V O L U M E 25, NO. 3, M A R C H 1 9 5 3
407
W
in concentrated acids by titration 71-ith fuming acids; “free anhydrides” in fuming acid; aniline in aniline .salts by acetylation (18).
I-
ACKNOW L E D G
W (L
3
l-
a (L
t5 a
-
\IENT h-I
I
Min. -4
+ TIME
Figure 6. Titration of 0.005 IV Sodium Hydroxide with 0.1147 iV Hydrochloric .4cid 1. NaOH, 1 m e q . (sens.
-
P-lhlin.+
TIME
= 1.0)
The authors are indebted t o the Atomic ComEnergy mission for partial support of this work.
LITERATURE CITED
h 3
Figure 5 . Titrations of Hydrochloric -kcid, immoniuni Chloride, and Mixtures of Hydrochloric Acid and rlmmonium Chloride in 60 Ml. of Solution with 0.994 A’ Sodium Hydroxide 1. HCI, 5 m e q . (sens. = 0.3) HCI, 5 m e q . a n d ”4C1, 5 m e q . (sens. = 0.3) NH4C1, 10 m e q . (sens. = 0.6) ( m u l t i p l y t e m p e r a t u r e s c a l e by two)
2. 3.
base ( 1 1 ) ; various acids of phosphorus with base (13, 1 4 ) ; trisodium phosphate with acid ( 6 ) ; aprotic acids (aluminum chloride) and bases (dioxane, ethyl acetate) in benzene (20). Precipitation Titrations. Zinc, lead, and magnesium with hydroxide (6); calcium, strontium, and mercury(1) and (11) with oxalate (9). Complex-Formation Titration. Cobalt(11),copper, and nickel n-ith ammonia (6);niercury(I1) with iodide; nickel, zinc, and cobalt(I1) with cyanide (11). Redox Titrations. Arsenite with bromate and with hypochlorite; oxalate, hydrogen peroxide, iron( 11),and ferrocyanide Kith permanganate (9). Miscellaneous Titrations. Acetic anhydride in acetic acidsulfuric acid acetylating baths by reacting the anhydride 11-ith aniline: acetyl nuniber.and iodine number in fat analysis; 11-ater
(1) Becker, J. il., Green, C. B., and Pearson, G. L., Bell System Tech. J., 26, 170 (1947). (2) Dean, P. A i . , and Sewcomer, E., J . Ana. Chem. Soc., 47, 64 (1925). (3) Dean, P. JI., and Watts, 0. O., Ibid., 46, 855 (1924). (4) Dowell, K. P., Elec. X f g . , 42, No. 2, 84 (1948). (5) Dutoit, P., and Grobet, E., J..chim.phys., 19, 324 (1921). (6) Grobet, E., Ibid., 19, 331 (1921). (7) Kolthoff, I. M.,and Stenger, 5’. R., “T’olumetric .halysis,” Vol. I, Ken. York, Interscience Publishers, 1941. ( 8 ) Lingane, J. J., ASAL. CHEW,20, 285 (1945). (9) hlayr, C., and Fisch, J., 2. anal. Chem., 76, 418 (1929). (10) Alondain-hlonval, P., and Paris, R., Bull. SOC. chim. France, 5, Ser. 5,1641 (1938). (11) Mondain-Monval, P., and Paris, R., Compt. rend., 198, 1154 (1934); 207,335 (1938). (12) Aiiiller, R. H., Ax.4~.CHEM., 21, 108 (1949). (13) Paris, R., and Robert, J., Compt. rend., 223, 1135 (1946). (14) Paris, R., and Tardy, P., Ibid., 223, 1001 (1946). (15) Paris, R., and Vial, J., Chim. anal., 34, 3 (1952). (16) Robinson, H. A , , T r a n s . Electrochem. Soc., 92, 445 (1947). (17) Roth, W. A., and Scheel, K., “Landolt-Bornstein PhysikalischChemische Tabellen,” 5th ed., Berlin, J. Springer, 1923. (18) Somiya, T., J . SOC.Chem. I n d . , J a p a n , 51, 135T (1932). (19) Thomsen, J., “Thermochemische Untersuchung,” 1‘01. I, Leipzig, Barth, 1882. (20) Trambouae, Y., Compt. rend., 233, 648 (1951). RECEIVED for review July 18, 1962. Accepted December 2 , 1952.
