Determination of Chloride Ion in Forrnamide Solutions AND LYLE R. I).iWSON University of Kentucky, Lexington, hj..
CARL BERGER
Necessity for determining chloride ion in formamide solutions prompted an investigation of argentometric methods both by the usual volumetric precipitation procedures and potentiometrically. Inaccurate results were obtained in the determination of chloride in solutions containing formamide by the Rlohr technique because the end point was indistinct and the blank large and variable. Preliminary data by the Fajans method were satisfactory, but further investigation is needed. The Fisher
Titrimeter, used with a silversilver chloride electrode immersed in a saturated solution of potassium sulfate in formamide, gave satisfactory results at low chloride concentrations in mixtures containing 8Oqo or more formamide. Evidence was obtained that the sil\ ersilver chloride electrode in formamide behaves essentially like a “reversible” electrode. Similar adaptations might be made in the determination of inorganic ions in other mixtures contairiing water and a nonaqueous solvent.
I
X THE course of an investigation on an electrochemical problem in this laboratory it became necessary to devise an analytical method for the determination of chloride ion in formamide solutions. This paper reports the results of esperimcnti using the Mohr and Fajans techniques and potentiometric methods with chlorides in formamide-water solutions. Formamide resembles water in that it possesses good solvent power for many inorganic salts. It is reported to have a large association factor ( 3 )and a dielectric constant much higher than water ( I ) . In addition to establishing a method for the analysis of solutions of chlorides in this solvent, the results reported herein provide interesting data regarding the apparent “reversibility” of the electrodes used with the Fisher Titrimeter, in solutions containing large percentages of formamide.
n i t h 0.100; S silver nitrate. Blanks were run loi each determination, Fajans Titration. To 20 ml. of pure formamide w&s added a known quantity of potassium chloride. To this solution were added 7 ml. of aqueous 4% dextrin and 5 or 6 drops of 0.1% dichlorofluorescein. This was titrated to a distinct pink end point with 0.1007 ,V silver nitrate. Blanks were not run with this procedure because the end point requires the presence of silver chloride.
MATERIALS AND APPARATUS
The source of chloride ion in these experiments was C.P. potassium chloride, which was purified by recrystallizntion from conductivit water. Silver nitrate, potassium dichromate, dextrin, and dic&orofluorescein, designated as c.P., were used without further purification. The main part of the work reported was done w-ith a Fisher Titrimeter using silver, silver-silver chloride, and calomel-silver electrodes which were obtained from the Fisher Scientific Co. Purification of Formamide. Procedures for the purification of formamide which are recorded in the literature (2, 4, 5 ) consist mainly of low pressure distillations or freezing techniques. At the University of Kentucky a combination of these techniques has proved successful in producing relatively pure formamide.
‘-GLASS
PRONGS
RUBBER BANDS
GROUND GLASS JOINT REMOVABLE CAP--/
Figure 1.
The first step involved fractional freezing of the commercial grade of the solvent, which was obtained from the Du Pont Co. This was accomplished by allowing two thirds of the formamide, placed in a specially constructed flask (Figure l), to crystallize a t Ion- temperature. The impure mother liquor was drained off after the solid had been loosened by pouring hot water into the central tube. Then the solid was allowed to melt and the procedure was repeated two or three times. The formamide was dried overnight with calcium oxide, then distilled slowly a t a pressure of about I mni. of mercury. (Six to 8 hours was required for this distillation.) The first and last 10% of the distillate were discarded and the formamide was stored in a “low actinic” glass-stoppered bottle through which dry nitrogen was bubbled for 0.5 hour.
