small voltage (0.15 voli,) was impressed across the detecting ele1:trodes. Procedure. A suitable-sized sample was transferred to a 100-ml. beaker, 10 ml. of buffer solution were added, and the solutic n was diluted to 50 ml. with distilled water. The p H a t final dilution was 8.9. A stirrer and the electrode assembly were dipped into the solution and the initial current in the G etection system recorded. Bromine (hypobromite) was generated and the current in the detection system recorded at suitable electrolysis intervals. The current-electrolysis time data were then plotted and an extrapolation to the end point was made. A blank titraticn was carried out in the same manner arid the time subtracted from the srimple titration. Calculations based or Faraday’s law were then made to obtain the amount of reactive species in the cell. RESULTS AND DI!ICUSSION
Although the indic,itor currents in basic solution were not stable enough for titration to a fixed current level, it was thought that a plot of the indicator current as a function of electrolysis time might be suitable for analytical purposes. Figure 1 shows a typical curve for the titration of ammoniuri chloride. The current prior to the cnd point of the
titration was essentially constant and then increased in a nonlinear fashion after the end point. However, an extrapolation to the end point was easily and reproducibly made. Since the current during the titration was constant, it was necessary to record only one or, a t most, two points prior to the end point. A blank titrat.ion was carried out in the same manner and the time was subtracted from the titration of the unknown. At a generation current of 19.3 ma., the blank titration was 3.5 seconds with the sample titration ca. 158 seconds. Table I lists the data for 10 determinations of ammonium chloride. The standard deviation for the 10 analyses was * O . l l with a mean of 10.3 micromoles of ammonium chloride. The experimentally-found mean deviated from the true value by 0.1 micromole. Table I1 shows the results for various concentrations of sulfamic acid. Included in the data are two sets of replicate analyses. I n one set of replicate analyses, the standard deviation was zt0.04 with a mean of 5.01 micromoles and in the other it was hO.09 with a mean of 10.63 micromoles. The former mean deviated from the true value by 0.03 micromoles and the latter by 0.03 micromoles also.
Table II. Direct Coulometric Titration of Sulfamic Acid with Hypobromite Ion Micromoles Generating Micromoles XHsSO, current, NHISOI ma. added found 4.98 4.98 4.98 4.98 4.98 10.66 10.66 10.66 10.66 10.66 14.22 10.66 7.11 3.56
9.65 9.65 9.65 9.65 9.65 22.0 22.0 22.0 22.0 22.0 19.3 19.3 19.3 19.3
5.03 5.06 5.00 4.97 4.97 10.55 10.75 10.58 10.58 10.71 14.18 10.68 7.48 3.80
LITERATURE CITED
(1) Arcand, G. M., Swift, E. H., ANAL.
CHEM.28,440 (1956). (2) Knvis, A. F., Microchem. J. 5, 559 (1961).
ALANF. KEUVIS GEORGE R. SWPP EUGENE S. GAZDA Central Analytical Laboratories Olin Mathieson Chemical Corp. New Haven, Conn.
Coulometric Titration of Ammonia with Hypobromite Using Direct Amperometric End Point Detection SIR: Willard and Cake (6) and Kolthoff and Stenge. (2) have developed volumetric riethods for the determination of ammonia by oxidation to nitrogen wii;h hypobromite. Since then Kolthoff, Stiicks, and Morren (3) have described zn amperometric end point detection ir the titration of 6 X lo-‘ to 4 X 10-6M ammonia in sodium bicarbonate solution. They employed a rotating platinum wire electrode at +0.2 volt. us. S.C.E. as the indicating electrode. Laitinen and Woerner (4) employed a similar technique in titrating ammonia with hypochlorite. ilrcand and Swift (1) developed a coulometric titration of 14 to 230 pg. of ammonia using electrogenerated bromine in a supporting electrolyte of pH 8.5. I n alkaline medium, bromine disproportionates to kiromide and hypobromite ions. Two platinum foil indicator electrodes with 150 mv. impressed between them were used for the arnperometflic end point. Arcand and Swift found that the indicator current resulting from a given quantity of bromine in alkaline solu-
