Colorimetric Determination of Primary Amine in Fatty Amine Acetates and Fatty Amines ALBERT J. MILUN General Mills, Inc., Minneapolis 73, Minn.
b
Primary amine in fatty primary amine acetates and in fatty primary amines of high molecular weight can be determined by a method based upon the reaction of the primary amine with salicylaldehyde. A bright yellow Schiff base is formed which has an absorption maximum a t 4 10 mp. Interference due to secondary and tertiary amines is avoided b y the presence of acetic acid. Unsubstituted amides, N-alkylacetamide, and nitrile also do not interfere. Data were obtained on known mixtures and a reproducibility check gave =k0,6% relative as the 99% confidence interval of the mean. The method has been applied successfully to the analysis of technical grade primary fatty amine acetates and fatty primary amines.
F
primary amine acetates of high molecular weight are produced commercially by neutralizing fatty amines with acetic acid. These primary amines contain between 87 and 98y0 primary amine, varying amounts of secondary amine, traces of tertiary amine, and small amounts of impurities such as nitriles, amides, acids, and socalled "unsaponifiables" carried over from the fatty acids used as raw materials. The amount of secondary amine is in the range of 0 to lo%, depending mainly upon whether the material has been distilled. These same impurities are present also in the corresponding acetates along with others such as N-alkylacetamide which might be formed during neutralization. For product quality control it is desirable, therefore, to have a method for determining the concentration of the major constituent, primary amine. Experience has shown that the procedure for determining primary amine as originally proposed by Wagner, Brown, and Peters (3) and adapted to high molecular weight fatty amines by Jackson ( 1 ) has certain drawbacks when applied to amine acetates. The latter method consists of two separate titrations with acid. One titration determines the total basicity and the other determines the secondary and tertiary amine after the primary amine is reacted with an excess of salicylalATTY
1502
ANALYTICAL CHEMISTRY
dehyde; the primary amine is then determined by difference. I n amine acetates the reaction between primary amine and salicyaldehyde does not go to completion because of the presence of the acetic acid. Thus, the secondary plus tertiary amine titrations are erroneously high. Also, with undistilled technical grade amines undesirably slow end points are experienced in this same titration of the secondary plus tertiary amine. When Jackson's procedure was used on amines, the Schiff base resulting from the reaction of the primary amine and salicyaldehyde had a bright yellow color. However, salicylaldehyde gave a yellow color with secondary and tertiary amines also, apparently due to the formation of the salicylaldehyde anion in the presence of the basic secondary and tertiary amines (2). Further investigation showed that secondary and tertiary amines give no color with salicylaldehyde in the presence of acetic acid, which, being more acidic than salicylaldehyde, prevents the formation of the colored anion. Based upon these facts, an analytical procedure was set up in which the intensity of the color due to the Schiff base fesulting from the reaction of primary amine with salicylaldehyde is measured a t 410 mp t o determine primary amine in mixtures of primary, secondary, and tertiary amine acetates. The method is also applicable to mixtures of the free amines.
APPARATUS
Spectrophotometer, Beckman Model
DU, with cell compartment thermostated a t 30" C. Slit widths of 0.6 mm. were used for measurements. Cells, 1 em. Constant temperature bath, 30" C. Weighing bottles, high form with stopper, Kimble No. 15145, height 80 mm., diameter 40 mm. REAGENTS
Salicylaldehyde solution.
salicylaldehyde is diluted to the mark with chloroform in a 100-ml. volumetric flask. Fresh solutions were prepared every fourth day. Acetic acid solution. Exactly 4 ml. of glacial acetic acid is diluted with chloroform to 200 ml. in a volumetric flask. Chloroform, analytical reagent grade. PROCEDURE
Weigh a sample containing up to 1.6 mmoles of primary amine acetate (or primary amine) into a weighing bottle. For samples containing over 85%. primary amine acetate (or primary amine), 1.3 to 1.6 mmoles are recommended for optimum results. Add 17 ml. of chloroform (14 ml. for amines) from a graduate to dissolve the sample. Pipet 3 ml. (6 ml. for amines) of the acetic acid solution, washing down the sides of the bottle during the addition, and swirl gently to mix the solution. Add 5 ml.
WAVE LENGTH (mp)
Figure 1, 1. 2.
