1895
V O L U M E 2 4 , NO. 1 2 , D E C E M B E R 1 9 5 2 To a second beaker, 10 ml. of sample, 10 ml. of 0.25 B ammonium hydroxide, and 10 ml. of sodium diethyldithiocarbamate are added. The solution is transferred to cell and photometer is read in 1 to 5 minutes. Photometer reading is translated t o parts per million of copper by reference to a standard curve calibrated under these same conditions and covering the range of 0 to 5 p.p.m. of copper as Cu. RESULTS AND DISCUSSION
Table I s h o w the concentrations of ions that can be tolerated without interference with the test. Up to 10 p.p.m. of iron can be tolerated, but above this concentration a precipitate forms on the addition of the ammonium hydroxide. Below 10 p.p.m., iron is not precipitated by the ammonium hydroxide, but is held in an apparent colloidal suspension.
Table 111.
Effect of Varying Quantities of Test Reagents
C u Present,
KHIOH,
3.00 3.00 3.00 3.00 3.00 3.00 3.00
Ml. 10 10 10 11 9 9 11
P.P. M.
Thiocarhamate, M1. 10 11
9 10 10 9
11
Cu Found, P.P. 11. 3.00 2.92 3.16 2.94 3.14 3.24 2.87
Cu Corrected for Error in Vol., P.P.X. 3 yo2 3.05 3.04 3.04 3.02 3.06
Table I1 shows the effect of the presence of 10 p.p.m, of iron as Fe a t low copper concentrations. Even a t these lov- concentrations of copper, iron does not interfere. The color produced in the test f o l l o ~ sBeer’s law. Maximum absorption is obtained at 448 mM as determined on a Beckman Model D U spectrophotometer. N o effect of temperature was found over the range of 20” to 36” C.
I n fifteen tests, by three different analysts, on samples containing from 0 to 4 p.p.m. of copper, the maximum error encountered was 0.05 p.p.m. Tests were also conducted to determine the error introduced by inaccurate measurement of the test reagents. Table I11 shows the data obtained with i l ml. variation in either and both test reagents. \Then the reading is corrected for the error due to the different volume, excellent checks are obtained. These results indicate that, using Xessler tubes for comparison, no error will be introduced by slight variations in the amount of reagents. This modified method of copper determination has also heen applied to a Klett-Summerson photometer employing a 420-mp filter and 13-mm. depth tpst cell. The same procedure is recommended except that 5 ml. of test sample, 5 ml. of ammonium hydroxide, and 5 ml. of sodium diethyldithiocarbamate are employed. A straight-line calibration curve is obtained. Recommended copper ranges for cells of different depths are listed below. Above 10 p.p.m. of copper, the line tends to flatten out and is no longer a straight line. Cell Depth, hlm. 20 10 5
Copper Range, P.P.M 0-5
0-10 0-10
This method of copper determination has been studied by Subcommittee K-9 (5-12) of the American Water TVorks Associntion and recommended for the next (tenth) edition of “Standard Methods for the Examination of Water and Sewage.” LITERATURE CITED
(1) American Public Health Association, “Standard Methods for the Examination of Water and Sewage,” New York, 9th ed., pp. 47-9,
1946. RECEIVED for review M a s 14, 1952. Accepted August 6, 1962.
[End of Symposium on Analytical Methods for Industrial Wastes]
High Sensitivity Recording Polarograph MYRON T. KELLEY AND HUGH H. MILLER Oak Ridge l\-ational Laboratory, Carbide and Carbon Chemicals Co., Division of Union Carbide and Carbon Corp., Oak Ridge, Tenn. The polarograph described is intended to extend the useful range of polarography to the 10W6M region, as the analysis of such dilute solutions is frequently valuable in chemical anal) sis of radioactive materials. The instrument described has useful current sensitivity ranges from 0.01 to 10 p a . for full scale deflection of the recorder. It is provided with a linear compensator to minimize the effect of condenser current on the recorded polarograms and also a “curve follower” w-hich can be used to subtract the peculiar, nonlinear blank curves which are obtained at high sensitivities. The instrument has been used for the determination of the diffusion current for lead in solutions of 2 X lo-’ M to 2 X 10-OiMwith an estimated precision within +205i& The diffusion current is shown to be proportional to concentration in this concentration range, although the diffusion coefficient seems to be somewhat different than at higher concentrations.