Differential Titration of Amines JA3IES S. FRITZ, Iowa State College, Ames, Iowa
S
EF’ERAL recent papers have dealt with the titration of
organic bases in acetic acid, dioxane, and other solvents (1, 2, 6, 7 , 9, 10). These are excellent general methods but do not wrve to differentiate various types of amines. The use of acetic aiihydride permits the convenient determination of tertiary :imines in the presence of primary and secondary amines (12). TIeatment of a mixture with salicylaldehyde followed by titration in benzene-2-propanol (13) or in ethylene glycol-2-propanol ( 1 1 ) has been used to determine primary amines. It is now proposed that differential titration in nonaqueous solvents be used to
distinguish quantitatively between amines of different basic strength. THEORY
Although it has been widely and successfully used as a solvent for titration of amines of all types, acetic acid cannot be used to differentiate aliphatic and aromatic amines. This is illustated by titrating a mixture of pyridine and butylamine in acetic acidacetonitrile (Figure 1). This curve shows only a single break despite the considerable difference in basic strength of these two amines (in water the p K values differ by 5.5 units). An explana-
408
ANALYTICAL CHEMISTRY Although several excellent procedures are available for the acidimetric titration of amines in nonaqueous solutions, none of these permits the differential titration of amines of different basic strength. This paper discusses the theory of such titrations and proposes simple potentiometric and indicator methods for differential titrations in nonaqueous solvents. These methods permit analysis of aromatic-aliphatic amine mixtures and certain other mixtures such as anilineo-chloroaniline, pyridine-caffeine, and aniline-sulfathiazole. Preliminary treatment with salicylaldehyde permits the determination of both primary and secondary aliphatic amines by a single titration. These general methods should find application in the analysis of a large number of specific amine mixtures.
tion of this is that both h s e s react nearly completely n i t h the solvent,
B
+ HAC +B H + + Ac-
and it is the acetate ion that is titrated. This phenomenon has been termed the “leveling effect” ( 6 ) . The requirements of a good solvent for differential titration of bases can be listed: 1 . Available in fairly pure condition a t a reasonable price. 2. Dissolves amines readily. 3. KO pronounced basic properties. This is necessary in order that weak bases ran be titrated. TTater and alcohol are too strongly basic to be used as solvents for the titration of aromatic amines 4. S o pronounced acidic properties. Solvents such as acetic acid level most amines to about the same basic strength. 5. Sufficiently high dielectric constant so that the titration can be folloJved potentiometrically.
