Polarographic Determination of Dimethylamine F. L,ENGLISH Chambers Works, E. I . d u Pont de Nemours & Go.,Deepwater, N. J . A procedure is presented for the determination of dimethylamine in crude liquors containing, in addition to dimethylamine, ammonia, monomethylamine, and trimethylamine. The sample is treated with nitrous acid which destroys ammonia and most of the monomethylamine, convertsdimethylamine to the nitrosamine, and leaves trimethylamine substantially unaffected. After adjusting to a pH of 0.8, the nitrosamine is determined polarographically; the error is of the amount present. about *l%
A
POLAROGRAPHIC technique for the estimation of small amounts of dimethylamine has been reported by Smales and Wilson ( 5 ) lvhich depends upon conversion to the nitroso derivative. Their procedure involves a distillation which is undesirably long for a control method and does not completely eliminate interference by mono- and trimethylamine. The authors have found that the interferwce by monomethylamine is due to the by-product formation of pome nitromethane during the nitrous acid treatment, The procedure detailed herein eliminates the distillation as well as interference by the other aminrs in the concrntrations normally cncountrred in crude reactor samples.
point with the iodine solution to destroy the excess hydrosulfite. Add 2.5 ml. of concentrated hydrochloric acid and 1 ml. of gelatin solution, and dilute to 100 ml. in a volumetric flask. Electrol\-ee in the customary manner, using an initial potential of -0.3 volt and a sensitivity setting to give a step height of 100 to 200 mm. The curve is well defined, and, although the top plateau is inclined at a much greater angle to the horizontal than is the condenser current line, it is reproducible and accurately measurable. The final solution is stahle and may be retained even ovcrnight hefore polarographing. EXPERIMENT 4 L
APPARATUS
Preparation of Pure Materials. ilnhydrous mono-, di-, and trimethylamines of 97 to 99% purit were dissolved in water, acidified with a slight excess of hygochloric acid, evaporated until crystals began to form, cooled slowly to 5" C., filtered, washed with a mixture of 3 volumes of acetone to 1 of ethyl alcohol, and dried a t 100" C. Further purification of each mas then effected as follows: Monomethylamine. Crystallized three times from denatured alcohol (2B formula, 95% ethyl alcohol plus 0.5% benzene). Dimethylamine. Crystallized twice from a 1 to 1 by volume acetone-ethvl alcohol mixture. Trimethylamine. Crystallized once from ethyl alcohol and twice from 1 to 1acetone-alcohol. All were washed after each filtration with a mixture of 5 volumes of acetone to 1 of alcohol. They were finally dried at 100' C. and stored in paraffin-sealed bottles; they are all somewhat hygroscopic. These products gave no test for ammonia with Francois reagent and purities ranging from 99.9 to 100.27, by chlorine and total base determination, and, in the case of monomethyl, by the Van Slyke ( 4 ) method. They were used in this form in the experimental work, not as the free bases. Bmmonia. ;Inalytical reagent grade ammonium chloride was used without further purification.
A hlodel XX visible recording polarograph, manufactured by E. H. Sargent & Co., Chicago, Ill., was used. The polarographic cells were of the H-shaped type recommended by Lingane and Laitinen (a), one arm serving for the sample solution and the other for a saturated calomel cell anode. These cells were mounted in an air-agitated, vibration-insulated ( 1) water bath, thermostatically regulated to 25" =t 0.05" C. The capillary passed 1.366 mg. of mercury per second and the average drop time, over the -0.6- to -1.2-volt range and in the supporting electrolyte, of the analysis was 4.33 seconds, giving an r n * 4 1 / 6 constant of 1.57 mg.213 second-1'2. REAGEBTS
Sulfamic Acid (SH2HS01). Dissolve 15 grami in nntcr to 100 ml. Sodium Hydroxide. 30% by weight. Sodium Hyposulfite. Commonly known as hydrosulfite, Na&Od. High grade commercial material is satisfactory. Iodine. Approximately 0.2 S,prepared by dissolving 50 grams of potassium iodide and 26 grams of iodine in 50 ml. of water and diluting to 1 liter. This solution need not be standardized. Gelatin. 0.5 gram of Knox's KO.1 gelatin dissolved in 50 ml. of warm water. This should be freshly prepared daily.
