Rapid Argentimetric Determination of Halides by Direct Potentiometric Titration V. J. SHINER, JR., and M O R R I S L. S M I T H hd.
Department of Chemistry, lndiana University, Bloomington,
:Z direct potentiometric titration method allows quantitative determination of chloride, bromide, and iodide in a solution of acetate buffer containing a few drops of liquid nonionic detergent. For concentration levels as low as O.O0201M, the error is less than 0 . 1 7 ~ while ~ at the limiting concentrations of and 10-6M, respectively, for chloride, bromide, and iodide, the error is less than 1%. The method is extremely rapid and is applicable to the direct determination of mixed halides.
Preparation of Silver Electrode. The method of Clark ( 2 ) was used to prepare a stable reproducible electrode. This involved soaking the electrode in a 1 to 1 solution of concentrated nitric acid and water, to which a little sodium nitrite had been added, until the surface % a s bright. It was then rinsed liberally with distilled water and polished with a fine grade of emery cloth. For continued stability the electrode was polished from time t o time or whenever the surface became darkened. The equivalence point potential \vas observed to vary over a range of 2 t o 3 mv. for different electrodes, but was essentially constant for a given electrode. T n o of these prepared in this laboratory have given the same equivalence point potential over a period of 10 months.
URIXG the course of investigations in this laboratory on the reaction rates of some organic halogen compounds, the need for a rapid, simple, accurate procedure for the determination of halide ions arose. S o n e of the numerous halide determination procedures which have been described in the literature seemed to meet the desired requirements. The main limitations of argentimetric methods such as those of Jlohr, Volhard, and 1,iebig are their insensitivity and/or the excessive manipulation and care involved in preparation of samples which make them impractical for rapid analysis. Several potentiometric methods, hoth direct and differential, seemed to offer the most promise. Salomon ( 4 )was the first t o publish a procedure involving direct potentiometric titration with silver nitrate, but this was primarily 3 demonstration and the det'ails are not clear. Also no mention \vas made of the application of the procedure to the analysis of miscd halides. The methods described by Wade ( 5 )and Masten and Stone ( 3 )meet the requirements of accuracy but require too much time. The procedure described by Blaedel, Lewis, and Thomas ( 1 ) is sufficiently rapid and accurate, but it does not possess the desired variability in range of concentration. The method of Clark ( 2 ) appeared the most generally applicable, but the time required for the cell to reach equilibrium near the equivalence point is excessive and each analysis requires a plot of potential against volume of added titrant to determine the equiv:ilence point, since the system is not stable for any appreciable length of time. I t was found that these tn-o objections to Clark's procedure can be overcome by the addition of a few drops of liquid, nonionic detergent to the titration solution and t h a t with a f p w further simple modifications the method became entirely sxtisfactory.
Table I. Total 311. AgSOz 0.00 3.00 6.00 8.00 9.00 9.50 9.70 9.80 9.90 9.95 10,oo 10.03 10.10 10.20 10.50 12.00 15 00 20.00 0.00 3.00 6.00 8.00 9.00 9.10 9.20 9.30 9 40 9.50 9 BO 9.70 9.80 9.90 9.95 10.00 10.05 10.10 10.50 12.00 15.00 20.00
EXPERIMENTAL
Apparatus. The indicator electrode was a silver rod 3 mm. ill diameter and 10 cm. long, which dipped into the titration beaker. -4salt bridge connected the titration beaker to a small flask containing 1M pot,assium chloride and a Beckman saturated calomel electrode. This reference half cell was immersed in a thermostat set a t 25.00" & 0.01" C., although such thermostating is not necessary unless determinations of concentrations of the order of 10-4M or less are being made. The beaker was surrounded by a black cloth bag t o keep out light and its contents were stirred rapidly during titration with a magnetic stirrer. The potential between the silver and calomel electrodes was measured with a Leeds & Northrup Type K potentiometer and a Leeds 8: Xorthrup Model 2430 C galvanometer having a sensitivity of 0.0035 pa. per mm. Reagents. The standard solutions of silver nitrate, sodium chloride, potassium iodide, and potassium bromide were prepared from oven-dried analytical reagent grade chemicals and deionized water. Reagent grade sodium acetate and acetic acid were used to prepare the buffer solutions. The detergent used was Tergitol Non-Ionic TPX, kindly supplied by Carbon and Carbide Chemical Co.
Titrations with Silver Nitrate E. M. F., MY.