P
OLAROGRBPHY is readily adaptable to the analysis of radioactive materials because of the few manipulations required. This laboratory has been interested in applying this method to highly radioactive samples where the size of the sample must be severely limited. When such is the case (as with samples less than 0.1 ml. in volume), one must either adopt micropolarographic techniques or analyze very dilute solutions with more conventional polarographic cells (1 to 10 ml. of solution). The latter system was chosen for study a t this laboratory because
of mechanical difficulties in handling micropolarographic equipment behind barricades used for radiation shielding. The aim in this work was to obtain readily measurable waves a t concentrations of 10-6 M or loxver. The diffusion current t o be expected a t such concentrations is of the order of 5 X 10-8 microampere. On any of the commercially available polnrographic instruments, this represents a deflection of less than 1 mm. or essentially an unmeasurable current. Attempts to increase the sensitivity of available recording instruments by increasing
1896
ANALYTICAL CHEMISTRY
the resistor across which voltage drop is measured led to an increase in the dead space of instruments using a low-impedance electronic recorder of conventional type. As the advantages of the pen recording instruments with galvanometers are selfevident, a recording instrument has been used for this work. DESCRIPTION OF INSTRUMENT
The instrument is conventional in many respects-i.e., it uses a motor-driven Helipot to supply the polarizing voltage, and the cell current is measured by recording the potential drop across a known resistor by means of a Brown recorder. However, it incorporates several features which are not found in commercial instruments.
A preamplifier is used with the Brown recorder, so that it will operate from current-measuring resistors u to 1 megohm without appreciable dead space. A recorder witg a 1-mv. range for full scale is used in order to hold the potential drop in the measuring resistors to a minimum. A damping circuit employing high resistance and relatively low capacity (10 pfd.) is used, so that the time constant is almost independent of the value of the current-measuring resistor. This also avoids the necessity of using electrolytic capacitors, which sometimes show objectionable leakage effects. A linear compensator is incorporated similar to that proposed by Ilkovir and Semerano ( 2 ) which bucks out the so-called “condenser current.” Over considerable portions of the potential range the condenser current increases almost linearly; hence an increasing potential of opposite sign applied to the recorder can remove most of the effect of this condenser current. A device known as a “curve follower” is incorporated for the purpose of subtracting the blank due to the supporting electrolyte. Without such a device, the blank effects can sometimes completely disguise the wave due to a very small concentration of a reducible ion. The instrument is built in three separate units connected as shown in the block diagram in Figure 1. The polarizing unit TO POLAROGRAPHIC C E L L
tl
0
c
POLARIZING UNIT
-
is conventional, with the exception of the “residual current compensation” mentioned. Figure 1 shows the potential source, E,, uith the motor-driven voltage divider. Resistor R, determines the current sensitivity, while resistors R,I and R,z determine the extent of linear compensation applied to counteract the condenser current. C D and RD form an RC damping circuit to reduce the magnitude of recorder oscillations, especially a t high sensitivities. The block diagram shows only the slide-wire of the curve follower, which is described in more detail be lo^. The Brown recorder is conventional except for the preamplifier. The polarizing unit is shown in detail in Figure 2. A battery, El, supplies a fixed potential, measured on voltmeter Vl, to a motor-driven Helipot, &A, from which the polarizing voltage fqr the polarographic cell is drawn. The 10-turn Helipot, &A, is turned by a 1 r.p.m. Bodine motor, with provisions for reversing and a friction clutch to allow7 a quick return to zero potential. A second potential source, E*, can be used to supply an initial potential up to 3 volts, either positive or negative. Thus the total useful voltage range is from +3 to -4.5 volts. The current through the polarographic cell is measured by recording the potential drop in a series of standard ( & l % ) resistors, Rn to R22. Damping is supplied by the