745
V O L U M E 20, NO. 8, A U G ' U S T 1 9 4 8 The volume of the solution displaced is the volume of the pigillent plus the volume of the adsorbed Hornkem. The volume of r,hr solution displaced by 10 grams of silica, as experimentally dewrmined, is 10,/2.372 = 4.2158 ml. This is actually the volume (If 10 grams of silica plus 0.01 gram of adsorbed Hornkem. The assumed volume of the adsorbed Hornkem is 0.01 /0.8 = 0.0125 nil. The difference, 4.215s - 0.0123, gives 4.2033 ml. as the volume of the silica. I t s density then becomes 10/4.2033 = 2.379 gram nil.-', which corresponds to the value 2.372in Table I. .i similar calculation for the zinc tetroxychromate pigment gives thc rrue density as 4.167, corrcqmnding to 4.000 in the t,xhlc. The correctious ttrtb based on a guess as to the density of the d s o r b c d layer, and the nearer the density of the adsorbed layer is to the density of the sprrounding solution, the smaller will be the .orrection. I t follows that the values for the specific gravities tn Tahle I are hased on the assumption that the densities of the idsorbetl layers are the same as the densities of the surrounding inliitionc xiid t h a t thri method must be applied to very finely
divided pigiiieiits with reservations. The same reservations must he considered when the pigment is immersed in a pure liquid. The pigments considered in this report are all essentially hydrophilic. With hydrophobic pigments other wetting agents might have t o be employed. If water-soluble Retting agents having a specific affinity for hydrophobic pigments are not available, it might be necessary to use oil-soluble wetting agents of the required characteristics dissolved in some organic solvent. S o solution is suitable for use with pigments that havr appreciable .solubilities i n the solvent. LITERATURE CITED
(1) Baker and Mar t i n , IYD.ENG.CHEM..AKAL.ED.,15,279 (1943). (2) Gooden, I b i d . , 15, 578 (1943). (3) Moran, Ibid., 15, 361 (19431.
.
Colorimetric Determination of Certain Dinitro Aromatics In the Presence of the Corresponding Mononitro Compounds F. L. ENGLISH E. I . du Pont de Nemours & Co.,Deepwater P o i n t , 5. J .
Procedures are presented for the estimation of small amounts of dinitrobenzene. dinitrochlorobenzene, dinitronaphthalene, dinitrotoluene, and dinitroxylene in their mononitro derivatives and for dinitrocresol in mononitrotoluene. The first five depend upon the color formed when an acetone solution of the sample is treated with dilute alcoholic sodium hydroxide; the last, upon the color of the sodium salt of dinitrocresol in aqueous solution. In all cases, the color intensity is read with a photoelectric colorimeter. A single determination can be run in LO to 15 minutes and results are reproducihle within ahnut *3@ of the amount o f dinitro compound present.
1'""
well-known color ccltctiolis of certain tiiriitro ttroinativ compounds when treated in acetone solution with alcoholic aodiuni hydroxide were intended, as originally published ( 3 ) . for qualitative purposes only. Preliminary experiments with these reactions shon-ed that in general the hue, intensity, and 4tability of the color formed are largely influenced by the conwntration of sodium hydroxide and, to a lesser extent, by thr amount of water present. T o attain quantitative reproducibilit? it is therefore necessary t o exercise proper care in the preparatiori of the caust,ic reagent. Even with accurately prepared solutions of sodium hydroxidc \ n ethanol (denat,ured,2B formula) somewhat erratic results R-erv obtained and in many cases the colors were too fugitive for quantitative purposes. The substitution of ammonia for sodiuni hydroxide was ineffective; the colors formed, if any, were of too l o x tinctorial power, while potassium hydroxide yielded colore of such a low order of stability as t o be entirely hopeless from the tLrialytica1standpoint. I t was found, however, that if the caustir reagent is prepared from sodium niorioxide ( S a 2 0 ) (obtainable from 6:. I. du Pont de Semours & Co., Tnr., R 8: H Chemical Division, Siagara Falls, S . Y.)instead of the hydroxide, the resulting ,*olorsare sufficiently stable for a colorimeter reading, presumably because of dehydration of the alcohol. Solutions of sodium hydroxide in methanol give feeble colors, if any. The different &nitro compounds studied require somewhat different concenrrationn of sodiiim hydroxide to produce niaxirnurn color.
