reaction, is added from a pipet. The flask is stoppered and allowed t o stand for 30 minutes. A known volume (an excess) of 0.02N cerium(1V) sulfate solution is added, and the solution is titrated with iron(I1) sulfate, using o-phenanthroline iron(I1) sulfate as the indicator. For these titrations a buret graduated to 0.02 ml. should be used. It is desirable to run two blanks. Blank 1 is a solution of cerium(1V) sulfate of the same volume as t h a t required for the sample. This is titrated with standard iron(I1) ammonium sulfate mixture. Blank 2 is a solution prepared by using the same volumes of sodium nitrite and cerium(1T’) sulfate solutions as required for the sample. The excess of cerium(1V) sulfate in this solution should not be less than 5 ml This amount was determined as the smallest excess that mould gire a sufficient concentration to ensure quantitative oxidation. Calculations. I n cases where t h e sample titration, in terms of iron(II), is less t h a n blank 2 . t h e sample contains no sulfamic acid b u t does contain free nitrite ion. Then %; HXO? =
(Bz - V ) X N X 2.351 grams of sample
where Bz = nil. of blank 2 V = ml. of iron(I1) solution for test sample N = normality of iron(I1) solution K h e n the sample titration, in terms of iron(II), requires more solution than blank 2, sulfamic acid is present. Then X 4.855 B,) X yosulfamic acid = i T ’ grams of sample Blank 2 should be, in terms of iron(II),
Table I.
NaNO1,
famic acid that can be detected is the amount n-hich is eauivalent to the difference between blahk 1 and blank 2 . Data obtained for the duplication of blanks 1 and 2 are given in Table I.
Dudication of Blanks
Ce(IV), 111. Blank 1
Fe(II), Ml,
10 10 15 15 20 20
10.50 10.45 15.00 15.00 20.90 20.95
DISCUSSION
The recovery of sulfamic acid from solutions containing a known amount in the 1 t o 10 mg. range was around 99%. The values obtained are summarized in Table 11. This method can be applied to solutions containing small amounts of either nitrous or sulfamic acid. requires no complicated apparatus, and can readily be applied t o control Lt-ork.
Blank 2
M1. 5
5 5 10 10
in
5.02 5.04 10.04 5.00 5.02 10.00 10.04 4.98 5.02 10.00
10 10 15 15 15 20
Table It.
1
8
0.97 0.97 1 (74
9.70
(1) Baumgarten, P., Krummacher, A. H., Ber. 67, 1257-60 (1934).
Determination of Sulfamic Acid in Known Solutions
Sulfamic Sample iicid Taken, SaKOz, KO. Rlg. RI1. 2
LITERATURE CITED
Ce(IV),
Fe(II!,
5
in
111. 6 10 6.12 7 20
20
25
15.8.5
3
5
311. 10 10
about one half of blank 1. This is desirable, as the calculations involve the difference ( B , - V ) or (17 - &), which must be neither too large nor too small. Blank 1 supplies data for the calculation of the normality of the iron(I1) solution. The total amount of SUI-
Sulfamic Acid Found, N g . 0.96 0 98
Error, %
1.95
-1.0 f1.0 +0.5
9.74
+0.4
IT.W., Arnold, E. -I., ANAL. CHEX. 19, 336-7 (1947). (3) Carson, W. S . , Jr., Ibid., 23, 1016(2) Bowler,
19 (1951). (4) lleuwesen, 8 . ,Merkel, H., 2 nnorg. Chem. 244, 89-93 (1940).
RECEIVEDfor review Alarch 14, 1957. Accepted July 10, 1957.
Titration of Nitroaromatic Amines as Acids JAMES S. FRITZ, ANTHONY J. MOYE, and MARLENE JOHNSON RICHARD lnsfitufe for Atomic Research and Deparfrnent of Chemistry, Iowa Sfate College, Ames, Iowa Anilines substituted in the 2, 4, or 2, 4, and 6 positions with a t least two nitro groups, or with one nitro group and one or more chloro groups, can b e titrated as acids in pyridine, with triethyl-n-butylammonium hydroxide as the titrant. Diphenylamine derivatives with a t least one nitro group in the 4 position can b e titrated in a similar manner, Trinitrotoluene and trinitrobenzene can also b e titrated as monobasic acids. In several cases a selective determination of the different components in a mixture is possible.
