Titration of Traces of Ammonia after Kjeldahl Distillation SIR: I n a recent publication (4) we described the determination of traces of nitrogen in petroleum b y a method employing a Kjeldahl distillation and a colorimetric finish. Although the latter w s sensitive enough, it required critical attention to details. Also, it !vas often necessary first to evaporate and aliquot the distillate, so that the level of the ammonia would be within the limits to n hich the colorimetric method applied. We have simplified the determination considerably b y finishing i t acidimetrically. I n general, the procedure is as follo~vs: The ammonia is distilled as described. To prevent traces of alkaline spray from being carried over during the distillation, a connecting bulb with two inner tubes curving in opposite directions is used. The receiver contains 25 ml. of 0.15% boric acid solution. To avoid loss of ammonia by sudden pressure surges, the delivery tube is fitted t o the receiver by means of a slotted stopper. After 100 ml. has been distilled, the ammonia is titrated directly with 0.0lA’ acid in the presence of methyl redalphazurin mixed indicator (1). I n the low concentration of boric acid, the indicator gives a very sharp color change with 1 drop of titrant. We have found sulfamic acid to be an ideal titrant. It can be prepared and used as a primary standard, costing less than 1 cent a liter for 0.01N solution, and any hydrolysis on standing (NH2-
I
6 PH 5
4
5
0 ML..
10
OF T I T R A N T
Figure 1. Titration of ammonia with sulfamic acid
+
S0,H H20 = XHdHS04) does not affect its utility as a titrant for ammonia. Sulfuric acid and potassium biiodate may also be used, but the former requires standardization and the latter is considerably more expensive. Figure 1 shows a recorded automatic potentiometric titration of the ammonia -boric acid system with sulfamic acid; it indicates how closely the indicator change (simultaneously observed by the analyst) corresponds to the actual point of inflection. The suitability of the described titrimetric finish was further demonstrated by the results of treating duplicate aliquots from processed samples. These showed a n average difference betn-een the colorimetric and volumetric
methods of less than 1 y of nitrogen over the range of 50 to 250 y. To establish some indication of the maximum amount of ammonia that could be recovered by the described method, n e distilled five portions of standard ammonium chloride solution (each containing the equivalent of 24.995 mg. of nitrogen) and titrated the distillate. Recoveries ranged from 99.i6% to 100.04%, with a n average of 99.9i70. These results suggest that the quantity of boric acid used by some investigators (2, 8) is unnecessarily high. Indeed, a concentration of boric acid much higher than Fve specify is undesirable in the titration of traces of ammonia, because the end point becomes less distinct. LITERATURE CITED
(1) Fleischer, H., IND. ENG. CHEM., ANAL. ED. 15, 742 (1943): 16, 273
(1944). (2) Hillebrand, W.F., Lundell, G. E. F., Hoffman, J. I., Brigftt, H. A,, “-4pplied Inorganic Analysis, p. 784, TT’iley, NPWYork. 1953.
(3) Machimk