(Figure 3). With only a few eaceptions the points fall almost on n straight line. The slope of this line is about 100 mv. per p K unit, compared with a theoretical slope of 59 mv. per pK unit for such a curve in water (4). This means that less difference in basic strength is required to obtain separate end points in acetonitrile. It will be noted that this curve shows a marked tendency to level out with amines having a p& less than about 6. This might be due to decreased sensitivity of the indicator electrode in this region or it might indicate a leveling effect due to the acid properties of the solvent. I n a similar plot using acetic acid instead of acetonitrile, Hall (3)obtained a curve of similar shape in which amines having a pKb less than about 9.25 in water a11 appeared to be of essentially the same strength in acetic acid. 700
-PYRIDINE
500
I
UWJ
-
zook 100
I
I
I
400
I
-200 I 0
(
I
I
1
2
4
6
8
Figure 2. Figure 1. Titration of Butylamine Plus Pyridine in Acetonitrile-Acetic Acid
j
I
1
10 12 14 16 M L HC104 (DIOXANE1
700 0
~
22
4
Titration of Butylamine Plns Pyridine in Ace tonitrile
600
Acetonitrile TYUE found to fulfill all of these requirements. Since perchloric acid in acetonitrile is not stable on standing, dioxane is used as the solvent for the titrant. Figure 2 shows the two sharp breaks obtained nhen a mixture of pyridine and butylamine is titrated in acetonitrile. [Khile this work was in progress, Pifer and Wollish ( 8 )reported that two breaks are obtained in the titration of certain bases if dioxane is used as the solvent, while only a single break is obtained if acetic acid is present. As dioxane has a very lorn dielectric constant, the potentiometric titration requires a special cell in which the electrodes are placed very close together to overcome the high resistance of the solution.] I t ivas desired to learn whether there is any simple relationship between the strength of organic bases in acetonitrile and the strength of these bases in water. To check this the potential (in millivolts) a t the mid-point of the titration of amines in acetonitrile was plotted agaiiwt the pKb values of these amines in water
20
18
1
I
I
- NITROANILINE,
V O L U M E 25, NO. 3, M A R C H 1 9 5 3
409 acetic acid. Cool, add 2 drops of methyl violet, and titrate to a light blue-green end point.
600
Calculations
I
500-
Milliequivalents B , = ( m - fnb)-V hlilliequivalents B2 = ( n - nb).\where BI and B, are tm-o bases of different strength, B, having the greater basic strength, V L = milliliters of perchloric acid from start of titration to first end point m5 = milliliters of perchloric acid at first end point in blank n = milliliters of perchloric acid to titrate from first to second end point 12.5 = milliliters of perchloric acid to titrate from first to second end point in blank S = normality of perchloric acid used
I
400-
I~ETHANOLAMINE
100
-
i
J
PROCEDURE B
Dissolve the sample containing 0.6 to 1.0 meq. of total amines in 20 ml. of acetonitrile. Add 6 drops of eosin and titrate with 0.1 perchloric acid in dioxane to a pale yellow end point. Add 2 drops of methyl violet and 20 ml. of acetic acid, and continue the titration until a blue-green end point is reached. A hlank is determined by the method in procedure .4. Calculations are identical n-ith those in procedure A .
Figure 4. T i t r a t i o n of E t h a n o l a m i n e plus Aniline in Ace tonitrile
500
-100'-
I
-
PROCEDURE C
Procedure -1is folloned. except that 0.1 S perchloric acid in acetic acid is used as the titrant. The blank is determined by titrating a 20-ml. portion of acetonitrile with perchloric acid in acetic acid. This blnnk is subtracted from the first end point only.
I
Figure 5 . T i t r a t i o n of Cinchonine in Acetonitrile APPARATUS
Beckman PIT incter (Model G) or similar titrator. Buret, 10 nil., easily read to 0.01 ml. Glass electrode, Beckman Catalog No. 1190-42 Caloinel electrode. Beckman C atalog No. 1170.
0
2
RE.AGENTS AND SOLUTIONS
Acetic acid, reagent grade, glacial. Acetic anhydride, reagent grade, 90 to 95%. Acetonitrile. Eastman white lahel or equivalent. p-Dioxane. Eastman white lahe1 or equivalent. Eosin Y. saturated solution in acetonitrile. Methyl violet, 0.27, solution in chlorobenzene. Perchloric acid in diosane. Dissolve 8.5 nil. of 7 2 7 , perchloric acid in 1 liter of diosane. Perchloric acid in p-dioxane. Dissolve 8.5 ml. of 72% perchloric acid in 1 liter of dioxane. Perchloric acid in glacial acetic acid. Dissolve 8.6 ml. of 72y0 perchloric acid in 60 ml. of glacial acetic acid. Add 22 nil. of acetic anhydride and let stand overnight. Dilute to 1 liter with glacial acetic acid. Potassium acid phthalate, primary standard grade. PROCEDURE A
Dissolve the sample containing 0.6 to 1.0 meq. of total amines in 20 rnl. of acetonitrile. Using the millivolt scale of the pH meter, titrate potentiometrically with 0.1 K perchloric acid, in p-diosane. A blank must be run on each hatch of acetonitrile. T o do this, a 20-mI. portion of acetonitrile is titrated potentiornetrically with 0.1 A' perchloric acid in dioxane. To standardize the titrant, dissolve about 100 mg. (accurately wighed) of potassium acid phthalate in 25 ml. of hot glacial
4
.6
.8 ML
ID HC104
I2
14
1.6
18
20
22
24
(DIOXANE)
Figure 6. T i t r a t i o n s in Acetonitrile 1. 2.
