Determination of Ammonium Salts in Acid Mixtures Containing Guanidine Salts R. W. WILLIAMS, HARRY STALCUP, and MAE I. FAUTH
U. S. Naval Powder Factory, Indian Heod, Md.
b A direct method has been developed for the determination of ammonium salts in complex acid mixtures which may also contain other nitrogen compounds such as guanidine salts, nitric acid, and nitroguanidine. The reaction of ammonia with formaldehyde which is utilized, involves the formation of one equivalent of acid for each equivalent of ammonium salt present. Guanidine salts, nitroguanidine, and alkali sulfates and nitrates do not interfere.
T
investigation Tyas concerned primarily with finding a direct method for measuring the ammonium salts present in complex acid mixtures which contain other types of nitrogen compounds such as guanidine salts and nitroguanidine. A typical sample of this nature has the following composition. HIS
Component Ammonium hydrogen sulfate Water Sulfuric acid Guanidinium hydrogen sulfate Nitroguanidine Nitric acid
% 24.02 15.67 42.82 16.94 0 05 0.20
The reaction of ammonia and formaldehyde in aqueous solutions has been studied by many investigators. Baur and Reutschi ( I ) showed that practically complete reaction takes place in aqueous solution and that the over-all reaction could be represented by the equation,
Hexamethylenetetramine Boyd and Winkler (2) conducted further kinetic studies of the reaction and concluded that the reaction was much more complex than indicated b y Reaction 1 and that products other than hexamethylenetetramine were formed. Kolthoff (6) found that excess alkali or formaldehyde gave rise to a titration error which increased with contact time. The proposed method does not involve either of these sources of error, as the titration is carried out immediately after addition of the formaldehyde and no excess of alkali is involved.
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ANALYTICAL CHEMISTRY
Other investigators (3, 1I ) have shown that the reaction between formaldehyde and ammonium salts in both aqueous and acidic medium yields hexamethylenetetramine. The reaction conditions apparently were such that the formaldehyde functioned as a condensing agent which Polley, Winkler, and Nicholls (9) contend would favor hexamethylenetetramine formation as shown in Equation 1. Knudsen (6) and Plochl (8) have indicated that formaldehyde may act as a reducing agent, in which case they suggest that the salts of methylamine are formed: KH4C1-
HCHO
CHaNHz.HCl+ (CHa)zNH.HCl -L (CHB)~N.HCI n-HCOOH (2)
+
The use of the reaction with formaldehyde in the determination of ammonia is mentioned by Grissom (4), Marcali and Rieman (7), and Rusconi (10). Because of the presence of sulfuric acid, nitric acid, gauanidine salts, and nitroguanidine, the Kjeldahl, Nessler, and titrimetric methods were not suitable for the acid mixtures encountered in this laboratory. The formaldehyde method has been modified to overcome the interference due to guanidine salts and nitroguanidine. PROCEDURE
A sample of approximately 5 grams, which contains 15 to 25% of ammonium salts, is weighed into a 250-ml. glass beaker specially designed for potentiometric titrations. A four-hole rubber stopper containing the two electrodes, glass and calomel, is inserted into the top of the beaker. A small funnel is placed in one of the holes (0.25-inch diameter) and approximately 100 ml. of water is added to the beaker contents. Mixing is accomplished by
Table I.
means of a magnetic stirrer and a Teflon-enclosed stirring bar placed inside the beaker. The solution in the beaker is neutralized to p H 5 with approximately 5.Y sodium hydroxide poured through the funnel, and finally to exactly p H 6.0 with 0.5N sodium hydroxide. This final titration I S accomplished by inserting the tip of the buret containing the 0.51V sodium hydroxide through the fourth hole (0.25-inch diameter) in the rubber stopper. The funnel is washed with distilled mater and replaced in the rubber stopper. Fifteen milliliters of fornialdehyde (35 to 40%) solution prei-iously neutralized to p H 8.4 are then slowly added through the funnel while the standard 0.5.V alkali is being simultaneously added in such a manner that the pH of the solution in the beaker does not drop below pH 6. The solution IS finally neutralized to p H 8.4. hll. KaOH X N of NaOH X F wt. of sample % compound F for ammonium ion = 1 804 F for ammonium sulfate = 6 607 F for ammonium hydrogen ~ulfate =11 511 DISCUSSION
When the sample was neutralized with alkali before addition of the formaldehyde, it appeared that conditions were established which favored the reaction shown in Equation 2. It also appeared possible that ammonium salts may react with formaldehyde x i t h the liberation of free acid, in addition to forming hexamethplenetetramine.