AE.XI.F./O.lO 311.
Titration of 10 XI1. of Sodium ChlorideQ 140.21 155.93 173.50 184.49 207.08 235.08 2.36 240.08 2.50 243.98 3.90 248.24 4.2t1 251.79 7.10 256.80 10.02 260.41 7.22 263.57 6.32 269.00 5.43 280.46 3.82 290.96 2.00 251.70 370,OO Titration of 10 Ml. of Potassium Broniidea 1.83 1 1 fiR
27.15 45.60 66.55 69.74 73.09 76.95 81.30 86.68 93.07 102.94 114.56 136.10 165.45 197.00 222.67 241.87 290.60 328.38 352,62 370.00
3.19 3.35 3.86 4.35 5.38 6.39 9.87 11.62 21.54 58.70 62.55 51.34 38.40 12.18
Titration of 10 M1. of Potassium Iodideb 0.00 -220.40 3.00 - 210.00 6.00 - 193.50 8.00 - 173.80 9.00 -152.50 9.20 - 145.00 9.30 - 140.00 5.00 9.40 - 133.78 6.22 9.50 125.68 8.13 9.60 114.10 11.55 9.70 - 96.50 18.60 9.75 - 75.50 21.00 9.80 - 00.80 74.70 9.85 114.00 229.60 9.90 186.00 72.00 10.00 210.00 24.00 10.50 234.20 12.00 310.70 15.00 348.50 20.00 369.00 a Both solutions were 0.0200M. b 0.0197hf solution potassium iodide; 0.0200M solution silver nitrate.
-
1043
1044
ANALYTICAL CHEMISTRY
Preparation of Salt Bridge. The salt bridge was a U-shaped glass tube 10 mm. in diameter, 15 em. tall, and 12 em. wide. It was inverted and filled with a hot liquid 37, solution of agaragar in 1 X potassium nitrate. After cooling to room temperature this solution formed a thick gel and solidified. This bridge served as the electrical connection between the calomel reference half cell and the titration vessel. When carefully prepared, these bridges served for months nithout alloaing the diffusion of chloride ion from the reference half cell to the titration vessel. Titration Medium. The titration medium was a buffer solution 0.2,U in sodium acetate and 0 . 2 X in acetic acid. It is not thought that this is the only medium in which the method would be successful, but because the acetate buffer was convenient for quenching the samples \T ithdram-n during the kinetic investigations, it was the only solution that was used. Hox-ever, in the titration of the mixed halides, it is necessary to use a buffer that is 0.6M in both acetic acid and sodium acetate. According to Clark ( 2 ) there are several other salts that could be used for this purpose. General Procedure. A given volume of the halide solution was delivered into 100 ml. of the buffer solution by a pipet. A few drops of detergent were added and the entire titration beaker was covered with a black cloth bag. Standard silver nitrate solution was delivered into the beaker from a 10-ml. microburet, graduated in 0.05 ml., while the solution was stirred efficiently. The potentiometer was set on the previously determined equivalence point potential and the end point was indicated when the galvanometer light reached zero. The average determination required less than 30 seconds. Determination of Equivalence Point Potential. The equivalence point potential is that potential a t which the rate of change of cell voltage with addition of titrant is the greatest. This was determined b y observing and plotting the potential change attendant upon each constant small addition of titrant. The peak of such a plot gives the equivalence point potential. The results of some typical determinations are shown in Table I for chloride, bromide, and iodide. The equivalence point potential was found to be unaffected by the detergent concentration in the range from 1 to 20 drops per 100 ml. RESULTS AND DISCUSSIOK
mental values of these quantities is shown in Table 11. Considering that the accuracy of the calculated values is limited by , the agreement is sat,isfactory. the accuracy of the K s . p values, The results of a number of titrations of 0.020051 solutions are shown in Table 111. As seen in the table, the results are accurate and reproducible with the standard deviation of the order of 0.0001 meq. or O.lY0 of total halide present.
