1045
V O L U M E 28, NO. 6, J U N E 1 9 5 6 Table \-, Determination of JIixed Halides by Single Titrationa with Silver RTitrateb .Meq of Each Halide Taken in Mixture 0.1000 0.1000 0,2000 0.2000 0.2000
0.2000
0.4000 0 4000 0.4000 I
Chloride Found, hleq. 0.1000
0.0999 0.2001 0.2001 0.2000
Bromide Found, hleq. 0.1001 0.1000 0.2000
e . 2000
0.1999
0.1998 0.2001
0,4002
0.4003
0.3999 0.3998
0.4000 0,4000
0.3999 0,4000 0.4001 a Titration carried o u t in 100 ml. of 0.0.W acetate buffer b 0.0225.21 solution.
Iodide Found, hleq 0.1000 0.1000 0.2000 0 1998 0.2002
0.2000
0.4000 0.4000
0.3998 0.3999
constant among titrations and to avoid errors due to the effect of changing ionic strengths on the endpoint potentials. I t vas found that additional detergent (10 to 12 drops) facilitated the approach of the cell to a stable potential. The procedure then consists of simply setting the potentiometer first on the silver iodide equivalence point and adding silver nitrate until the galvanometer reaches zero deflection. This gives the iodide titer. The titers for bromide and chloride are subsequently determined in sequence by titrating the cell to their respective end point potentials. Sample results, such as those shown in Table V, indicate the error in these determinations t o be about 0.10/,, which is a marked improvement over previously used methods. Nature of Precipitate. The main objections to the direct potentiometric methods previously reported in the literature were overcome by the addition of a few drops of liquid detergent. At the end of the titration, there is no visible precipitate in the beaker, but rather a slightly turbid emulsion Apparently the
detergent coats each microscopic particle of the silver halide almost instantaneously upon formation and prevents any coagulation of the precipitate. This maintains a very large surface area of silver halide and the equilibrium between precipitate and solution is achieved rapidly. Indirect evidence that supports this explanation of the function of the detergent \vas obtained from experiments on the Volhard determination of chloride. I n this classical method the silver chloride is effectively removed from (iontact Ivith the solution, by addition of an occluding agent such as nitrobenzene, in order that a thiocyanate ion excess can readily build up in the solution to form the red ferric comples a t the equivalence point. Hexever, the authors have observed that the addition of liquid detergent to the titration medium during a \-olhard chloride determination using nitrobenzene caused erroneous results and that t'he red endpoint color faded very rapidly. Or, in short, the detergent served as an anticoagulant promoting contact between the precipitate and the solution, giving false results. ACKNOWLEDGRIENT
This work \vas supported by the Petroleum Research Fund of the ; ~ I I E R I C A X CHEMICAL SOCIETY.The authors also n-ish to thank Ward B. Schaap for several valuable suggestions. LITERATCRE CITED
(1) Blaedel, W.J., LeiTis, IT. B., Thomas, J. W., .IXAL. CHEY.24, 509 119521. (2) Clark, IT.. J . Chem. SOC.1926, 749. (3) Masten, AI. L., Stone, I(.G., SAL. CHEX 26, 1076-7 (1954). (4) Salomon, E., 2. Elektrochem. 4 , 71-3 (1897). (5) Wade, P., A n a l y s t 76, GOG-9 (1961).
RECEIVED for revien October 28, 1955. Accepted Fehruafy 2, l95R
Chromatographic Separation of Some Aromatic Nitrogen Compounds W. R. EDWARDS, JR., 0.S. PASCUAL, and CILTON W. TATE Louisiana State University, Baton Rouge, La.
A chromatographic and spectrophotometric procedure for the separation, identification, and estimation of five pairs of analogous nitro and nitroso compounds has been described previously. The present paper presents additional information which facilitates the development of similar methods for the analysis of solutions containing a variety of other organic nitrogen compounds, either singly or in mixtures. It includes R values of over 30 compounds, some determined with more than one combination of adsorbent and developer, in order to permit a favorable choice of conditions.
T
HE readiness Tvith which many aromatic nitrogen compounds undergo changes through oxidation, reduction, thermal decomposition, and mutual interaction frequently makes identification of the products desirable. This same reactive versatility may make it difficult to identify small amounts of some materials in the presence of others by ordinary chemical methods. I n such circumstances, chromatography can be of great help. The chromatographic and spectrophotometric characteristics of a number of nitro and nitroso compounds have been described, as well as a procedure for their separation from typical mixtures and for their subsequent identification and estimation (f). To estend these methods to a variety of other aromatic nitrogen compounds, it was necessary to determine the chromatographic characteristics of these materials, and in some instances
theii spectrophotometric qualities. The present paper offers some of this information Aided by these data, and subject to the limitations of such methods, it should he possible t o achieve the separation from solutions, and a t least the partial identification, of the listed compounds. Such procedures are most efficient nhen: ( a ) the number of solutes present is not large; ( h ) they do not diffei nidely in concentration; ( e ) concentrations are not high: ( d ) R values (Z), proportional to their rates of flox through chromatographic columns, differ substantially; and ( e ) R values do not approach zero or unity too closely. To facilitate development of procedures which will satisfy criteria d and e, data obtained on different adsorbents with different developers have been included, Because R values indicate the relative positions and the extent of separation of the zones of different solutes, when chromatographed under conditions similar to those under which the values were measured, they may suggest favorable conditions for the isolation of such solutes from a variety of mixtures, and may also foreshadow the degree of success t o be expected in such processes. They cannot be trusted to predict the precise point a t which any zone will appear, because the standardization of adsorbents is only approximate. Also, because each solute modifies the qualities of its solvent, it ~villalter the chromatographic behavior of other solutes present. JThen the solutes resemble each other in structure or polarity, but differ greatly from the solvent, this effect may be substantial. It was observed that a small amount of nitrobenzene increased materially
ANALYTICAL CHEMISTRY
1046 Table I.
Chromatographic and Spectrophotometric Characteristics R Values" B
Color of Zone on Silicic ilcid Before After streaking streaking o-Nitroaniline 0.340 0.659 0.891 Yellow Yellow Yelluw p-Nitrophenol b Colorless 0,333 113 YellowC Yellow 0:k37 0.358 p-Hydroxyazobenzene I52 Dk. .vellowC Yellow 0 , 1 2 0