D
a study of various nitro derivatives of aniline for possible use as acid-base indicators, it was found URIXG
that certain compounds of this type can be titrated quantitatively as acids. The titrations were carried out potentiometrically in pyridine, by using triethyl-n-butylammonium hydroxide ( 4 ) as the titrant. It then became of interest t o study in some detail the potentiometric titration of nitro aromatic compounds that are acidic yet do not have a functional group that is geiierally considered to be acidic. This paper presents the results of that study. Some analytical use has been made of the acidic properties of nitroaromatic amines and polynitroaromatic compounds. Moss, Elliott, and Hall ( 6 ) used 1,3,5-trinitrobenzene as an indicator in the titration of phenol with $0-
dium aminoethoxide in ethylenediamine. Fritz and Keen (3) found that 2-nitroaniline behaves as a weak indicator acid in ethylenediamine and is suitable for use as an indicator in the alkalimetric titration of phenol. Takiura and Takino (‘7) used 4-nitro-i’-aniinoazobenzene as an indicator in the same titration. Brockmann and 1Ieyer ( I ) titrated several aromatic polynitro compounds potentiometrically as acids in ethylenediamine. They found that 3dinitrobenzene gives two definite potentiometric breaks, the second coming after 2 equivalents of base (sodium aminoethoxide) per mole of 3-dinitrobenzene has been added, that picric acid consumes 3 equivalents of base per mole, VOL. 2 9 , NO. 1 1 , NOVEMBER 1957
1685
and that 2,4,6trinitrophenetole quires 2 equivalents per mole.
re-
REAGENTS
Triethyl-n-butylammoniuni hydroxide, 0.1N. Reflux equivalent quantities of triethylamine and n-butyl iodide. Convert the air-dried quaternary ammonium iodide to the hydroxide by agitating, for about 90 minutes, 30 grams of the iodide with 90 ml. of absolute methanol and 27 grams of powdered silver oxide. Filter the mixture under nitrogen and dilute the filtrate to 1 liter with anhydrous benzene. Standardize by titration of benzoic acid in pyridine. For accurate work restandardize the titrant every few days. Benzoic acid, reagent grade. Nitrogen, prepurified. Pyridine, reagent grade. Samples of nitro compounds, analyzed as received; mostly 98 to 100% pure. APPARATUS
Glass electrode, Beckman general purpose, No. 1190-80. Calomel electrode, Beckman fiber type, No. 1170, or sleeve type, No. 117071. Replace the aqueous potassium chloride solution by a saturated solution of potassium chloride in methanol. Buret, 10-ml. Normax. Potentiometric titrator. Any good direct-reading instrument such as the Precision-Shell Dual Titrimeter or Beckman Model G p H Meter. Magnetic stirrer.
additional electron-withdrawing substituent tends to increase further the acidity of an aromatic acid. An examination of the titration curves in Figure 1 shows some possibilities for differentiating titrations. When a mixture of 2,4,6-trinitroaniline and 2,4dinitroaniline is titrated, separate end points are obtained for each compound (see Figure 2). It should also be possible to titrate 2,4,6-trinitroaniline in the presence of any of the other nitroanilines listed here; and to titrate 2,4-
r--
I
I
,
I
I
i
Figure 1. Titration of substituted aniline derivatives in pyridine
1. 2-Nitroaniline 2. 2-Sitro-4-chloroaniline 3. 2,6-Dichloro-.l-nitroaniline 4. 2-Chloro-4-nitroaniline 5 , 2,4-Dinitroaniline 6. 2,4,6-Trinitroaniline (picramide)
I
- loooo
PROCEDURE
Dissolve a 0.4- to 0.8-meq. sample of the compound to be titrated in 40 ml. of pyridine, and titrate potentiometrically with 0.1N triethyl-n-butylammonium hydroxide. Determine the end point of the titration by plotting the potential against the volume of titrant. Determine a solvent blank by potentiometric titration and subtract from the volume of titrant used to titrate the sample.
dinitroaniline, 2,6dichloro-4-nitroaniline, or 2-nitro-4-chloroaniline in the presence of 2, 3-, or 4-nitroaniline. Nitrodiphenylamines. Figures 3 and 4 show curves for the titration of some nitrated diphenylamines in pyridine. I n general, these diphenylamines are more acidic than the corresponding phenylamines, owing to the electrophilic nature of an additional benzene ring. The titration curve of 4-nitrodiphenylamine has a good potential break a t the end point, but 2-nitro-
1
2
3
ml
4
5
6
7
8
9
OF 0 1 N ET~BUN'OH
Oi
-100
SCOPE
Nitroanilines. The titration curves in pyridine of several mono- and polynitro derivatives of aniline are shown in Figure 1. 2- and 4nitroaniline are too weakly acidic to be titrated. A slight inflection is obtained a t the stoichionietric point when 2-chloro-4-nitroaniline is titrated; 2-nitro-4-chloroaniline has a sharper potentiometric end point. The presence of another chloro substituent, as in 2,6-dichloro-4-nitroaniline, increases the acidity still further. 2,4Dinitroaniline has about the same acid strength as 2,6-dichloro-4nitroaniline, but 2,4,6-trinitroaniline is a stronger acid than either of these. These effects are in accord with the well known fact that the nitro group has stronger electron-withdrawing properties than the chloro group, and that each 1686
ANALYTICAL CHEMISTRY
-8
-900 00
IO
20
rnl.
