for a 90-mg. soil sample would be approximately 0.2%. The presence of 8% gibbsite in a soil would result in an absolute error of 2.8%. Allowances would have to be made in the case of tropical soils high in gibbsite, diaspore, bauxite, or bayerite. Figure 8 shows thermogravimetric curves for a number of silicate minerals and gypsum. Biotite, tremolite, and vermiculite (curves 1, 2, and 3) show high temperature losses. The presence of appreciable amounts of the first two minerals would result in inaccurate carbonate values. Large amounts of allophane would give erroneous results for organic matter. For subsoils developed on saline parent materials containing 1 to 2% gypsum, the absolute error in the hygroscopic moisture determination on a 90-mg. sample would range from 0.2 to 0.4%. Jt7here crystals of gypsum can be detected in the soil, (Bcshorizon), by the naked eye, corrections in the hygroscopic moisture values should be made. The presence of 20% illite would cause a positive absolute error in the organic matter determination of approximately 0.6%. While kaolinite and bentonite would not interfere with organic matter determinations, if present a t the 20% level, they could cause absolute positive errors in carbonate measurements of 1.5 and 0.770, respectively. However, kaolinite is seldom a significant constituent of calcareous soils. Hydrous silicates such as biotite, vermiculite, and allophane (Figure 8, curves 1, 3, and 4) when present in large amounts might cause high values for hygroscopic moisture. The data on a variety of soils, as taken directly from thermogravimetric curves, agreed satisfactorily with those obtained by established chemical methods. Where it is known that large amounts of interfering constituents such as those mentioned are present, caution should be exercised in interpreting the thermogravimetric curves.
SUMMARY
The pyrolysis curves of most soils examined showed plateaus starting a t 150’ to 180’ and extending to 210” to 240’ C., indicative of either hygroscopic moisture, or hygroscopic moisture plus easily volatile organic compounds. I n general, thermogravimetric values fell in between those obtained by the Karl Fischer titration and oven-drying a t 105’ C. This aspect needs further clarification. Organic matter started to burn off between 210’ and 240” C. I t s pyrolysis was usually complete a t 500” C. I n organic soils and those containing less than 15% clay, a relatively close estimate of the organic matter content rould be made from a pyrolysis curve. K h e n the clay content varied from 15 to 40%, the loss in weight a t 500’ C., read from a pyrolysis curve, usually gave an estimate of the organic matter content which was in satisfactory agreement R ith dry combustion and wet oxidation data. When soils contained more than 40% clay, it was not possible to distinguish between Lveight losses due to the pyrolysis of organic matter and those due to the elimination of the lattice water of clays. The thermobalance has proved useful for the detection and quantitative estimation of carbonates in pure carbonates, in soils to which carbonates were added, and in soils in which they occurred naturally. The data suggest the possibility that lattice water may be quantitatively determined at least in pure clays and in simple mixtures. Because lattice water came off at different temperatures with different clays, it may be possible to use this characteristic as an additional means of identification. -4 preliminary thermogravimetric study of a soil will give valuable information as to its constitution. I n addition, the thermobalance has proved a useful tool for the intelligent definition of
conditions of analysis by other methods. ACKNOWLEDGMENT
The authors are indebted to the National Soil Survey Committee (Canada) for supplying most of the soil samples and analytical data on clay contents, and to Ralph Grim of the University of Illinois for supplying the kaolinite. They also thank Richard Levick for technical assistance. LITERATURE CITED
(1) ,Assoc. of Offic. Agr. Chemists, “Offi-
cial .\lethods of .4nalysis,” 8th ed., p. 28, Washington 4, D. C., 1955. (2) Rarlow, J. W., Can. Food I n d . 28, 11 (1952). (3) Bouyoucos, G. J., Soil Sci. 42, 225 (1936). (4) ,Chemistry Division, Science Service, Canada Department of Agriculture, “Chemical Methods of Soil Analysis,!’ rev. ed., Otta,wa, 1949. ( 5 ) Dwal, C., “Inorganic Thermogravimetric Analysis,” p. 31, Elsevier, Xex York, 1953. (6) Gibaud, hfichelle, Geloso, 31. hf., Chinz. anal. 36, 153 (1954). (7) Hoffman, I., Schnitzer, M., Kright, J. R., Chem. & I n d . ( L o n d o n ) 261, 1958.
(8) Mahin, E. G., Carr, R. H., “Quantitative iigricultural Analysis,” 1st ed., pp. 249-51, McGraw-Hill, Xetv York, 1923. (9) Ibid., pp. 81-5, 249. (10) National Soil Survey Committee (Canada), unpublished report. (11) Neuss, J. D., O’Brien, AI. G., Frediani, H. A., A N ~ L .CHEU. 23, 1332 (1951). (12) Peech, Michael, Alexander, L. T., Dean, L. A., Reed, J. F., “AIet’hods of Soil Analvsis for Soil-Fertilitv Investigations,” U. S. Dept. Agr. ”Circ. 757, 6 (1947). (13) Van Slyke, D. D., Folch, Jordi, J . Biol. Chem. 136, 509 (19401. (14) Waksman, S. A., “Humus,” 1st ed., p. 243, Baillikre, Tindall and Cox, London W. C. 2, 1936. RECEIVEDfor review July 29, 1957. Accepted October.27, 1958. Contribution KO. 372, Chemistry Division, Science Service.