RC combination, Re and C1, with a time constant up t o 20 seconds. The network between the two large electrolytic condensers, C1 and C3,provides a means of setting the zero of the instrument and also is useful to buck out constant currents due to causes other than the diffusion current of the ion desired. The range of RIO (a 200-ohm Helipot) is about 30 times the full scale deflection of the recorder. The true zero is a t midscale or about 5.0 on the Helipot dial. The remaining resistor network, consisting of R?B,R,, Ra, and to Rao, constitutes a linear compensator to buck out the “condenser” current a t high current sensitivities with the d r o p ping mercury electrode. With Ra out of the circuit (on point 1 of switch Sa)and S3closed, this network supplies a potential to the recorder which is opposite in sign to the normal component of cell current and increases with the increase of potential from (which is directly coupled to R*A). The degree of compensation can be adjusted by setting R3 (a 10-turn Helipot), or the compensation can be eliminated by opening 83. Rc can be shunted across three different fixed taps on R?B (at 2, 4, or 6 turns) so that correction can be applied according to two different linear formulas,,- changing at- a preselected point. To date, this feature has not proved very useful, because the blank curves do not show abrupt changes of slope which can be closely approximated by two linear curves. The curve follower has proved more useful to compensate for nonlinearity in the blanks.
I SLIDE WIRE
CURVE FOLLOWER
BROWN RECORDER
Figure 1. Block Diagram of Polarograph
The recorder used in this instrument i8 a Brown recorder modified by the addition of a preamplifier (Figure 3) as described by Rosenbaum and Stanton (4). The circuit of Rosenbaum and Stanton has been changed in the f o l l o ~ ~ ing respects:
J
Figure 2. ORNL Polarograph Polarizing Unit
V O L U M E 2 4 , NO. 1 2 , D E C E M B E R 1 9 5 2
1897
TO T E R M I N A L ' E " ON MAIN TERM STRIP IN BROWN RECORDER
TO TERMINAL'B" ON MAIN TERM, STRIP IN BROWN RECORDER
-
'/4 o
ah
w
BROWN
R I
470
IW
L-----d'
Polarographic Diffusion Current of Lead in Very Dilute Solutions [Dropping mercury electrode
0
__
e-
I t2OOV
c 3
-
C ID
20pt
I t2oOv -
0.47pf
450"
WHITE
The input resistor is changed to 1.2 megohms, as that is sufficiently high for this purpose and less undesirable pickup results with the lower resistor. Along with this the grid resistor of the first stage is changed to 3.3 megohms and the coupling condenser to 0.05 pfd. to maintain the same time constant as the original circuit. A second twin-triode (12AX7) is added in the preamplifier, as it was found that the signal level after the first two stages was 80 low that appreciable pickup occurred in transferring to the Brown amplifier a t this point. By transferring the function of
Pb + + Conon.
O.lpf
2700 2 v
Cla
Table I.
5000 4 w
R2
c2
I
20pt 450"
TO TERM1NAL"F" ON MAIN TERM. STRIP IN BROWN RECORDER
40m0
R 3
I
(m2/3tl/48
Diffusion Current, Micromoles/Liier pa. X l o 3 0.24 1.33 0.48 2.83 0.72 3.56 1.20 5.44 2.39 11.48 7.20 39.4 72" 568 7200 5830 RUB without curve follower.
= 2.17) ] 'd/C
aa./Miliimdle/Liter 5.54 5.90 4.94 4.53 4.80 5.47 7.89 8.10
the first 7F7 in the Brown amplifier to the 12AX7 in the preamplifier alternating current pickup difficulties were greatly reduced and the modifications necessary in the regular Brown amplifier were reduced to installing a plug connecting to pin 6 of the first 7F7 (which is removed). As in any other high-gain amplifier, alternating current pickup elimination was troublesome. The two chief precautions observed were the use of a single ground point on the chassis for all grounds in the first two stages, and the use of a Brown converter (chopper) which had the connections for the driving coil entering from the top rather than through the usual pin connections in the base. It was found that 12AU7 tubes manufactured by General Electric were less microphonic than others tried. 12AY7 tubes made by this manufacturer can be used interchangeably in the first stages. Figure 4 shows a very simple circuit used to introduce more damping on the motor of the Brown recorder, as this was found desirable to eliminate overshoot when a high speed recorder was used. The curve follower is used only a t the higher sensitivities for the subtraction of the blank curve from that obtained on a sample. This device, first described by Coor and Smith (I), is constructed by modifying a Brown recorder as indicated in Figures 5 and 6 .