3.0
4.0
50
6.0
7.0 8.0 9.0
OF 0.1 N Et,BUNtOH-
Figure 2. Titration of picramide and 2,4-dinitroaniline (upper) and 2,4-dinitroaniline in presence of 2-nitroaniline (lower)
diphenylamine shows only very weak acidic properties. 2,4-Dinitrodiphenylamine is strongly acidic. The titration of 2,2’,4,4’,6,6’-hexanitrodicurve phenylamine is very interesting, in t h a t two definite inflection points are observed. It is possible to titrate 4-nitrodiphenylamine in the presence of 2-nitrodiphenylamine. Potentiometric titration of a mixture of 2,4-dinitrodiphenylarniiie and 4-nitrodiphenylamine gives two end points, the first for the dinitro amine and the second for the mononitro compound (Figure 5 ) . Polynitroaromatics. Both trinitrotoluene a n d trinitrobenzene have a
- 1100
L
-12000
1
2
3
Figure 3. Titration of substituted diphenylamine derivatives in pyridine
4 5 6 0.1 N Et3BUNtOH-
rnl. OF
1. 2-Nitrodiphenylamine 2. 4Nitrodiphenylamine 3. 2,4-Dinitrodiphenylamine
sharp end point when titrated in pyridine with triethyl-n-butylammonium hydroxide. The end point occurs when 1 equivalent per mole of the titrant has been added. 2,CDinitrotoluene shows definite acidic properties when titrated under these same conditions, but the
Table I.
Quantitative Data for Single Titrations
Titrated Compound 2-Chloro-4-nitroaniline 2-Nitro-4-chloroaniline 2,4-Dinitroaniline 2,6-Dichloro-4-nitroaniline 4-Chlor0-2~6-dinitroaniline 2 4 6-Trinitroaniline 4~~itrodiphenylamine 2,4-D.initrodiphenylamine 2,2’,4,4’,6,6’-Hexanitro-
diphenylamine 2,4,6-Trinitrot,oluene 1,3,5-Trinitrobenzene
cFigure 4. Titration of 2,2’, 4,4‘,6,6’ hexanitrodiphenylamine in pyridine
-
yo Found 100.5, 100.1 101.4, 100.9 99.2, 100.0 100.3, 100.4 100.5, 100.5
100.1 100.7 99.9, 99.3 9 8 . 8 , 99.5 99.3, 99.4
2-Bromo-4,6-dinitrotolu-
100.4, 101.0 99.8, 100.1 101.8, 101.1
ene 2,4-Dinitrotoluene
103.5, 105.3
Mixture 2,4,6-Trinitroaniline 2,4Dinitroaniline 2,PDinitroaniline 2-Nitroaniline 2.4.6-Trinitrotoluene ’ 2,4-Dinitrotoluene
101.9 99.8 100.9
...
’
98.7
lOi :0
2,4-Dinitrodiphenylamine
4-Nitrodiphenylamine
Figure 15. Titration of nitrated d i p henylamine mixtures in pyridine
*
-8001 -90%
2.0
40
6.0
ml
8.0 10.0 120 14.0 of O I N E t , B u N + O H -
2,4
16.0
0.59
l o o k
18.0
- DINITRODIPHENYLAMINE
4- NITRODIPHENYLAMINE
1 903-
/
2- NITRODIPHENYLAMINE
IO001 0.0
1.0
I
2.0
I
3.0
I
4.0
I
5.0
I
6.0
I
7.0
I
0.0
3
ml. of 0.1N Et3 Bu N’OH-
Figure 6. Titration of polynitroarornatics in pyridine I
I
1. 2,4,6-Trinitrotoluene 2. 1,3,5-Trinitrobenzene
3. 2-Bromo-4,6-dinitrotoluene 4.