Preliminary runs made in tetramethylammonium chloride showed that the reduction potentials of these amines are too close together for quantitative separation, which is in agreement with published data (S). Treatment of dimethylamine with carbon disulfide and sodium hydroxide t o form dimethyldithiocarbamate produced a solution that yielded two polarographic waves, one a t El/s = -0.77 volt and the other a t -1.35 volts. These were later shown t o be due, respectively, to some reaction product of carbon disulfide, possibly trithiocarbonate, and to carbon disulfide itself. A usable wave from dithiocarbamate was not found, even in the absence of excess carbon disulfide. Attention was then directed to the reaction with nitrous acid which destroys ammonia and monomethylamine, forms the nitroso derivative of dimethylamine, and has been stated not to react with trimethylamine. Introductory experiments on dimethylamine led to the following observations:
PROCEDURE
The sample should be in aqueous solution containing 15 to 25% of total bases. Transfer an aliquot, not exceeding 5 ml. and containing 0.05 to 0.13 gram of dimethylamine, to an 8-inch test tube, dilute to 5 ml., and set in an ice bath. Add 3.0 * 0.05 grams of sodium nitritt and 5 ml. of glacial acetic acid, and place in a water bath a t 25 * 2" C. for 10 minutes. Immerse the tube to within about 2 inches ( 5 em.) of the top in an ice bath and, while swirling vigorously, add dropwise from a pipet sufficient sulfamic acid solution (about 17 ml.) to destroy the exceSs nitrous acid as indicated by test with starch-potassium iodide paper. An excess of sulfamic acid will do no harm. Add a drop of phenolphthalein indicator solution, titrate to neutrality with the sodium hydroxide solution, and add 0.1 ml. excess. Establishment of the correct alkalinity a t this point is vital. Immerse the tube in a water bath a t 60" C. and after it has come to temperature add 1.0 * 0.05 gram of sodium hydrosulfite. Stopper the tube, invert it several times to dissolve the hydrosulfite completely, return it to the 60" C. bath for 5 minutes, cool to 25" C., and dilute to 100 ml. in a volumetric flask. Pipet 10 ml. into a beaker, add 50 ml. of water, 1 ml. of glacial acetic acid, and 1 ml. of starch indicator solution, and titrate to an exact end
1. Sitrosation in mineral acid is prohibitively slow, but is rapid in acetic acid. 2. Dimethylnitrosamine is not reduced polarographically in neutral or alkaline solution but produces a definite wave in acid. The plateau is not parallel to the condenser current line but in344
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V O L U M E 2 3 , NO. 2, F E B R U A R Y 1 9 5 1 Table I.
ilkaline R e d u c t i o n of Y i t r o n i e t h a n e
SaaSzOa,
5 S KaOH,
0 00 0 25
0 00 0 50
Gram
n
.F,O
0.76 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00
111.
n .in
0.50 0.50 0.20 0.7: 1.00 0.20 0.30 0.50 0.70
Temp.,
c.
23 2: 2 23 23 2 .j 2 .i 23
;.
40 40 40 40
-4pprox. PH 1'2 12 12 ~~
12 10 12 i 13 + 10 11 12 13
Step Height, Mm. 88 27 15 14 3
:
32 73 1 1 4 47
clincd to it a t angles of 50" t o 40", depending upon the height of' thc wave and othcr factors. Change in acid concentration or substitution of methanol or dimethylformamide for some of the water as solvent affect the plateau angle but little. The inclined plateau is, however, reproducible and accurately measurable. :