tion was not sufficiently reproducible for use as a means of determining the end point. Therefore, they used the current in the alkaline solution only to indicate when a slight excess of bromine had been generated. Then an indirect method was employed to find the end point; the solution was acidified with perchloric acid and additional bromine was generated a t 1-second intervals. The current readings were plotted against the generation time and a straight line resulted. A blank was run in the same manner and an arbitrary current reading in the acid solution was chosen as the end point current. The authors have experienced difficulty in trying to employ the method of Arcand and Swift in the titration of ammonia solutions obtained from biological samples. We found that the indirect end point determinlttion was not reproducible. The current readings due to the excess bromine in acid solution appeared to be very dependent on the amount of excess bromine generated before the solution was acidified. I n a blank showing a difference of only 1.7 seconds to generate excess bromine at
4.825 ma. before acidification, the generation-time readings differed by 7 seconds to reach the same current reading in acid solution. illso, a given current reading depended upon strict reproduction of the stirring rate and volume. The authors observed the same difficulty experienced by Arcand and Swift in trying to generate to an arbitrary current value directly in alkaline solution. However, in preliminary titrations we noted that the generation time needed for the current readings to increase in alkaline solution appeared to correspond closely to the end point value and was reproducible. Therefore, a direct, conventional end point This detection was investigated. method has been employed to determine as little as 1.4 pg. of ammonia. EXPERIMENTAL
Reagent grade chemicals were used without further purification. Solutions of ammonium sulfate were prepared by dissolving Mallinckrodt “Analytical Reagent” ammonium sulfate in an appropriate volume of water. The VOL. 35, NO. 13. DECEMBER 1963
2217
ammonium sulfate had previously been dried a t 110" for 2 hours. The stock buffer solution employed was prepared by dissolving 38.1 grams of sodium tetraborate decahydrate in 1 liter of water and adding 727, perchloric acid to adjust the pH to 8.6. The coulometric titrations were made with a Sargent Coulometric Current Source, Model IV, a t a generating
Table 1.
Table II. Automatic Titrations
353.4& 354.1 354.P 354.0 34.04' 34.07n 34.84" 34.66' 34.90" 34.84 29.30 6.3P 6.3P 6.28" 6.4g4 6.38 3.29" 3.17a 3.14' Ave. 3.20 2.89 3.17 Sam les added to :is descriged in text.
221 8
I o
Manual Titrations
XHa, fig. Error, Taken ma. Found fig. 354.3 353.4a -0.9 19.30 144.3 144.1 -0.2 4.825 35.43 +O 03 4.825 35.46 35.43 4.825 35.75 f0.32 35.43 35.57 4.825 $0.14 35.43 4.825 -0.26 35.17 35.43 35.40 4,825 -0.03 Ave. 35.47 $0.04 34.78 34.785 0 4.825 34.78 34.6gn -0.09 4,825 34.78 34.72a -0.06 4.825 -4ve. 34.73 -0.05 28.86 27.95 -0.91 19.30 28.86 28.64 -0.22 4,825 Ave . 28.30 -0.X 14.43 14.09 -0.34 19.30 7.09 7.29 $0.20 4.825 7.09 7.15 4,825 f0.06 Ave. 7.22 f0.13 6 38 6.46' $0.08 4.825 0.38 6.46" $0.08 4.825 Are. 6.46 $0.08 4.825 2 89 3.12 $0.23 4.825 2.89 3.14 +0.25 4.825 2.89 3.26 $0.37 4.825 "89 3.14 +0.25 4,825 Ave. 3.17 +0.28 1.44 1.76 $0.32 4.825 a Samples added to apretitrated solution as described in text.
354.3 354 3 354 3 Sve. 34.78 34.78 34.78 34.78 34.78 Ave. 28.86 6.38 6.38 6.38 6.38 Ave. 3.19 3.19 3.19
30
-0 0 -0.2 +0.2 -0 3 $0.06 $0.19 $0.06 -0.12 $0 12
19.30 19.30 19.30 4.825 4.825 4.825 4.825 4.825
f0.06 +0.44
0 0 -0.10 $0.10 0
+O.lO -0.02 -0.05
4.825 4,825 4.825 4.825 4.825 4.825 4.825 4.825 4.825
+O.Ol
+0.28
4.825 pretitrated solution
ANALYTICAL CHEMISTRY
SECONDS a1 4 . 8 2 5 ma
Figure 1 . A. 6.