Butylamine plus salicylaldehyde Butylamine plus dibutylamine plus salicylaldehyde
Calculations llilliequivaients B1 = ( m - m b ) S hlilliequivalents B2 = ( n ) s SCOPE
Titration of aliphatic-aromatic amine mistures in acetonitrile results in two distinct breaks. Typical curves (obtained using Procedure -4)are shown in Figures 4 and 5 . Once a given mixture has been titrated potentiometrically, visual indicators can often be used for subsequent titrations (Procedure B). Eosin Y changes sharply from pink to pale yellow when aliphatic amines have been titrat,ed. hlethyl violet is then added and the titration is continued, a color change from violet to blue green marking the second end point. Aliphatic primary and secondary amines can be differentiated if the mixture is first treated with salicylaldehyde (13). Acetonitrile is added and the mixture is titrated potentiometrically n-ith perchloric acid in dioxane. Salicylaldehyde reacts with the
ANALYTICAL CHEMISTRY
410
primary amine to form a weaker base, but one which can still be titrated. The basic strength of the secondary amine remains essentially the same. The first end point represents the secondary amine content while the difference between the first and second end points measures the primary amine content (Figure 6). The use of salicylaldehyde to distinguish between primary and
secondary amines n-as proposed by Wagner, Brown, and Peters (13) and later used by Siggia, Hanna, and Kervenski ( 1 1 ) but in neither of these methods are both end points sharp. Aromatic amines and weaker bases such as negatively substituted aromatic amines can often be differentiated. I n this case somewhat better results are obtained using perchloric acid in glacial acetic acid as the titrant (procedure C). This is permissible because aromatic amines are too weak to be leveled by acetic acid. Typical mixtures nhich can be titrated are aniline-ochloroaniliue, aniline-sulfathiazole, and pyridine-caff eine (see Figures 7 and 8). Here the breaks are too gradual to permit the use of visual indicators. The individual titration curves of some 55 amines published by Hall (3)are a convenient help in predicting TT hich mixtures of n-eak amines can be successfully titrated. Quantitative results of some niivtures are given in Table I. It nil1 be noted that the precision and accuracy decrease as the sharpness of the end points decreases.
-ANILINE
DISCUSSION
i450t / 0 0 4
2 0ML. HCIO4 (DIOXANE) 50
1.0
50
40
Titration of Mixtures in Acetonitrile
Figure 7.
A blank determination on each batch of acetonitrile is necessary because basic impurities are likely to be present. The curves in Figure 9 show the blank obtained using perchloric acid in dioxane and in acetic acid as the titrant. A11 titrations in acetonitrile should be carried out at room temperature, as there may be disturbing side reactions of elevated temperatures. Small amounts of water decrease the sharpness of the end point markedly for titrations carried out in acetonitrile. Figure 10
___ 700 600 500 v, 400
5
9- 300 J
5 zoo
400t/
I
100
0 2.0
LO
30
46
0 I 2 3 4 5 6 7 8 9 1011 12131415
ML.
WL.
Figure 8. Titration of Mixtures in Acetonitrile Table I. Proce- Taken, dure Mg.