Fractional Freezing Apparatus
Titration with Fisher Titrimeter. The first set of deterniinations was run with calomel and silver electrodes. I n this series of experiments known amounts of potassium chloride were added with a few milliliters of concentrated nitric acid to varying amounts of formamide and the solutions were titrated with 0.1 N d v e r nitrate. Different amounts of formamide Rere used to ascertain whether the solvent caused anomalous behavior of the calomel electrode. The second set of experiments, in which silver and silver-silver chloride electrodes were used, nas done in the same manner. In this case, honever, the anomalous behavior of the silver chlorlde electrode was avoided by immersion in a formamide solutioni.e., instead of using a saturated aqueous solution of potassium sulfate to act as a salt bridge for the silver chloride electrode, the electrode was immersed in a saturated solution of potassium sulfate in formamide. I n the usual analytical procedure for determining chlorides in water solution a few milliliters of nitric acid are added a t the outset. The same procedure was followed for analvsis in formamide in all escept two cases (Table I T )
EXPERIMENTAL
Standard solutions were prepared by dissolving known quantities of potassium chloride in formamide These solutions were analyzed by titration with standard aqueous silver nitrate s o h tions. Mohr Titration. To 20 ml. of pure formamide were added a known amount of potassium chloride and I ml. of aqueous 5% potassium chromate. This was titrated to a red-bronm end point
994
V O L U M E 2 4 , NO. 6, J U N E 1 9 5 2 Table I.
995
Chloride Determination in Formamide by Mohr and Fajans Methods
KCI, KC1 1Ipthod Added, Found, Used G G. hlohr 0.1225 0.1152 0.2240 0.2174 Mohr 0,1298 0.1298 Faians Fajans 0.1435 0.1433
Formamide, bI1. 20 20 20 20
Absolute Error, Mg. -7.3 -6.6 0.0 - 0 2
E I Iu r c
-6 0 -3.0 0 00 - 0 14
DISCUSSION
The Mohr technique yielded such inaccurate results that work this method was discontinued. The values obtained are shown in Table I. I t is of some theoretical interest to reflect upon the factors contributing t o the unsatisfactory results obtained with the Mohr mrthod. Most of the error, it appears, is due to the indistinct end points. Lloreover, the extremely large blank obtained (as much as 1.15 nil. of 0.1 A' silver nitrate) casts doubt upon the applicabilit,y of this technique in formamide, since it indicates a far greater solubility for silver chromate in formamide than in water where the blank is of the order of 0.15 to 0.30 ml. of 0.1 N silver nitrate, depending on the individual analyst. The accuracy of the Mohr method depends on the insolubility of the silver chromate precipitate. Because of these factors, this technique appears t o hold little promise of accuracl-. The Fajans procedure is based on the principle that suspensions of silver chloride in the presence of an excess of halide ion acquire a negative charge owing t o adsorption of the halide, whereas the charge becomes positive in the presence of an excess of silver ions. Dichlorofluorescein, which acts as an adsorption indicator, gives a color change on the surface of the precipitate when the charge on the suspended material changes. The d a h given in Table I indicate that the analysis may be carried out accurately in formamide. No blank can be run, because the color change involves a change in charge on a silver chloride precipitate. The experimental data on the analysis of chlorides by potentiometric titration are of interest from both practical and theoretical etandpoints. 011
Table 11.