+
2(X?&)zSOc 6 HCHO + (CH2)aN4 2HzS04
+
+ 6HD
(3)
Regardless of which reaction route was taken, one equivalent of acid \vas
Determination of Ammonium Salts in Acid by the Formaldehyde Method
yo Ammonium Salts Found in Sample (as NH4HS04) 23.74 23.90 23,88
Recovery of (NH4)zS04Added to Sample Grams found recovery Grams added 100.1 0.7012 0 7020 0.7050 99.8 0 7034 99.7 0.7019 0 7001
Table II. Recovery of Ammonium Hydrogen Sulfate in Synthetic Acid Mixture
KH,HSO,, Grams
Added 1 0010 1.0010 1.0010
XHaHSOd, Gram Found 0.9961 0.9982 0 yotiti
XHaHSOd,
% Recovered 99 6 99.8 99 tj
formed ior each equivalent of aminonium salt present. -15 the proposed method depends on measui ement of the liberated acid, it was necessary first to neutralize the free acids already present in the sample. Seutralization of the sample to a slightly acid pH was also necessary to prevent possible loss of ammonia resulting from the presence of the alkali. The equivalence point for the acid niivture occurred a t pH 6.0, vdiereas after addition of formaldehyde it was a t p H S.4. When the acid was neutralized to a p H of 6.0 before addition
of the formaldehyde (pH 6.0) and then titrated to pH 8.4 after the formaldehyde addition, calculated recovery values were obtained for the ammonium salts as shoivn in Tables I and 11. Only a slight excess of formaldehyde vias necessary to drive the reaction to completion. As the reaction mas almost instantaneous, the approximate amount of formaldehyde necessary for samples containing varying amounts of ammonium salts could be found by adding small increments of the formaldehyde solution and then titrating the liberated acids. This procedure was continued until no more acid was liberated including completion of the reaction. Although no kinetic studies were undertaken and no final proof was developed for the course of the reaction, the following equations for the reactions involved with ammonium sulfate and ammonium acid sulfate (neutralized with alkali) appear feasible: (IjH4)zSOi f 4HCHO + (CHaSH2)2.HzSOi 2HCOOH (4)
+
+
2NH4SaSO4 4HCHO -+ ( C H I S H ~ ) ~ . H ~ ~Ka2S04 O,
+
+
SHCOOH (5)
The method is simple, convenient, and suitable for use in the control laboratory. LITERATURE CITED
(1) Baur, E., Reutschi, W.,Helv. C h i m Acta 24, 754 11941). ( 2 ) Boyd, 11.L., Winkler, C. A., Can. J . Research 2 5 , 387 (1947). (3) Dering, H. O., Kelly, bl. D. (to
Superfine Chemicals, Ltd.), Brit. Patent 396,467 (-4ug. 10: 1933). (4) Grissom, J. T., J . Ind. Eng. Chem. 12, l i 2 (1920). (5) Kolthoff, I. M., Pharna. Weekblad 58, 1463 (1921). 16) Knudsen, P., Ber. 47, 2694 (1914). (7) Marcali, K., Riemsn, W.,ANAL. CHEM.18, 709 11946). ( 8 ) Plochl, J., Ber. 21, 2117 (1888). (9) Polley, J. R., JTinkler, C. 4.,Kcholls, R. V. V., Can. J . Research 25B, 525 (1947). (10) Rusconi, A., Chimica(Milan) 5, 107 (1950). (11) Schieferwerke Ausdauer A-G, Brit. Patent 286,730 (March 10, 192i). for review December 15, 1956. RECEIVED Accepted April 11, 1957.
Theory of Gradient Elution through Ion Exchangers HELMUT SCHWAB’, WILLIAM RIEMAN 111, and PHILIP A. VAUGHAN Ralph
G. Wright
Chemical laboratory, Rufgers University, New Brunswick, N.
b Equations were developed to predict the position of the peak in the graph of a gradient elution. Elutions of chloride, bromide, and oxalate ions were performed with sodium nitrate as eluent, first with nongradient technique to evaluate the necessary parameters, then with gradient technique to test the equations. Agreement within the experimental error of locating the peak was obtained with both linear and exponential variation of the eluent concentration. The principles should be applicable to elutions with varying pH or varying concentration of a complexing agent.
A
gradient elution has been the subject of numerous recent investigations, both practical (1, 4, 7 , 9, 10) and theoretical (5, 6), no satisfactory equations have been available for calculating where the position of the peak will appear. This paper presents the derivation of such equations and experimental data to attest their validity. LTHOUGH
Present address, National Cash Register Co., Dayton, Ohio.
J.
The equations n ere derived by means of the plate theory of exchange chromatography which has been used successfully to calculate the minimum column height required for a desired separation (Z), to calculate the positions of the peaks in elutions with stepwise changes in eluent concentration ( 8 ) , to elucidate the chromatographic behavior of condensed phosphates ( 2 ) and glycols ( I d ) , and to determine the best conditions for the separation of condensed phosphates (8, 11) and glycols (IS). Gradient Devices and Equations. T h e device with a constant-volume mixing chamber (3, 5 ) , sketched in Figure 1, delivers eluent whose concentration follows the equation [El] = M2- ( M 2-
Jfl)e-d/r7R
(1) where [El] denotes the concentration of eluent when C$ ml. have flowed from the mixing chamber, MI the initial concentration of eluent in the mixing chamber, TIR the volume of the mixing chamber, and M z the concentration of eluent in the reservoir. If MI is zero, as in all the work reported in this paper, Equation 1 becomes
[El] = J f 2 ( 1 -
e-$/VR)
(2)
Gradients that follow Equation 2 are hereinafter called exponential gradients. The simple device of Bock and Ling (3) with vessels of equal cross sections, Figure 2,B, is designed to deliver eluent according to the equation (‘If, - Jfl) (1 -
or
if M i is zero. LTnfortunately,this device does not deliver exactly linear gradients, unless the liquids in the two chambers have about the same density. Because this requirement was not fulfilled in the work described here, the device of Bock and Ling was not used. The device sketched in Figure 2 4 , delivers eluent whose concentration follo~vs( 1 ) the equation d
[El] = .Ila - (Ma - AI2) ---e-4/”x VR (;$fa- 11fl)e-4/vR VOL. 2 9 , NO. 9, SEPTEMBER 1957
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