ds and D in Table V cannot be made because of uncertainty as to the numerical value of the shape factors involved (12). Only for the reheated zinc oxide, sample KH-1604, is the average particle size appreciably larger by the nitrogen method than by the ultramicroscopic method. For this oxide the value for D , calculated from N by direct microscopic examination is likewise somewhat smaller than that obtained from the nitrogen isotherms. The explanation for this is probably to be found in the technique used in preparing samples for microscopic or ultramicroscopic exI , amination. I n the former instance the rubbing down process involved in preparing the materia1 l to0 ‘200 300 400 500 600 71 for slides will tend to break up groups of particles PRESSURE M M . Hg that have fused together during the period of reheating the zinc oxide to high temperature. The FIGURE 6. ADSORPTION OF NITROGEP; ON PAPER AND ON CUPRENE AT LIQUID NITROGEN TEMPERATURES dispersion technique required for preparing the 2. 1. 0.542 13.41 gram gramsofof cuprene paper D evacuated at 25O C. ultramicroscopic samples is even more likely to 3. 13.41 grams of paper D evacuated at 110-115° C. break up such aggregates, since the zinc oxide for 4. 8.02 grams of paper A evacuated at 25O C. such samples is actually compounded into rubber 5. 8.02 grams of paper A evacuated at 110-115° C. Calculated surface areas for samples for curves 1, 2, 3 4 , and 5 are 1.59, 20.7, 0.654, and then recovered by dissolving away the rubber. 0.606, and 0.568 s q . meters per grad, respectively. It is reasonable to-expect, tgerefore, that the particle size will be smaller by the ultramicroyields values for N . The average diameters of the particles scopic method for this one sample than by the nitrogen expressed in microns are calculated from values for S or N in an adsorption method. The fact that the direct microscopic obvious manner by the equations included in Table V, using value is also smaller than that obtained by the nitrogen for the density of zinc oxide the value 5.60 grams per cc. adsorption method is understandable as a balance between Several interesting comparisons can be noted. I n the first two factors involved in the former, the breaking up of place, the average diameters as judged by the nitrogen adparticles in preparing the slides, and the missing of some of sorption method are in all cases smaller than those obtained the particles because of the impossibility of observation beby adsorption from solution by a factor of 2 or 3. This is not low 0.2 micron. Ordinarily the breaking up of agglomerates would not unexpected and conforms to the results of measurements pn titanium dioxide, barium sulfate, zirconium silicate, and affect the apparent particle size as judged by the low-temgraphite whose surface areas had also been measured by the perature isotherms, because the individual particles in agadsorption of salicylic acid from solution. On all the samples glomerates are sufficiently loosely held together to give ready of zinc oxide except KH-1604, the reheated oxide, the average access to the nitrogen molecules used as measuring units. diameters obtained by the nitrogen adsorption method are Only when the agglomeration consists of an actual sintering smaller than those yielded by the direct microscopic counts of together of a number of particles as by a high-temperature either N or S. This, too, would be expected both because treatment such as used on sample KH-1604 will the area of the direct microscopic methods probably do not take into the agglomerate be considerably less, as judged by the lowaccount particles smaller than about 0.2 micron and because temperature isotherms, than the sum of the surface areas of the “sorption surface”, by including the internal surface of the particles that together make up the agglomerate. any pores that may be present, will tend to be larger than The results shown in Table I1 are self-explanatory. T h e the external surface measured microscopically. reason for the larger discrepancy between the values for The values for the particle diameters of zinc oxide obtained graphite obtained by adsorption from solution and from the
‘6
TABLE V. SCRFACE AREAMEASUREMEXTS ON ZINC OXIDE Pigment KadoxBlsckLabel-15 XX Red-72 XX Red-78 Reheated zinc oxide
NO. F-1601 K-1602 G-1603 KH-1604
Direct Microscopic dr S 0.28 3.87 0.34 3.27 0.79 1.37 1.86 0.58
Adsorption of Methyl Stearate
ds 0.19 0.24 0.55 4.50
S 5.54 4.56 1.97 0.24
Adsorption of Nitrogen da
S
0.115 0,124 0.28 1.68
9.48 8.80 3.88 0.66
Surface area, square meters per gram. Average diameter (microns) related t o S by S -6 pda * N Number of particles per gram X 10-10. D Average diameter related to N by N = p D , . In present table D is expressed in microns. p Density of ZnO particles in grams per cc. = 5.60. S
di
Number of Particle Measurements Direct Microscopic Ultramicroscope D N X 10-10 D N x 10-10 0.21 0.25 0.49 1.40
2040 1140 I53 6.6
0.135 0.16 0.26 0.82
7260 4200 1075 32.0
ANALYTICAL EDITION
January 15, 1941