Exactly
5 ml. of Matheson, Coleman, and Bell
Absorption curves
Schiff base from octadecylamine and salicylaldehyde, 0.424 mg. per ml. Salicylaldehyde, 5 mg. per ml.
of the salicylaldehyde solution from a pipet in the same manner as the acetic acid solution and swirl gently to mix the solution. Place the bottle in a 30" C. constant temperature bath. One hour and 20 minutes after adding the salicylaldehyde, quantitatively transfer the bottle contents to a 500ml. volumetric flask, rinsing the bottle thoroughly with chloroform. Add chloroform to the volumetric flask only to within approximately 1.5 inches of the mark to allow for subsequent thermal expansion of the solution. Place the flask in a 30" C. constant temperature bath for 10 minutes, make up to volume with chloroform, and mix. Rinse and then fill a spectrophotometer cell with the solution (this can be accomplished conveniently with a 10-ml. volumetric pipet). Place the cell in the spectrophotometer and allow it to sit in the spectrophotometer compartment for 5 minutes. One hour and 40 minutes after addition of the salicylaldehyde, measure the absorbance against chloroform a t 410 mg. The timing should be within several minutes of that prescribed. With samples containing appreciable color use a comparable concentration of sample in chloroform as a blank. Calculate per cent primary amine acetate (or primary amine) according to the following equations: a=-
A
ex1
where A = absorbance c = concentration in grams per liter I = length of cell in em. a X 100 X molecular weight 70
=
e
Calibration. Determine the absorbance of a sample of pure primary amine acetate or primary amine as described above. Calculate the molar absorptivity ( E ) as follows : e =
A
x molecular weight C X L
DISCUSSION
Salicylaldehyde reacts with primary
alkyl amines according to Equation 3 to form a Schiff base. H
/"=" H C=SR /
The absorption spectrum in chloroform from 360 to 440 mp for the Schiff base resulting from the reaction of octadecylamine and salicylaldehyde is shown in curve 1, Figure 1. The broad band peaking a t 410 mp is suitable for quantitative measurements. Curve 2 in Figure 1 is the absorption spectruni of salicylaldehyde in chloroform. Acids tend to reverse the reaction in Equation 3, the tendency increasing with increasing strength of the acid. Thus, formation of the Schiff base is slower with amine acetates as compared with the free amines The rate of reaction is increased by a n excess of sslicylaldehyde and also by heating. Figure 2 shows the effect of these variables on the rate of color development measured a t 410 mp for a sample of primary tallow amine carried through the determination. At 30" C. the reaction is rather slow, the color intensity leveling off a t approximately 1 hour and 40 minutes after addition of salicylaldehyde. Heating speeds up the reaction appreciably; however, the reproducibility of the color intensity is not so good as that obtained for longer reaction time a t 30" C. Doubling the concentration of salicyaldehyde gave higher absorbances. However, 5 ml. of the salicylaldehyde solution was chosen for the determination because it gave a negligible blank and satisfactory absorbance values.
TIME (HRS.) AFTER ADDITION OF SALICYLALDEHYDE
Figure 2.
Rate of color development with primary tallow amine
30' C. X Heated an steam bath for 5 minutes after adding salicylaldehyde A 30' C., 10 ml. of salicylaldehyde
An excess of acetic acid is ntldcd to the amine acetate or amine t o inhibit any yellow color nhich might be caused by the formation of the salicglaldehyde anion with secondary or tertiary amines. Variations betn een 10 and 85% excess of acetic acid had no effect on the color intensity. Chloroform was chosen for this determination because of the ready solubility of the high molecular weight fatty amine acetates and fatty amines in this Eiolvent. I n order to prevent errors due to the relatively high coefficient of thermal expansion of chloroform, the dilution is carried out in a bath thermostated a t 30" C. To obtain best precision from day to day, the cell compartment of the spectrophotometer was thermostated also and the filled cell was allowed to stand in the compartment for 5 minutes to come to equilibrium temperature in order to aroid errors caused by the changes in the volume of the solution with changes in temperature. A straight line was obtained when absorbance was plotted against concentration of amine on samples of octadecylamine (0 to 0.34 gram) carried through the described procedure. Table I gives the molar absorptivities of samples with varying molecular weight. The agreement indicates that a standard molar absorptivity can be used for primary amine acetates and primary amines of different molecular weight.
Table I.
Molar Absorptivities
Molecular Weighta
Molar hbsorp-
Sample tivity Octylamine 134 200 Decvlamine 160 202 Dodkcylamine 786 202 Te tradecylamine 212 201 Hexadecvlamine 249 200 Distil1ed"hydrogenated tallow amine 269.5 20 1 Distilled tallow amine 268 20 1 Octadecylamine 270 200 Octadecylamine acetate 329 203 a Determined by potentiometric titrations with perchloric acid in glacial acetic acid.