ANALYTICAL CHEMISTRY
1898 The pen of the Brown recorder is replaced by a photocell unit which "observes" the chart through a microscope objective. The photocell amplifier is adjusted so that the output is zero when the photocell sees an area half black and half white. When the field is predominantly black, the output drives the unit to the right, while a predominance of white drives it to the left. The photocell unit thus will follow the right-hand edge of a broad black line. A suitable curve is derived from the polarogram obtained on a blank by tracing over the tips of the oscillations onto a sheet of unruled recorder paper by use of a light box or a similar technique. If this curve is then run through the curve follower a t the same time that a polarogram is being recorded, the curve follower subtracts a t each point a potential equal to the ordinate of the blank curve. I n this way a blank curve which is far from linear can be made to approximate a straight line. APPLICATION OF INSTRUMENT
Some typical blank curves are shown in Figure 7. Curve A shows a blank on 1 M potassium nitrate with no compensation of any kind, It is obvious that no polarographic waves could be measured on this sort of a curve. Curve B shows the same blank E'
CURVE FCILCWER CXT
BROWM AMPLIFIER
I
Eo
0
UNB4LANCEO VOLTAGE
E"
SC TEILNLG
P
IN
rn
t
OUT
0-1 rn" ERROR
EC BROWN RECORDER CHART ~ F O , V TO , RDINAR~LY CONNECTED TO BROhN AMP INPUT
USING CURVE FOLLOWER A N 0 COMPENSATOR
Figure 5. Block Diagram of Curve Follower
I
-0 6
I
-05
I
-04
I
-03
I
-02
I
-0 I
0
VOLTS
Figure 7. Polarograms Illustrating Use of Compensator and Curve Follower at Very Low Concentrations
12
P 40332625 S E L RECT
/
IOOOmmf MICA
-87.5 4 W + 54VOC
I
I
BI
u 2.4
I UNBAL4NCED
L1 AND L Z ARE , 5 VAC. 2 5 O m a , 1TE.M*51-79
Figure 6. Circuit Diagram for Curve Follower
MICROMOLES/LITER Pb+'
Figure 8. Relationship of Diffusion Current and Lead Concentration in Potassium Nitrate
V O L U M E 2 4 , NO. 12, D E C E M B E R 1 9 5 2 a t the appropriate setting of the residual current compensator. Here reduction waves could be measured on some linear portions of the curve, but this blank still leaves much to be desired. Curve C shows the result obtained when a trace derived from curve B is used on the curve follower. Here the blank is fairly linear from zero to a potential of -0.5 volt. The instrument has been applied to the determination of lead in a potassium nitrate medium. As shown in Figure 7 by using the appropriate compensating curve on the curve follower, a blank curve approaching a straight line can be obtained. Using this same compensating curve, polarograms were drawn on a series of lead concentrations in 1 M potassium nitrate ranging from 0.2 X M to 2.0 X 10-6 M . One of these curves is shown in Figure 7 (curve D) and the measured diffusion currents (measured by extrapolation to the half-wave potential) are tabulated in Table I. The same data are shown in graphical form in Figure 8. The figures in Table I show that the diffusion current a t these low roncentrations is very nearly a linear function of concentration. The precision of measurement is relatively poor, but it is
1899 hoped that further study of the factors influencing polarography in these very dilute solutions will permit an improvement in precision to a t least & 10%. The figures a t higher concentrations are given in Table I to show that the diffusion current constant, id/^, appears to change with large concentration changes but to be linear over a small range. The diffusion constant predicted by the IlkoviE equation (3) for this electrode was 8.14 pa. per millimole per liter, agreeing very well with 8.10 observed a t 0.72 X 10-3 M . LITERATURE CITED
(1) Coor, Thomas, Jr., and Smith, D. C., Rev. Sci. Instrurne?its, 18, 173-81 (1947). (2) Ilkovi?, D., and Semerano, G., Collection Czechoslov. Chem. Communs., 4, 176 (1932). (3) Kolthoff, I. M,, and Lingane, J. J., “Polarography,” New York, Interscience Publishers, 1941. (4) Rosenbaum, F. J., and Stanton, L., ANAL.CHEM., 19, 794-8 (1947). RECEIVED for review July 25, 1952. Accepted August 29, 1952. Presented before t h e Division of Analytical Chemistry a t t h e 118th Meeting of t h e AMERICASCHEMICAL SOCIETY,Chicago, Ill.