gas phase than for the other three materials is not clear. It presumably points to a large “internal” surface area in cracks or crevices that are inaccessible to large organic molecules in solution and yet are covered by nitrogen molecules during the low-temperature gaseous adsorption measurements. The areas calculated from the butane isotherms are usually considerably smaller than those obtained from nitrogen isotherms. This is consistent with previous experience with this gas (2, 3, 6); the explanation of the smaller values is not yet certain. Small symmetrical molecules appear in general to be preferable to large long molecules for surface area measurements. The area of the standard sample of cement had been determined by the usual liquid flotation method and was given to the authors as 0.1890 sq. meters per gram. This is smaller by a factor of about 6 than the value 1.08 sq. meters per gram obtained in the present adsorption studies. The authors prefer not to discuss the possible causes of this discrepancy until more work has been done comparing the two methods. However, in view of the general agreement of the nitrogen adsorption method with ultramicroscopic methods for carbon black and zinc oxide, the error is probably to be looked for in some of the assumptions made in the standard determinations by elutriation methods. Close examination of the adsorption isotherms on the zinc oxide samples and on acetylene black will reveal some shape peculiarities analogous to those noted previously for certain iron synthetic ammonia catalysts ( 2 ) . The linear portion of the isotherm whose lower extremity is about 100 mm. in all the present work extends a few hundred millimeters only and then either increases or decreases in slope abruptly before joining the higher pressure part of the curve that is convex to the pressure axis near the saturation pressure. It is not possible to state as yet the cause of these shape irregularities above 400 mm. However, on all of these materials the VcT, values obtained from plots of Equation 1 are in good agreement with the point B values. The runs on lithopone, paper, and cuprene merely serve to illustrate the possible utility of the new method for measuring surface areas of miscellaneous industrial materials. Although due caution should be observed in interpreting the results of similar surface area measurements on materials on which the method has not yet been tried, the authors see no reason to doubt the wide applicability of the low-temperature adsorption method in determining the relative and even the absolute surface areas of a variety of materials [for a critical discussion of the method see (41. So far, out of the several hundred different materials studied only charcoal (2) and dehydrated chabazite give other than the S-shaped a d s o r p tionisotherm. For reasons that are not entirely clear but appear to be concerned with pore diameters, the one sample of charcoal and the numerous samples of chabazite that have been tried yield Langmuir-type adsorption curves that do not become convex to the pressure axis as pressure a p proaches the liquefaction value but approach asymptotically a limiting adsorption value.
Acknowledgment The authors wish to extend their thanks to B. L. Harris for his assistance in making the experimental adsorption measurements on the zinc oxide samples.
Literature Cited (1) Brunauer, S., and Emmett, P. H., J . Am. Chem. SOC., 57, 1754 (1935). ( 2 ) Ibid., 59, 2682 (1937). (3) Bruuauer, S., Emmett, P. H., and Teller, E., Ibid., 60, 309 (1938). (4) Committee on Contact Catalysis, National Research Council, 12th Report, Chap. V, Ken, York, John Wiley & Sons, 1939.
33
Dodge, B. F., and Dunbar, A. K., J . Am. Chem. SOC.,49, 591 (1927).
Emmett, P. H., and Brunauer, S., Ibid., 59, 1553 (1937). Emmett, P. H., and Brunauer, S., Trans. Electrochem. Soc., 71, 383 (1937).
Emmett, P. H., Brunauer, S., and Love, K., Soil Sci., 45, 57 (1938).
Ewing, W. W., J . Am. Chem. SOC., 61, 1317 (1939). Gehman, S. C., and Morris, T. C., IND.ENQ.CHEM.,Anal. Ed., 4, 157 (1932).
Giauque, W. F., Johnston, H. L., and Kelley, K. K., J . Am. Chem. SOC., 49, 2367 (1927). Green, H., J . Franklin Inst., 204, 713 (1927). Lineweaver, Hans, J . Biol. Chem., 122, 549 (1938). News E d . (Am. Chem. S O C . )18, , 492 (1940).
Colorimetric Method for Determination of Nitrite MARTHA B. SHINY Renziehausen Diabetic Foundation, Children’s Hospital of Pittsburgh, Pittsburgh, Penna.
A method employing sulfanilamide and N-(1-naphthyl)-ethylenediamine dihydrochloride for the determination of nitrite is proposed. These reagents have been found superior to sulfanilic acid and a-naphthylamine, formerly employed, in that the color developed is clearer, reaches its maximum intensity more rapidly, and remains stable for a longer time. A standardized solution of sulfanilamide is substituted for sodium nitrite as a primary standard to obviate the difficulties arising from the instability of the latter.
P
ROCEDURES for the colorimetric determination of nitrite in foods, water, and sewage have been based on the diazotization of sulfanilic acid by the available nitrite and the subsequent coupling with an agent such as a-naphthylamine ( I , 2, 6). As employed, these methods are open to two objections: (1) The coupling of diazotized sulfanilic acid with a-naphthylamine is relatively slow, requiring from 10 to 30 minutes for full color development ( 2 ) . With anaphthylamine acetate the color must be read within 30 minutes ( I ) . ( 2 ) Primary nitrite standards are unstable and difficult to prepare. It has been found possible to circumvent. these difficulties in part by replacing a-naphthylamine with N-( 1-naphthyl)ethylenediamine dihydrochloride, the coupling component suggested by Bratton and Marshall ( 3 ) for sulfanilamide determinations. It has the advantage of being water soluble, decreases the time required for color development to 2 minutes, gives a final color that remains constant for several hours, and is less sensitive t o variations in pH, reacting equally well in acid concentrations ranging from 0.1 to 1 Y. I n place of sulfanilic acid, sulfanilamide (p-aminobenzenesulfonamide) has been used. Sulfanilamide of a high degree