2,I-Dinitrotoluene
VOL. 2 9 , NO. 11, NOVEMBER 1957
1687
stoichiometry appears to be erratic. Because of unsteady, drifting potentials, exceptional care and attention are required to obtain a titration curve for this compound. 2-Bromo-4,6-dinitrotoluene can be titrated stoichiometrically in pyridine. Titration curves for these compounds are given in Figure 6.
the tetraalkylammonium hydroxide is added to the benzene ring.
hexanitro compound, but it is necessary to n-ait 20 or 30 seconds after ad-
v1
h-02
DISCUSSION
Quantitative data for the titrations described above are listed in Table I. I n general, the precision and accuracy are satisfactory but are not so good as obtained for the titration of other types of acids (such as phenols, enols, and carboxylic acids) under the same conditions. A tendency toward slightly high results was noticed. It seems probable that in the case of nitroanilines and nitrodiphenylamines, one of the hydrogen atoms of the amino group is acidic. ’
This type of reaction has been well described by Neisenheimer (5) and was referred to by Brockmann and Meyer ( 2 ) in explaining the results of their titrations of polynitro compounds with sodium aminoethoxide. Trinitrotoluene may react in the same manner as trinitrobenzene, or a hydrogen may be extracted from the methyl group. When 2,2’,4,4’,6,6’-hexanitrodiphenylamine is titrated, two types of re-
NH
’
[R4S] +
SO2
NH
v II
yo2
--
etc.
A-
/\
0
The acidity increases as additional nitro or other electron-withdrawing groups are introduced into the aromatic ring. I n the case of trinitrobenzene, it seems likely that the hydroxyl ion from
0
(1)
actions take place. First, the acidic hydrogen is titrated rapidly with the tetraalkylammonium hydroxide. After the first end point has been reached, a second mole of titrant reacts with the
dition of each increment of titrant in order to reach equilibrium. The reaction between the first and second end points probably involves addition of base to a n aromatic ring (see Reaction
‘4. LITERATURE CITED
(1) Brockmann, H., Meyer, E., Chem. Ber. 87,81(1954). ( 2 ) Brockmann, H., Meyer, E., .Vaturwissenschaften40,242 (1953). ( 3 ) Fritz, J. S., Keen. R . T., ANAL. CHEM.25, 179 (1953). ( 4 ) Fritz, J. S., Yamamura, S. S., Zbid., 29, 1079 (1957). (5) Meisenheimer, J., -4nn. Chem. Lzebigs 323, 219 (1902).
( 6 ) Moss, M. L., Elliott, J. H., Hall, R. T., ANAL.CHEAT. 20, 784 (1948). (7) Takiura, K., Takino, H , J . Pharm. soc. Japan 74, 971 (1954).
RECEIVED for revien February 13, 1957. Accepted July 8, 1957. Contribution 559. Work performed in the Ames Laboratory, U. S .4tomic Energy Commission.
High Frequency Titration of Lead DONALD T. SAWYER’ and PAUL S. FARRINGTON Deparfment of Chemistry, University of California, los Angeles, Calif.
b The use of a high frequency oscillator as an end point detector for the determination of lead has been investigated. A titration procedure has been developed which uses high frequency end point detection with dichromate as the titrant. The method is rapid, and is as accurate as other volumetric techniques for lead.
A
many methods have been developed for the determination of lead, the number of good volumetric procedures is limited (IO). I n the chromate method, which is probably one of the best of the volumetric methods, although slow and tedious, lead is precipitated as the chromate, excess iodide LTHOUGH
Present address, Division of Physical Sciences, University of California, Riverside, Calif. 1688
ANALYTICAL CHEMISTRY
is added t o the acidified precipitate, and the liberated iodine is titrated with standard thiosulfate. Kolthoff and Pan have described an amperometric method using dichromate as the titrant (19). A potentiometric titration of lead with the disodium salt of ethylenediaminetetraacetic acid has been described by P;ibil and coworkers (14). The high frequency technique is only a special form of conductometry; however, absence of contact between the electrodes and the solution eliminates errors caused by coating of the electrodes by a precipitate and other surface phenomena. The general applicability of the high frequency method to precipitation titrations ( S , 5 ,6,13) suggested an investigation of a new volumetric technique for the determination of lead. A rapid method has been developed which uses potassium dichromate as the titrant, and a high frequency oscillator for
end point detection. I t gives accurate results for samples containing from 10 to 200 mg. of lead. EXPERIMENTAL
A Beckman Conductometer (not commercially available) was used as the high frequency oscillator for end point detection. The instrument, which uses a Q-type circuit. is described by Willard, Xerritt, and Dean (16). Instruments with similar circuits have been dereloped by Hall (8, 9) and by dnderson and coworkers ( I ) . Because the response sensitivity of the Q-type instrument is a function of the ionic strength of the solution, it is necessary to calibrate it. The calibration curve obtained for the Beckman instrument is shown in Figure 1. The ionic strength of an unknown solution can be determined by the coupling condenser reading, which will indicate whether the solution is in a