Potentiometric Determination of Chloride in Formamide
(Silver a n d calomel electrodes with Fisher Titrimeter) KCl KC1 Formamide, Absolute Found, G. M1. Error, M g . Error, % .4dded, G. 30 -0.5 -0.33 0.1471 0.1466 35 +0.2 +o I 1 0 1717 0.1719 -2.4 0.0898 60 -2.2 0.0920 t l . 1 0.1428 70 +1.4 0.1442 90 +3.2 -2 8 0.1153 0.1185
The data in Table 11, obtained using a calomel-silver electrode couple, show the effects of increasing the volume of formamide and the volume fraction of formamide. The volume fraction equals the amount of formamide present a t a given time divided by the total amount of mixed solvent present a t that time. Thus on addition of 15 ml. of 0.1 -Yaqueous silver nitrate to 100 ml. of formamide solution, the volume fraction of formamide rvpressed in per cent is
voltage readings and irregular variations in the e.m.f. accompany increases in the formamide content. Conjecture might lead one to presume that the increased irregularity could be due to a difference in transference numbers on either side of the junction. This could cause a variation in the total potential of the system as a result of changes in the junction potential. The influence of stirring was approximately a constant factor, as all samples analyzed with the Fisher Titrimeter were stirred at a constant rate. In potentiometric analysis the criterion of a good end point is an abrupt change vertically in the e. m. f. curve a t the equivalence point of titration. An increase in formamide content definitely reduces the sharp vertical nature of the e. m. f. curve a t the rquivalence point; thus the voltage change per unit volume of titration solution near the equivalence point becomes less, resulting in decreased accuracy of the titration. The next step involved the use of the more conventional silver and silver-silver chloride electrodes for determination of chloride. Because the construction of the eilver-silver chloride electrode is similar to that of the calomel-;.e., the electrode is surrounded by it salt solution, which in turn forms a junction with the solution heing analyzed-it was expected that a higher concentration of formamide would have a n adverse influence on analysis for chloride under these conditions. All the analyses recorded in Tables I11 and I V were made a t volume fractions of formamide above
SO%. Table 111. Potentiometric Chloride Determination in Formamide (Silver and silver-silver chloride electrodes with Fisher Titrimeter) KC1 KC1 Formamide, rlbsolute Added, G. F o u n d , G . Ml. Error, Mg. Error. % +0.8 0.0754 90 0.0762 +1 1 +7.2 +3 6 90 0.2090 0.2018 -2 0 -1 3 90 0 1563 0.1583 -0.6 -1 3 25 0.0455 0 0461 -1.0 -0.76 90 0.1333 0.1323 -0.8 - 0 58 90 0.1371 0.1863 +o 2 3 +O.l "5 0.0138 0.0439 ,7 j -0 15 -0.1 0.0648 0.0647
Table I11 presents results of the analysis of chloride solutions using the typical silver chloride electrode surrounded by a saturated aqueous solution of potassium sulfate. The first six determinations show the erratic results obtained by this technique. Considerable voltage drift and fluctuation were encountered, which could be avoided only by waiting for very long periods of time for equilibrium to be reached. The last tv o lines of data in the table show that nith sufficient patience and exact timing of the titrant addition, good results can br obtained. These determinations required approximately 2 hours each. Actually, the formamide content wap low in both solutions. In all cases the drift and overlapping of e m.f. values a t the equivalence point were difficult t o control. Table I V shows the effrct of substituting a saturated solution of potassium sulfate in formamide for the aqueous potassium sulfate
Table IV.
+ 15 x 100
_ _ loo _ _ ~
100
The data phon. that as the content of formamide increases, the rcsults bccome less reliable. It appears that formaxnick everts a disturbing influence on the junction between t h r main body of the solvent and the aqueous chloride solution in which the calomel electrode is immersed. A rather pronounced drift of
0
Potentiometric Chloride Determination in Formamide
(Silver a n d ~iriyrovedsil\.er-silver chloride electrodes with Fisher Titrimeter) KCl KCI Furinamide, Absolute r o u n d , G. Ml. Error, Mg. Error, 7% Added. G . -0.85 -0 2 25 0 0236 0.0234 -r) fi7 2G -0 2 0.0295 0 0293 +o iz 0 0867 0.0868 90 +o 1 1 2 5 + 0 12 0 1641a 0 1643 +o 2 - 0 06 -0.1 125 0.1651a 0,1650 -0.05 -0 1 12.5 0 1829 0.1828 -0.05 -0.1 12.5 0.2087 0.2088 Usual procedure not followed.
A N A L Y T I C A L CHEMISTRY
996 solution usually used with silver-silver chloride electrodes. Not only are the results uniformly good, but the amount of drift and fluctuation of voltage values is negligible compared to the technique described previously. .4s a result of this stabilization of the instrument, an analysis can be completed in 30 minutes. The data show that the limit of the accuracy of the method is 0.1 t o 0.2 mg. of chloride as potassium chloride. ACKNOWLEDGMENT
The authors gratefully acknowledge their indebtedness to G. R.