Determination of Small Amounts of Hydrocarbons in the Atmosphere PAUL P. MADER, MERLYN W. HEDDON, ROBERT T. LOFBERG, AND RUTH H. KOEHLER Los Angeles County .4ir Pollution Control District, Los Angeles, Calif.
h spectrophotometric method for determining small concentrations of hydrocarbons in the atmosphere has been developed. It employs the use of a specially built 100-cm. gas cell supplied with rock salt windows and inserted between the light source compartment and receiver compartment of a Beckman infrared spectrophotometer. Because of the large volume of the new cell, the entire gas sample can be swept into the light path of the instrument. The hydrocarbon sample is collected by a freeze-out tech-
T
HE;studies made by the Los Angeles Air Pollution Control District show hydrocarbons to be the largest single group of gaseous and vapor phase contaminants that are being discharged into the atmosphere. Olefins, branched-chain, and perhaps other hydrocarbons represent the original raw materials u hich, in the presence of air oxygen, ultraviolet radiation, ozone, and oxides of nitrogen, may be converted into oxygenated products ( f - 3 , 6). These hydrocarbons in their oxidized form contribute substantially to the smog conditions prevalent in the I,os Angeles area in days of low wind speeds and temperature inversion. I t has been demonstrated that oxidized forms of hydrocarbons reduce the visibility ( 3 , 5 ) in the affected area and produce eye, nose, and throat irritation, as ell as damage to leafy vegetables ( 2 , 4 ) . In view of the significant effects caused by the hydrocarbon o\idation products, it became desirable to find a method for the determination of small amounts of hydrocarbons in the atmosphere a t various times of the day. Specific measurements were needed in air pollution investigations to establish the correlation M hich might exist between the absolute amounts of hydrocarbons found in the atmosphere and the harmful effects experienced by humans and observed on vegetation.
nique, using a Shepherd sampler immersed in liquid oxygen. Based on the collection of the condensates of a 60-liter air sample, the instrument records the presence of 23 micrograms of hexane, or less than 0.1 p.p.m. of this hydrocarbon. The results obtained for known hydrocarbon concentrations are accurate and consistent. This method was successfully applied to the determination of hydrocarbons from air streams over polluted areas or inside industrial establishments.
Previous methods used for determination of minute amounts of hydrocarbons are based on the combustion of the collected sample to carbon dioxide, which is absorbed in an excess of barium hydroxide. The excess of barium hydroxide is back-titrated with a standard hydrochloric acid solution. The amount of carbon dioxide developed corresponds to the hydrocarbon content of the sample. The disadvantage of this method lies in the fact that the determination is executed against a background of 300 p.p.m. of carbon dioxide in the surrounding air, while the amounts of hydrocarbons evpected to be present in the atomosphere may be of the order of a few parts per million only. Any loose connection in the combustion train assembly or any air contact of the titration vessel receiving the generated carbon dioxide will lead to erroneous results. Frequent “blank” determinations must be performed. JThenever the combustion train is not in use for several days, it is necessary to heat the entire assembly for several hours to drive all accumulated carbon dioxide from the combustion tubes and the rest of the system. I n spite of these inherent shortcomings, this method, laborious and timeconsuming as it is, can still be used by a skillful and patient operator. -4fairly simple and straightforward method for determination