Coulometric titration of ammonia
Blank: 8.00 ml. of stock buffer solution, 6.00 ml. of 5M NaBr, and 16.00 ml. of water. Bromine generated at 4.825 ma. Test solution: 8.00 rnl of stock buffer solution, 6.00 ml. of 5M NaBr, 11.00 ml. of water, and 5.00 ml. of 8.47 X 10% ammonium sulfate. Bromine generated at 4.825 ma.
current of 19.30 or 4.825 ma. The generating anode and cathode were platinum foils of 2 sq. em. and 0.8 sq. cm., respectively. The cathode was isolated from the test solution by plating it in a glass tube fitted with a sintered-glass frit end and containing the stock buffer solution as catholyte. The indicating electrodes were two platinum foils ( 2 sq. em.) with 150 mv. impressed between them. The impressed potential was supplied by a Sargent Model XV Polarograph. -411 amperometric measurements were made by recording the current on this instrument either manually or automatically. During generation and current measurement, the solution was stirred with a magnetic stirring bar. The titration vessel mas either a 250-ml. beaker or a 50-ml. weighing bottle. Procedure. I n titrations a t 4.825 ma., 8.0 ml. of stock buffer solution and 6.0 ml. of 5 M sodium bromide were added to a 50-ml. weighing bottle. Standard ammonium sulfate solution was pipetted into the bottle and water was added t o bring the volume to 30.0 ml. I n titrations a t 19.30 ma., either the above procedure or 15.0 ml. of buffer and 15.0 ml. of 5M sodium bromide were added t o a 250-ml. beaker. Standard ammonium sulfate solution was pipetted into the beaker and water was added to make a total volume of 75.0 ml. Manual plots were obtained by generating bromine at 10-second intervals until a slight rise in the current readings indicated the approach of the end point. The current rise was then recorded a t I-second intervals until sufficient points were obtained to determine the end point break. Automatic plots were obtained by allowing the chart to record the current continuously until a sharp rise in current indicated the end point. The end point break was obtained by extrapolating the straight
lines in a manner analogous to that employed in the manual plots (See Figure 1). The divisions on the chart paper mere calibrated in terms of seconds. There was always a slight initial rise in current before the levellingoff observed by Arcand and Swift. RESULTS AND DISCUSSION
-4 typical blank and test solution curve is shown in Figure 1 with current readings being obtained manually. I n the 30-ml. volume, a large blank reading equivalent to about 2.9 pg. of ammonia was found. The major portion of the blank impurities appeared to be in the sodium bromide solution since a decrease in the buffer concentration by a factor of ten decreased the blank reading by only 13%. Arcand and Swift observed a blank reading of about one-third this value. The reaction a t the indicator electrodes is sufficiently rapid to allow current to be recorded automatically on the polarograph, thus decreasing the titration time conqiderably. An added advantage of direct end point detection is that the blank can be pretitrated to a sufficient rise in current to extrapolate the end point, and the excess hypobromite noted. At this point sample can be added directly and titrated by adding the quantity of excess hypobromite initially generated t o the end point value. I n this manner a series of samples can be titrated in succession in the same generating solution. The disadvantage of a large blank and the pipetting errors involved in reproducing a reagent blank are also eliminated. I n applying this method the end point value of the first
.ample folio\\ ing Inetitration of the blank was usually in (mor \Thile -ubsequent samples were satisfactory. Therefore, the procedure followed was to pretitrate the blank to a slight excess and then add an timmonia sample of approximately the same size as the expected teyt sample and pretitrate it, noting the excess of hypobromite generated beyond the end point. Then the sample of a serics of samples n a s added and titiated a. a l m e I n the-e p-etitration procedures, a large exceqs of hypobromite -hould not be generated before adding the sample, as this may lead t o a positiL e error (1). I n the manual titrations, variations of stirring rate and total volume should have no effect on the m d point break, although the generating; reagents should be pipetted to reproduce the blank impurities. I n applying the automatic plot, however, theqe should be re-
produced fairly clowly with a volume duplication of i l ml. appearing t o be sati-factory. For a series of titrations in the same solution, satisfactory end points are obtained if the stirring rate is increased in approximately the qame proportion as the volume increase when a new sample is added. In these titrations the recommended p H should be adhered to within -0.1 p H unit as discussed by Arcand and Swift (I). I n any event where small ammonia samples are titrated with hypobromite, which is a strong oxidant readily reduced by many different impurities, the analyst should always run a blank to determine the titration error due to impurit,ie,q. The data of this inveqtigation are shown in Tables I and 11. A series of samples ranging from 354.3 to 1.44 pg. of ammonia vcre titrated either
manually 01 :iutoiiiatically u-ing both the direct and pretitration mcthotli. Automatic titration appears t o give results slightly superior to the manual titrations. LITERATURE CITED
(1) Arcand, G. )I., Swift, E. H., . 4 s a ~ . CHEM.28,440 (1956). (2) Kolthoff, I. AI., Stenger, V. A., ISD. ENQ.CHEN.,AKAL.ED. 7, 79 (1935). i.7) Kolthoff. I. bl.. Stricks. W.. Morren. (4) Laitinin, H. -4.; Roerner, D. E., ANAL.CHEJI.27,215 (1955). (5) Willard, H.H.,Cake, W. E., tJ. A t l z . C h e m SOC.42, 2646 (1920). GARYD. CHRISTIAS EDWARD C. KNOBLOCK WILLIAMC. PTJRDY Division of Biochemistry Walter Reed Army Institute of Rcrearcli Washington 12, D. C. and DeDartnient of Chemistrv Unhersity of Maryland ' College Park, Md.