The colorinlater ubed 111 this \vorii was a Cenco-SheardLSanford Photelometer (Catalog No. 12,336) provided with 1-cm. fused glass cells and the Cenco light filters normally supplied with the instrument. .\ calibration curve for each determination wae prepared. Because all the methods are identical in principle and manipulation (except for dinitrocresol which is treated wparately) , differing only in the amounts of sample, reagent, etc., used, the following procedure is expressed in general terms \r-ith a subjoined table giving the required quantity of sample, t h v aliquot u w t , arid vixagent romposition for each specific case. SOUlUXl HYDROXIDE REAGENT
Lsing a %nil. graduated cylinder, measure int,o a drv S-ounce bottle the desired amounts of 95% ethanol and acetone, add the sodium monoxide (not peroxide), swirl gent,ly for 2 minutes, cool to 20" to 25" C., add sufficient acetone (also a t 20" to 25") to bring the volume to 200 ml., shake vigorously for about 10 seconds, and filter through a folded paper into a dry bottle. The filt,rate must be perfectly clear and only slightly yellowish. This reagent is not stable for longer than about one hour. PROCEDURE
l'ipet (or weigh, in the case of dinitronaphthalene) the desired amount of sample into a 100-ml. volumetric flask, allowing the pipet to drain 30 seconds with its tip touching the flask neck. Dilute to 100 ml. with acetone. Into a 50-mi. beaker pipet 25 ml. of the sodium hydroxide reagent, add the proper aliquot of the
ANALYTICAL
746
sample solution, stir briefly, and immediately read the color intensity on the colorimeter, using the appropriate light filter and some of the sodium hydroxide reagent in the comparison cell. Obtain the corresponding percentage of dinitro compound from the calibration curve.
CHEMISTRY
removal of the meta from 3' xylene by selective sulfonation. These three series of standard samples were analyzed by the colorimetric method previously described (Table 11). The 3" and meta figures are so nearly the same that their average may be taken to serve for both without significant error, but a separate curve is required for the ortho-para. A series of known composition mixtures of mono- and dinitrotoluene was prepared in similar manner, and three analyses were made of each. The results are given in Table I11 to illustrate the degree of concordance that can be expected in multiple determinations upon a given sample.
.
Table I.
Determination
Sample and Reagent Quantities
Sample
Aliquot
MZ.
MZ.
Sodium Hydroxide Reagent AceAlcotone hol NazO
MI. 10 10
MI. 10 10 10 15 15 15
1 DNB 1 1 DNCB 2 1 10 DNT 1 1 15 D N X meta and 3') 2 1 15 4 D N X lortho-para) 1 15 DNN 5 gram Maximum transmittances of these filters were 410, 525, microns, respectively, for the blue, green, and red.
Light Filter
G. Green0 Green Red 3 Red 3 Red 3 Blue and 610 milli2 2 2
% DKT Present 0.0 1 .o 2.0 3.0
NOTES
The preparation of the sodium hydroxide reagent is fairly critical. The initial volumes of acetone and alcohol should be accurate within 0.5 ml. and the sodium monoxide weighed on a rough balance within 0.1 gram. Sodium peroxide should not be used because of its violent reaction with alcohol. These procedures will easily detect 0.05% of the compound sought and cover the range up to about 5%. However, the sensitivity and scope can be changed within wide limits by suitable variation in sample size and aliquot taken. The colorimeter readings should be taken as soon as possible (preferably within 0.5 minute) after the sample aliquot is added to the reagent because the colors change fairly rapidly. In the early experimental work, it was found that the different isomers of a given dinitro compound yield more or less different colors in both hue and intensity when treated by the above procedure. For this reason, it is essential that the calibration curve be prepared on the basis of standard samples containing the same isomers and in substantially the same proportions as will be present in the samples t o be analyzed. To accomplish this, the following general procedure, illustrated in the case of nitroxylene was adopted.