Table I1 gives the results obtained on known mixtures and technical grade samples. Amides and nitriles do not interfere in this determination. The 99% confidence interval for the mean of 12 determinations carried out over a period of 6 days was =t0.60/, relative. The formation of the yellow Schiff base on reacting primary amines with VOL. 29, N O . 10, OCTOBER 1 9 5 7
1503
Table II. LT0,a
1 2 3 4 5 6 7 8 9 10 I1 12
Analysis of Mixtures
Composition of Mixture, % Primary Secondary Tertiary 0 0 89.9 60.1 28 .?I
sa
a
88 5 95 8 98 0” 98 12
100 0 10.1 39.9 71.5 2.8 5.0 0.6
Determined % Primary
0 100 0 0 0 0 0 0
0.3 0.1 90.0 60.3
28.4 83 .0 88.6 93 9 97 3 97 8 85 5 11.5
Ob
Samples 1 through 5 are synthetic mixtures of octadecylamine, dioctadecylamine, and trioctadecylamine. Samples 6 through 8 are technical grade undistilled and distilled tallow amines previously analyzed by titration according to Jackson ( 1 ) . Samples 9 and 10 are technical distilled tallow primary amine acetates made from amines essentially free from secondary amine. Sample 11 is a technical undistilled tallow primary amine acetate. Sample 12 is a crude .V-octadecylacetamide. * Per cent primary amine acetate determined by titration in glacial acetic acid with perchloric acid. a
salicylaldehyde is also a convenient method for quickly checking whether a n alkyl amine is primary in contrast to secondary or tertiary. The test involves the addition of salicylaldehyde to a solution of amine in chloroform, followed by acetic acid. A persisting bright yellow color indicates a primary amine. LITERATURE CITED
Jackson. J. E..
. ~ A L . CHEW 2 5 .
1764 (1953). ’
Morton, R. h.,Stubbs, A . L., J . Chem. Soc. 1940, 1347.
Wagner, C. D., Brown, R. H., Peters, E. D., J . A m . Chem. SOC.69. 2611
RECEIVED for review July 13, 1956. Ac-
cepted May 23, 1957. Paper S o . 201, Journal Series, Research Laboratories, General Mills, Inc.
Direct Co Io rimetric Determination of Nitrocellulose in Lacquers M.
H. S W A N N
Coating and Chemical laboratory, Aberdeen Proving Ground, Md.
The yellow color formed by reaction
of nitrocellulose with alkali in the presence of acetone provides a simple and rapid method for the direct analysis of lacquers.
T
yellow to red color that develops when certain dinitro and trinitro compounds react with alkali in the presence of acetone (1) was investigated for application to nitrocellulose analysis. A quantitative colorimetric method was developed that is directly applicable to the measurement of nitrocellulose in lacquer vehicles without interference from solvents, plasticizers, or coating resins. with the exception of some rosin products and phenol condensates. The method can be used to determine nitrocellulose in lacquers n.ith less effort and higher reproducibility than either the volumetric ( 2 ) or gravimetric ( 3 ) method. No special precautions are necessary and the solvent uqed also serves as reagent. HE
ANALYTICAL PROCEDURE
Approximately 2 ml. of clear lacquer vehicle are weighed into a 25-ml. glassstoppered graduated cylinder, dissolved in acetone (ACS reagent grade), and diluted to volume. An aliquot con1504
ANALYTICAL CHEMISTRY
taining 40 mg. or less of nitrocellulose is transferred to a ground-joint Erlenmeyer flask of 125-ml. capacity and diluted to 10 ml. with acetone. Ten milliliters of 10% aqueous potassium hydroxide are added along with a Berl saddle and the sample is refluxed for 1 hour in a water bath under a aatercooled condenser, The sample is then promptly cooled to room temperature. transferred to a 50-ml. volumetric flask, rinsed, and finally diluted to volume with a mixture of acetone and water (2 parts of acetone to 1 part of water by volume). The absorbance of the yellow solution is measured a t 425 mp and the nitrocellulose content of the aliquot is determined from a working curve prepared in like manner from pure nitrocellulose. DISCUSSION
All varieties of lacquer-grade nitrocellulose tested developed identical color intensity. These included l/,second. 1/2-second, 5/6-~ec~nd, and 69-seeond viscosities. S o coating materials could be found that developed color under the test conditions, with the exception of certain rosin products and phenol-aldehyde condensates. Interference from these resins is slight, but significant and qualitative tests to verify
their absence are advisable before application of the colorimetric method. Free aldehydes offer slight interference, but this can be prevented by oven-drying of the sample aliquot a t 105’ C. prior to color development, and resins containing free aldehydes are not usually found in nitrocellulose lacquers. If clear, colorless vehicles cannot be separated from pigmented lacquers by high-speed centrifuging, it will be necessary to prepare a blank from a duplicate aliquot, omitting the use of alkali and diluting to 50 ml. with acetone only. The color develops rapidly a t first and analysis can be made with slight loss of accuracy after only 10 minutes of reflux. An additional hour of reflux beyond the recommended time gives but slight additional color and is not necessary, but it is advisable to time the 1-hour reflux period accurately. A straight-line graph is obtained from absorbance readings if samples do not exceed 40 mg. of nitrocellulose. Cloudiness. which may form if unsaponifiable resins are present, can be removed by filtration of a small volume of solution Albefore absorbance is measured. though the color is stable, all glassware should be emptied and rinsed without eucessire delay to arroid alkali attack.
