it is 7 X lop9 microcurie per nil. An increase in sensitivity b y a factor of 100 can be achieved with the greater geometry of a n internal proportional counter and the decreased background made possible b y special shielding and anticoincidence circuits. $t these low concentrations, however, radioactive contamination of reagents becomes a serious problem, and it is necessary to run blanks and to subtract their count rate from the sample count rates. ACKNOWLEDGMENT
The authors would like t o thank J. H. Edgerton for performing many of the ionic analyses.
LITERATURE CITED
Brooksbank, W,A,, Emmons, A. H., Cost, J. IT., Reynolds, S. A, Atomic Energy Commission Rept. ORNL-816 (1950). Collins, IT’. D., Lamar, TV. L., Lohr, E. IY., U. S. Geological Survey, Water Supply Paper 658, 55 (1932). Corl-ell, C. D., Sugarman, S . , eds., “Radiochemical,3udies: The Fission Products, pp, 1417-757, McGra\v-Hill, New York, 1951. Ibid.. w. 1459. Ibid.; b. 1642. Duncan, J. F., Johns, T. F., Johnson, K. D. B., IlcKay, H. A. C., Maton W. R. E., Pike, E. IT. A , , Walton, G. X,, J . SOC.Chcm. I n d . (London) 69, 25 (1950). Emmons. Ai.H., Lauderdale, R. A., .Vucleonics IO, S o . 6. 22 (1952).
(8) Geilmann, IT.,Gebaur, K , 2. a n d . Chern. 139. 161 (1953). ’ ( 9 ) Ibid.,142, 241 (19i4). (10) Hahn, R. B., Straub, C. P., J . .lnz. Tl’ater Tt’orks A4ssoc. 47, 335
(1955). (11) Iileinberg, J., ed., Atomic Energy Commission Rept. LA-1721 (1954). (12) O’Leary, W. J., Papish, J., I s u . ESG. CHEX, ASAL. ED. 6, 107 (1‘334).
(13) Ring, S. A , , ASAL. CHEM 28, 1200 (1956). (14) Setter, L. R., Goldin, 4.S., Nader, J. S.,Zhid., 26, 1304 (1954). (15) Kilkinwn, G., Grummitt, iV. E., Siicleonics 9. So. 3. 52 (1951). (16) Yogoda, H., Partridge, H. h l . , J . -4m. Chem. SOC.52, 4857 (1930).
RECEIVED for review October 24, 1956. Accepted March 20, 1957.
Calibration of an Integrating Motor for CouIometric Titrations N. H.
FURMAN and A. J. FENTON, Jr.
Frick Chemical Laborafory, Princefon University, Princefon,
Extensive calibration of an integrating motor revealed that an empirical relationship exists between the motor calibration factor and the value of the series resistor. This relationship was found to b e linear over a reasonably wide range of current levels and resistances. Constancy of motor factor calibration with time was found upon successive calibration trials. The use of such an empirical relationship permits calculation of motor factors a t many combinations of resistances and current levels with an estimated accuracy of 1 to 2 parts per thousand. The only prerequisite is two direct measurements of the motor factor calibration.
R
a low inertia integrating motor has been reported to be useful and accurate as a current integrator in coulometric titrations ( 2 , 3). The motor is designed as a voltage integrator; hence in current integration applications the voltage drop across a series resistor is fed directly into the motor terminals. Calibration of the motor at each desired current level is necessary; this constitutes a major drawback to its usefulness in more than routine work. The authors’ experience with this integrating motor indicates, however, that a single relationship exists between the value of the parallel resistor and the motor factor calibration. An empiriECEXTLT
N. J .
cal equation may be calculated and is valid over a wide range of current levels, if the voltage input is within the linear velocity region of the motor. Amick (1) in this laboratory has shown that motor velocity is directly proportional t o input voltage in the range 12 to 24 volts.
ri
APPARATUS
The electrical circuit used throughout this investigation is shown in Figure 1. The constant current s u_m- ~” l vhas been described (4, 5 ) . The motor integrator is Nodel 903. Electro Method< Ltd., Stevenage; Herts, England. This model has a nominal ~ o l t a g e of 24 volts. Other models having nominal voltages of 1.5, 6, and 12 volts are available from the manufacturer. A \\ ooden motor mounting was used to eliminate niagnetic effects. The resistances of the decade box (Otto Koulf, Berlin) were checked against precision resistors (0.05y0tolerance, General Radio Co., Cambridge, Mass.). The values of the decade box resistances mere found to be within the tolerance limits of the precision resistors. The synchronous motor clock was Model S-6, Standard Time Co. The current levels were accurately determined b y measuring the voltage drop across a suitable precision resistor (General Radio Co., 0.05% tolerance) with a Leeds & Northrup potentiometer (Catalog KO. 7 6 5 5 ) .
MOTOR
120 v A C. Figure 1 . Schematic diagram of electrical circuit
R,. Suitable precision resistor R?. Decade box (I - 9,999 s! RB. 600 carbon resistor 8,. DPDT toggle switch
PROCEDURE
Both the clock and integrating motor were started and stopped simultaneously when the starting switch was closed. From the measured motor count and elapsed time at a constant current, the motor factor was calculated from the relationship, factor (milliequivalents per count) = i (ma.) x t (seconds)/96,5OO X No. of counts. VOL. 29, NO. 8, AUGUST 1957
1213
Table 1.