Infrared Ancdysis of Alpha-Olefins SIR: The olefin groiili type analysis preyiously reported ( 2 ) has been in use in this laboratory for a number of years. Recently a need arose for the accurate quantitative determination of olefin group types in normal a-olefin rich mixtures. I t v;as particularly desirable to achieve the highe-t possible accuracy in the a-olefin determination. Since the required accuracy was beyond the limits of the group type analysis, it was necessary to introduce a number of modifications in the procedure to obtain a satisfactory analysii. The purpose of this cornniunication is to report these modifications. Simple averaging of the absorptii.itics of the characteristic a-olefin band (11.0 microns) to obtain a group type value results in errors too large for the present requirements This is illustrated in Table I. For esample, the absorptivity of heserie.1 deviates from the average by 14% ttnd decene-1 by 5.5%. Thus, it is nece:aary to calculate a statistically y1 eighted absorptivity to be used with each sample. -Accordingly, the following procedurcb n as developed. The absorbances for all the normal a-olefins to be incliidecl in the analysis were obtained a t all the analytical wavelength---e.g., l O . X , 11.00, 11.25, 12.03, 12.15, 12.33, 12.50, and 14.42 microns, on a Perkin-Elmer Model 21 infrared spectrophotorieter. A 0.15mm. cell was used and dilutions were made in carbon disulfide when necessary. The absorptivity of each of the pure a-olefins was calculated at their group absorptim wavelength,
11.OO microns. The talibration compounds were high-purity American Petroleum Institute sanipleq. In the present case, we included only the even-numbered a-olefins from Ce through Cz0. Octadecene-1 and eicosene-1 nere not available and their data here determined by extrapolation of the data for the lower molecular weight compounds. When the absorptivities of pure a-olefins are u-ed and the carbon number distribution of the sample is obtained from a high temperature programmed gas chromatographic analysiq, the statistical absorptirity at 11.00 microns is calculated for the particular sample under analvsic The absorbance of the sample a t 11.00 microns divided by its Gtatiqtical absorptivity yields the concentration of a-olefins in the -nrnp:': (in mole. per liter). To determine the other olefin groups which are present as impurities in the a-olefin cample, the statistical absorbances are calculated for the sample a t all the an-
Table II.
alytical a-avelcnyths assunling the sample contains 1 0 0 ~ onormal a-olefins. The statistically calculated absorbances are then substracted from the respective observed absorbances of the sample. These difference absorbance4
Table 1.
Absorptivities of a-Olefins at 1 1 .O Microns
Absorptivity, liter mole-' % Dev. c.rn.-' from av. 138 - 14
a-Olefin Hexene-1 Heptene-1 Octene-1 Sonene-1 Decene-1 Undecene-1 Dodecene-1 Tridecene-1 Tetradecene-1 Pentadecene-1 Hexadecene-1
I50
-6 2 -1 4 -2 G 15.5
I58 156 169
io
$6.2
1
166 165 I64 I65 I60 ~A.T.
$3.7
+3.1 $2.4 $3.1 1
io.
lW.1
Matrix Used in Group-Type Olefin Analysis
Olbsorptivities in liter mole-' cm.-*) A, l\Iicronti/
olefin group
10 35
11.00 11.25
Av. of 4
1 2 . 0 to 12.5
14.42
Trans RHc=CHR
l-inyl RHC=CH,
Vinylidene IL'Rf'C=CHIR
Trisubstituted R'R"C= CHR
147 2 4 20 4 46 1 01
.i 59
1 65
160 4 9 45
S 29 184 7
2 43
4 19 2 23 2 53 15 7
0.24
1.35
0.41
0 31
1 88
Cis RHC=CHI: 6.09 4 71
VOL. 3 5 NO. 13 DECEMBER 1 9 6 3
3 01 2 30
24.5
2219