Table 11. Calibration Data for Determination of Dinitroxylene i n Nitrated 3'. m- and o,p-Xylenes % DNX Present
30
Photelometer Reading hleta
Table 111. Calibration Data for Determination of Dinitrotoluene
Ortho-para
PROCEDURE
The xylene was nitrated with 115y0 (theory) of nitric acid, in the form of a mixed acid containing 26% nitric and 61% sulfuric acid, a t 45" to 50" C. The product, containing about 15% dinitro, was fractionally distilled through a 30-cm. (12-inch) packed column a t 1- to 5-mm. pressure until most of the mononitro had passed over. The residue was then transferred to a small flask and distilled practically to dryness a t 1-mm. pressure without fractionation. The dinitro content of this distillate was calculated from its total nitrogen value, determined by titanous sulfate titration, on the assumption that it contained only mono- and dinitroxylene. The mononitro distillate was then refractionated, a small foreshot and about 20% residue were discarded, and the dinitro-free middle cut was combined with the distilled dinitro to produce a series of standards oE kriowri dinitro content. Three such preparations were made, starting, respectively, with 3' distillation range commercial xylene containing all the isomers, purified m-xylene, and the mixture of ortho and para remaining after
4.0
6.0
Photelometer Reading
A 99.3
B 99.6' 66.2
66.0 44.0
44.3
30.0 20.2 10.1
30.0 20.5 9.9
C 99.5 66.0
44.5 30.0
20.8 9,6
It IS thus evident that in spite of the rather poor color stability, consistent results are readily possible. This conclusion is further confirmed by the fact that in the hands of routine analysts these methods have produced entirely satisfactory results for more than 3 years. The 99.5 reading instead of 100 for 0% dinitrotoluene is due to a slight color developed by mononitro and not to accidental contamination by dinitrotoluene, for exhaustively purified oand p-nitrotoluene both gave the same reading separately. In some cases, this effect is much more pronouncgd, pure mononitronaphthalene (approximately 9401, alpha and 6% beta isomer) giving a reading of 76.0. Incident to'the mononitration of toluene, a small amount of dinitrocresol is formed (1, 9, 4) which must be removed by an alkaline wash to avoid explosion hazard in the subsequent fractional distillation. The following procedure was devised to estimate traces of dinitrocresol in washed nitrotoluene and thus check the efficiency of the washing operation. Procedure for Dinitrocresol. Pipet 10 ml. of the clear oil into a 100-ml. volumetric flask, add 75 ml. of 0.5% aqueous sodium hydroxide solution, shake vigorously for 1 minute, dilute to the mark with more of the sodium hydroxide solution, mix thoroughly, let stand about 5 minutes to settle, and filter a portion of the aqueous layer. The filtrate must be perfectlv clear. Read its color intensity against the 0.5% sodium hvdroxide solution using the blue filter. The sample quantity specified is such as to make the analysis cover the range up to about 0.1%, 0.001 % being readily detectable. To prepare the standard samples for calibration, wash 1 liter of crude mononitrotoluene direct from the nitrator several times with water to remove the mineral acids, then n ith successive 50ml. portions of 1%sodium hydroxide until the extracting solution is only slightly colored. Combine the caustic extracts, acidify with hydrochloric acid, filter off the precipitated material, which will consist of all the nitrated phenol derivatives normally present in crude mononitrotoluene, and dry in a vacuum desiccator. Distill the washed mononitro oil through a short fractionating column a t 10- to 20-mm. pressure, discarding a 10% foreshot and 30% residue. Prepare a series of standard samples covering the desired range by dissolving mighed portions of the crude dinitrocresol in the distilled oil. LITERATURE CITED
(1) B e n n e t t and Youle, J . Chem Soc., 1938, 1816. (2) B e u l e , P. de, Bull. SOC. chim. Belg., 42,27 (1933). (3) Rudolph, Z. anal. Chern., 60, 239 (1921). (4) Titov, J . Gen. Chem. U.S.S.R., 10, 1878 (1940). RECEIVED Sovember 25, 1947.