.I .I C C
B
B B C C
25.4 37.9 38.0 35.9 36.2 30.0 20.1 23,3 21.5 26.7 42.8 22.7 44.5 50.2 18.7 57.2 18.8 89.6
Di-n-butylamine Pyridine Di-n-butylamine Pyridine Aniline o-Chloroaniline Aniline o-Chloroaniline 8-Phenylethylamine Aniline ,9-Phenylethylamine Aniline 8-Phenylethylamine Aniline Pyridine Caffeine Pyridine Caffeine
Figure 9. Blank Determination
-
HClOa, HC104, Found, Difference, 311. s Ng. 3Ig. 1.95 4.75 2.94 4.56 3.91 2.39 2.15 1.80 1.78
0,1005 0.1005 0.1005 0.1005 0.0996 0.0996 0.0996 0.0996 0,0998
3.53 2.44 5.66 0.43 2.36 2.97 2.32 4.61
0.0998 0.0998 0.0998 0.0998
..
..
0.0998 0.0998 0.0998 0.0998
C
2 4 . 1 Aniline 6 5 . 2 Sulfathiazole
2 . 4 8 0.0998 2 . 5 5 0.0998
C
25.5 61.5
2 . 7 5 0.0998 2 . 3 8 0.0998
Aniline Sulfathiazole
X 100
1. 20 m l . CHCN titrated with HClO'in acetic acid 2. 20 m l . CHsCN titrated with HClOi i n dioxane
Titration of Amine RIixtures
Compounds
HCIO4
25.3 37.8 38.2 36.2
-0.1 -0.1 +0.2 +0.3
36.3 30.4 19.9 22.9 21.5
+O.l +o 4 -0.2 -0.4
42.7 22.7 44.3 50.5 18.7 57.7 18 4 89.5 23 1 65.1 25.6 60.8
-0 1 10.0 -0.2 +0.3 10.0 +0.5 -0 4 -0.1 -1.0 -0 1 t o .1 -0.7
..
50.0 ,..
5 5 0 t
450500-
L
\
\ 05
IO
I5
ML
20
2
ADDED
Figure 10. Effect of .kdding IIlethanol and Water to 11 111. of 0.1 .V Perchloric Acid in Acetonitrile
41 1
V O L U M E 25, NO. 3, M A R C H 1 9 5 3 shows the effect of adding mater and alcohol to a solution of perchloric acid in acetonitrile. The titration of an aliphatic amine in acetonitrile with perchloric acid (in dioxane) gives a tremendous break of 700 to 800 mv. a t the end point. This is considerably greater than the t m a k obtained either in acetic acid or in water. The titrations reported in this paper were all done on a semimicro basis. These procedures are convenient, require only small quantities of chemicals, and are reasonably accurate. Somewhat greater accuracy can be obtained by doing these titrations on a macro scale. LITERATURE CITED
(1) Blumrich, K., and Bandel, G , Angew. Chem., 54. 374 (1911). 12) . 22. 578. 1028 11950). _ _ ,Fritz., ,T.~ S . ,.. A N ~ LCHEM.. (3) Hall, N.F., J . Am. Chem: So;., 52, 5115 il930). (4) Kolthoff, I. h l . , and Laitinen, H., “pH and Electro Titrations,” 2nd ed., p. 106, Iiew Tork, John JTiley & Sons, 1948. ~
~
~
( 5 ) Luder, W. F., and Zuffanti, S., “Electronic Theory of Acids and
Bases,” p. 104, New York, John Wiley & Sons, 1946. (6) Markunas, P, C., and Riddick, J. -4., ANAL.CREM.,23, 337 (1951). (7) Moore, R. T., McCutchan, P., and Young, D. 4., Ibid., 23, 1639 (1951). (8) Pifer, C. W.,and Wollish, E. G., Abstracts of Papers, l2lst Meeting of AYERICAX CHEMICAL SOCIETY, Buffalo, N. Y., 1952. (9) Pifer, C. W., and Wollish, E. G., ASAL.CHEM.,24, 300 (1952). (10) Seaman, IT., and Allen, E., Ibid., 23, 592 (1951). (11) Siggia, S., Hanna, J. S.,and Kervenski, I. R., Ibid., 22, 1295 (1950). (12) Wagner, C. D., Brown, R. H., and Peters, E. D., J . Am. Chem. Soc., 69,2609 (1947). (13) Ibid., p. 2611. R E C E I ~ Efor D review J u n e 16, 1952. Accepted November 17, 1952. Contribution 209 from t h e Institute for Atomic Research a n d Department of Chemistry, Iowa S t a t e College, Ames, Iowa. Work was performed in the Ames Laboratory of the Atomic Energy Commission.