KO
of
Date
Trials
2/13/56 11/16/56
3 3 6 1 1 1 2 1 6 3 3 6 3
Av .
2/29/56 3/ 2/56 3/ 6/56 3) 7/56 3/10/56
Av .
2/18/56 11/16/56
Av. 2/23/56
Calibration Data for an Integrating Motor
Resistance, Ohms
Current Level, Ma.
10,000 10 ,000
2.170 1.795
4,000 4,000 4,000 4,000 4,000
5.066 5.086 5.032 5.035 5.049
2,500 2,500
8.090 8.304
2 ,000
Exptl. Factor, Meq./ Count
x
10.67
108 0.1380 0.1382 0.1381 0.3421 0.3412 0.3416 0.3411 0.3413 0.3414 0.5447 0.5443 0.5445 0.6794
Av. Devn. between
Trials, P.P.T.
*l.O 0.7 0.9
0.1381
...
...
-0.06
0.90 &O 0.5 0.3 0.8
0.3412
0.9054
+o. 04 ...
1.357
-0.07
1.695 2.711
+ O . 06
4
1,500
13.82
0.9054
0.3
1,000 1,000 1,000
20.73 20.88 21.62
800 800 800
27.22 27.70 27.63
500
40.22
1.354 1.357 1.356 1.356 1.691 1.696 1.696 1.694 2.711
1.3 0.7 0.7 0.9 1.1 1.0 0.3 0.8 0.6
Av.
7%
0.5
3 5 3 11 3 3 3 9 4
11/16/56
103
...
2/18/56
Av.
x
... ...
2/12/56 3/14/56 11/16/56 . . 2/14/56 3/10/56 3/14/56
Theoretical Factor, Diff. in Me%/ Count Factor,
0.5443 0.6797
+ O . 04
...
Average deviation of six results shown.
At least 100 motor counts or 600 seconds were used in each calibration run. The leads to the electrolysis cell were either shorted or the current was discharged through a 1000-ohm resistance. No difference in result was observed. RESULTS
Table I shows the theoretical and actual motor factors obtained. The theoretical factors were calculated from the relationship :
(meq. per count)
-*:"- + 2.7 x 10-6
where R is the ohmic value of the parallel resistance. This relationship is b, where x is of the type, y = mx taken as 1/R, 1.354 is the slope, m, determined by the ratio of two points; and 0.0027 is a constant. The constancy of a given motor factor calibration with time is very good to excellent up to a t least 9 months. Variations are largely due to inconstancy
+
of line frequency and voltage, neither of which was controlled during these measurements. The effect of temperature variation is unknown, as all measurements were done a t room temperature in order to simplify the apparatus as much as possible. DISCUSSION
An empirical relationship of this type greatly extends the usefulness of this integrating motor, as lengthy calibration a t many current levels is unnecessary. Calibration a t two current levels employing two different resistors should be sufficient to establish the motor constants. By a simple calculation the motor factor may be accurately determined for any combination of resistance setting and current level. The only additional criterion is that the voltage input to the motor be within the linear velocity region. If the necessary motor calibrations are obtained with apparatus that is independent of current or frequency fluctuations, the motor integrator should vield titration results indeDendent of these common sources of error. LITERATURE CITED
Amick, R. M., senior thesis, Princeton University, May 1955. Bett, N., Nock, W., Morris, C., Analyst 79, 607 (1954). Parsons, J. S., Seaman, W., Amick, R. M., ANAL.CHEW27,1754 (1955). Reilley, C. N., Adams, R. N., Furman, N. H., Ibid., 24, 1044 (1952). Reilley, C. N., Cooke, W. D., Furman, N. H., Ibid., 23,1030 (1951). RECEIVEDfor review August 6, 1956. Accepted January 23, 1957.
Colorimetric Determination of 1,2-PropanedioI and Related Compounds LAWRENCE R. JONES and JOHN A. RlDDlCK Commercial Solvents Corp., Terre Haute, Ind. Propylene glycol dehydrates and rearranges in concentrated sulfuric acid to a mixture of allyl alcohol and the enolic form of propionaldehyde. The mixture forms a violet-colored complex with ninhydrin (triketohydrindene hydrate) in a strong sulfuric acid solution. The color is suitable for quantitative measurement of propylene glycol at a wave length of 595 mp. It follows Beer's law in the range of 5 to 50 y and has an expected
1214
ANALYTICAL CHEMISTRY
accuracy within +2% and a precision within k l % . This colored complex is specific for propylene glycol and its polymers in mixtures of glycols.
M
published methods on the determination of propylene glycol (1,%propanediol) have been modifications of the Malaprade (19) reaction, which involves oxidation of the vicinal glycols with periodic acid to aldehydes, OST
followed by various means of detecting the aldehydes. Warshowsky and Elving (16) and Cannon and Jackson (2) used the polarograph, Braumel(1) ultraviolet absorbance, and Reinke and Luce ( I S ) and Hoepe and Treadwell (8) volumetric procedures to measure the acetaldehyde. Dal Nogare, Norris, and Mitchell (5) and Jordan and Hatch (10) determined acetaldehyde as iodoform, while Desnuelle and Naudet (6) used the colorimetric procedures of Schryver (1.4)