Table 111. Titrationn of Halides with Silver Nitrateb Chloride, RIeq. Bromide, U e q . Iodide, Meq. Found Taken Found Taken Found Taken 0.0200 0 0200 o.o2on 0 . 0200 0 . 0200 0.02oo 0 1000 0.1002 0 1001 0.1000 0 . ion0 O.IOOO 0.2000 0.2003~ O . ~ O O O 0 ,Z o o i d 0,2000 0.2000 e 0.3000 0.3002 0.3000 0.3001 0.3000 0.3000 0.4000 0.4004 0.4000 0.3999 0.4000 0.4000 Titration carried out in 100 nil. of 0.2.V acetate buffer. b 0.0200.11 solution. C For a total of 22 determinations of this value o\-er 10 months. range of milliequivalent found was 0,2000 t o 0.2004; standard deviation equal to 0.0004 meq. d For a total of 23 determinations of this value over 10 months, range of iiiilliequiralent found was 0.2000 t o 0.2002; standard deviation equal t o 0.0001 meq. e For a total of 10 determinations of this value over 10 months, range of milliequivalent found was 0.2000 t o 0.2001: standard deviation equal t o 0.0001 meq.
The method was checked a t lower concentration ranges and These concentrations these results are summarized in Table IT'. are not necessarily the lower limits obtainable by this procedure, but were low enough to more than meet the present requirements. The authors feel t h a t this method could easily be developed to estend to lon-er concentrations, particularly in the case of bromide and iodide. The actual titration solution concentrations in the esperiments cited in Table I\' are of the order of 10-4, 10-5, 10-61.11, respectively, for chloride, bromide, and iodide.
For silver chloride the equation for the potential of the cell a t the equivalence point is given by: E = E o - 0.0591 log v'KT
(1)
With an excess of silver ion present the potential of the silver electrode would be written as:
E
=
Eo
- 0.0591 log a t g +
(2)
Subtracting Equation 2 from Equation 1 gives the change in potential that would be caused by changing the silver ion concentration from that a t the equivalence point to any arbitrary excess value: (3) Thus, from the Ks,,,,values, the standard potential of the silver electrode, the potential of the calomel electrode, and from Equations l and 3 the equivalence point potential, and the change in potential attendant on a n y given change in silver ion concentration can be calculated.
Table 11.
Halide
Comparison of Theoretical and Experimental Values EriuiI., Point E.M.F., A E . h f . F . per 0.10 111. 117.. a t Equiv. Point, 1 1 ~ . Theoret. Exptl. TIieoret. Exptl.
_ _
I n Table I1 the calculated and observed equivalence point potentials are given. 41so shown are the calculated and observed changes of potential attendant upon the addition of 0.10 ml. of 0.0200.V silver nitrate to 100 mi. of solution a t the equivalence point, The comparison betreen the calculated and the experi-
~~
Table IV. Titration" of Halides with Silver Nitrate Halide Halide concn., .If righ-01 concn., Af Halide taken, ml. AgNOa added, mi. S o . of determinations Range, ml. Std. dev., ml. (1 Titration carried o u t in
Chloride 10-8 10-3 10 00 10 01 9 9.97-10.03 0.06 100 nil. of 0.2.11
Bromide 10-4
10 - 4 10.00 9.99 10 9 97-10 02 o 05 acetate buffer.
Iodide 10-5 10 -5 10.00 10.00 7 4 49-10.01 0 01
Determination of Mixed Halides. The difference between the solubility product constants of silver chloride and silver bromide is apparently not large enough to allow mixtures of these two halides to be titrated accurately with silver ion. Hon-ever, Clark ( 2 )developed a procedure involving precipitation in the presence of certain added salts xhich largely circumvents this difficulty. Severtheless, the experimental objections mentioned above to the general Clark procedure apply also to his determination of mixed halides. I t was of interest to see whether the neTy potentiometric procedure would be useful in this analysis. The mixed halides were titrated in a solution 0.6X in sodium acetate and 0.GJI in acetic acid. I n this more conrentrated buffer the equivalence point potentials (determined as described above on the individual halides j n-ere slightly different from those found in the 0.2.11 buffer. For silver chloride the change was not significant, but the value for silver bromide changed from 197 to 145 mv. and the value for silver iodide changed from 114 to 90 mv. This change in the equivalence point potentials could account for most of the beneficial effect of added salt on the titration, first observed by Clark, and may be due to the effect of adsorbed ions on the precipitate or to coprecipitation but seems too large to be due entirely to ionic strength effects on the activity coefficients. The use of buffer solutions serves to keep the ionic strength sensibly
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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. T h e 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 t h a t t'he red endpoint color faded very rapidly. Or, in short, the detergent served as a n 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 i t 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. T h e 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, i t 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, i t 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, i t ~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 t h a t a small amount of nitrobenzene increased materially