Periodate Method for Manganese and Effect of Band Width $1. D. COOPER, Research Laboratories Division, General Motors Corp., Detroit 2, Mich. Two periodate methods for determining manganese have been investigatedthe perchloric reference method and the nitrite reference method. The Beckman spectrophotometer was compared with other types of photometers. The precision of the method was found to be within 0.2 to 0.4% when the Beckman instrument w-as used. The nitrite reference method is somewhat faster than the perchloric reference method, but a 2 to 3% error may be involved in the analysis of tungsten steels.
T
HIS investigation %-asconducted to establish the reliability of the periodate method when the Beckman spectrophotometer is used and to determine the limitations that have been alleged to exist when other types of photometers are used. A reflection grating instrument v i t h bands of 10 and 20 mp and six typical filters of varied band width and with transmittance maxima ranging from 525 to 565 mfi were employed. The precision of the method, expressed as the standard deviation, was found to be 0.2 to 0.4% with the Beckman blodel DU spectrophotometer. The study included stainless steel, in which manganese was determined in the presence of chromium. It was confirmed that spectral quality limits exist for filters that may be used in the analysis of high-alloy steel; beyond these limits errors amounting to 4 to 8% may occur. On the other hand, there is a wide spectral region within which commonly available filters give satisfactory results. PERCHLORIC REFERENCE METHOD
An excellent treatise on the subject of periodate oxidation reactions has been published by Smith (6). Willard and Greathouse ( 7 ) developed a visual comparison method for manganwe in steel based upon periodate oxidation as early as 191i. Their review of the literature on methods for manganese determination covers the period beginning in 18.15. Haywood and Wood ( 1 ) included in their manual a persulfate method, based upon the preparation of a reference solution by reducing the permanganate ions by adding sodium nitrite to a portion of the oxidized solution. A similar approach is used by some analysts in their version of the periodate photometric method, the nitrite reference method, discussed a t the end of this paper.
In the periodate method for manganese, which is recommended when high accuracy is required, the solution of the alloy is prepared by fuming with perchloric acid to oxidize trivalent chromium ions to chromate ions, leaving only the manganous ions whose spectral characteristics can change with periodate treatment. Then by using identical aliquots for the preparation of the reference solution and the test solution, the extraneous absorbancy is theoretically compensated. As a further precaution, and to obviate wide slit settings, the permanganate ion maximum occurring a t 545.0 mfi is used rather than the one a t 526 mp. This is significant when samples with high chromium to manganese ratio, such as stainless steels, are analyzed, because the region of chromate ion absorbancy is essentially avoided, The foregoing applies when the Beckman Model DU spectrophotometer is used for absorbancy measurement. The method, when applied in the analysis of samples with high chromium to manganese ratio, has been alleged to give low results when “wide band” photometers are used especially when filter instruments are used. With wide band instruments it is impossible to avoid the spectral region of chromate ion absorbancy without a pronounced loss of sensitivity. The premise that chromate ion concentration in the reference solution equal to that of the test solution does not provide suitable compensation is immediately suggested. The equipment used initially in this investigation was the Beckman Model DU spectrophotometer, band width of less than 1 mp; Cenco Spectrophotelometer (I), nominal band widths of 5 , 10, and 20 mp; and a filter photometer comprising the Beckman as a photometer and a filter with a maximum transmittance a t 540 mp. This phase of the investigation indicated that the use of the filter yielded results which were approximately 1% low when the ratio of chromium to manganese was 56 to 1.