Leader for making available the fractional freezing apparatua shown in Figure 1 which he designed and constructed. LITERATURE CITED
(1) Leader, G. R., J . Am. Chem. Soc., 73,856 (1951). (2) Magill, P.L., I d . Eng. Chem.,26, 611 (1934). (3) Rao, S., J. Indian Chem. SOC.,18, 337 (1941). (4) Smith, G. F., J . Chem. Soc., 53, 3257 (1931). ( 5 ) Verhoek, F., J . Am. Chem. Soc., 58, 2577 (19361. RECEIVED for review August 13, 1951. Accepted March 27,
1952.
Evaluation of Particulate Concentrations with Collecting Apparatus Examination of Air Pollution Levels in Los Angeles County STANLEY R. HALL The Albert L. Chaney Chemical Laboratory, Glendale 6, Calif. Air pollution control programs require some method of assessing the effectiveness of the corrective procedures. Human observations are subject to many limitations and their accuracy w i l l be seriously questioned as time passes. Long-period weather cycles, shift in population centers, industrial growth, and changes in chemical processes all produce subtle changes that cannot readily be detected by direct visual observations. Measurements based on a reproducible procedure offer a logical and acceptable means for evaluating pollution levels. Particulates are collected on flat filter disks and measured by means of a wyell-known apparatus. Limitations and advantages of a proposed scaleof valuesfor particulate
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N AIR pollution research or in the administration of an air pollution control program, it is essential to determine the air
contamination levels over extended periods of time for comparison with previous years or corresponding times in other meas. Gradual increases or decreases in air pollution levels over a n extended period of time are not easily detectable and are always debatable when general observations are used for such purposes. Varying meteorological conditions make the problem doubly difficult and render general observance almost worthless. Some reliable and integrated measuring procedure is required. The first indication of common city air pollution is the haze and decrease in visibility. Because visibility is affected by the relative humidity, light levels, clouds, fog or water droplets, and particulates, it cannot be used alone as an index of air pollution. It is further subject to the limitation of darkness. The use of particulate mass would seem to provide a much better approach t o evaluation of air pollution levels. If the particulates are collected from measured volumes of air, and their mace and general composition determined, a pollution level for the particulates is established. The composition identifies the general sources and any change in these sources will be reflected in future measurements. From the best information now available, It s e e m that most nonparticulate centaminants also rise and fall with the particulate level ( I , l a ) . For evaluation of air pollution levels, these mass measurements should be taken continuously over extended periods of time. Each
levels are discussed. Procedures for calibrating the scale of particulates in terms of carbon or other materials are given. An automatic collecting apparatus is briefly described. For research studies a curve of particulate level variation is extremely useful in analyzing other related data. iiir pollution control officials can use the procedures for obtaining accurate comparable data to evaluate the effectiveness of control measures. Particulate levels and their variations with time are obtained with a minimum of labor. Personnel may be more effectively used, as data may be collected continuously and stored. Later, the most important periods may be studied as time and importance permit.
sample should be for a relatively short period and then all the samples should be integrated and correlated against meteorological data. Collecting and analyzing so many samples could be extremely time-consuming and expensive. Any scale of mass values could be used. Such a scale of values should be easy to obtain, proportional to m s per unit of volume, and for convenience, be in terms of small numbers. If values are in mass per unit of volume, the data for one area may be readily correlated and compared with those from another area. They should in themselves be a reference value t o which the various pollution constituents may be related. The mechanism of determining their mass values should be adaptable to automatic recording devices. METHOD
The samples are collected by drawing the air through filter paper in much the same way as was done by Shaw and Owens (IO)and others (4, 8). A 25-cubic-foot volume of air is drawn a t a uniform rate through a 1-square-inch area of filter paper during 1 hour. (Only a uniformly white paper of high quality should be used. Selected lots of Whatman No. 52 and the equivalent acidextracted Type 540, 5.5-em. diameter, are satisfactory.) This leaves a dark deposit on the paper. This darkening of the paper is measured and used aa an indication of the mass collected. The darkening is measured by the light reflected from the de osit when compared to that reflected from the clean filter paper ~ y! a reflectance attachment of the Beckman DU spectrophotometer a t a wave length of 400 mp. The readings are recorded in terms of