Transistorized Switch for Derivative Polarography JAN KRUGERS’ laborafory for Analytical Chemistry, University of Amsterdam, Amsterdam, Holland
b The mechanical switching mechanism suggested by Ishibashi and Fujinaga for use in derivative polarography may b e superseded to advantage by an electronic switching system. Several modifications are suggested which may be important in connection with theoretical considerations and trace analysis. A more general use of the circuit is for converting a small direct 444
ANALYTICAL CHEMISTRY
current into an alternating current of any desired frequency between 0.1 cycles per second and 10 kilocycles per second.
I
method of derivative polarography, described by Ishibashi and Fujinaga (I), the voltage applied to the polarographic cell alternates between E and E - AE volts many times per secN THE
ond. 4 t the same time the connections of the galvanometer are reversed. Both current paths required to enable this are shown in Figure 1. Obvlously, when hE is zero, both currents have the same value. Their effects on the gahyanometer system cancel 1 Present address, T. N. 0. Studiecentrum Kernenergie (BRl), Mol, Belgium.
out and the reading of the meter will be zero. If, however, a suitable value of AE is applied a t E values corresponding to the sloping parts of the i-E curve, both currents do not cancel out. The value of the meter readings depends on both &!i' and the slope of the i-E graph. An i-E curve n h e n @.Eis constant is represented in Figure 2. The peak x alue, io, of the resulting current is situated a t EO which is near or a t the El,l value of the constituent under investigation. The concentration of this constituent is connected with the height of the peak. The method has two important ndvantages in comparison with classic polarography. Because the currents obtained in this \\-ay are several times larger than those obtained in other polarographic methods, the method is very sensitive. I n the second place i t is possible t o get two entirely separated peaks or indications of two peaks when other methods do not give two clearly separated steps. Hon.ever, the method has not found niany applications. The complicated snitching mechanism is a severe dratvback. The commutator used by Ishibashi and Fujinaga to realize the two currrnt paths of Figure 1 t n enty tinies per second is represented in Figure 3. The cylinder is rotated b y a synchronous motor a t about 1200 r.p.m. (the frequeiicj then is 20 cycles per second). The b h c k parts represent copper strips and the small circles the collectors. The letters correspond to those of Figure 3 . During one half cycle a connection is made betn een collectors a and e , as well as betveen I: and d. The current path through switches 1 and 2 of Figure 4 is then obtained. During the other half of the revolution a and b and also d and f , are connected. The current flows then through !SI itches 4 and 3, resulting in an applied voltage E - AE, while the connections of the galvanometer are also reversed. The PIT itching mechanism consisting of synchronous motor, commutator, and six collectors is complicated; the results are not always very good. K i t h the small voltages and currents used in polarography, the collectors represent a resistance, nhich is not alnays reproducible. Although a resistance up to 10 kiloohms is admissible (because the currents are small up to 50 pa.), this resistance has to be constant to ensure reproducible results. With very good collectors and careful maintenance it is possible to obtain satisfactory results p i t h this switching mechanism. Another disadvantage is the impossibility of changing the frequency n-ithin wide limits. The velocity of rotation is in general between 600 and 3000 r.p.m. Each current path is then realized 10 to 50 times per second. For theoretical considerations i t may be necessary to
change the frequency within wider limits. The electronic circuit described below fulfills all the functions of the mechanical switching mechanism. A special advantage is the constant contact resistance. This is also realized in a semielectronic switching mechanism ( 2 ) . The fully electronic circuit has the advantage of smaller dimensions and the
Figure 1.
Two current paths
Figure 2. i - E curve obtained in derivative polarography
Figure 3. Cylinder coupled with shaft of a synchronous motor This realizes the alteration of the two current paths of Figure 1
-
g:
E volt a E volt %+
Figure 4.
+ I
possibility of stepless changes of the frequency viithin wide limits; besides, there is little maintenance necessary. THE CIRCUIT
The same circuit as shown in Figure 4 is used; hoTvever, the switches are replaced by transistors, which are used as electronically controlled switches (Figure 5 ) . I n Figure 5 4 , the base of the transistor is negative with respect to the emitter, permitting a current, to flon through the emitter-base junction. To protect the transistor against damage, a limiting resistor, R, is used. For the OC 71 transistor (Philips or Ilullard) the resistance between emit,ter and collector is a fevv kiloohms hen R is 100 kiloohms. I n Figure 5,B. the base of the bransistor is positive n.ith respect to the emitter. The current noiv flows from the base to the emitter. For this current direction the resistance of the emitter-base junction is 1-ery large and the current will accordingly be very sinall. Consequent'ly R may be omitted here. For the same p-n-p transistor the resistance between emitter and collect'or is n o n about 1 megohm a t 25' C. TVit'li higher ambient temperatures the resistance is smaller. This reasoning shows that a transistor may be used as a switch offering not infinite resistance, but a high resistance in the "off" position. B y changing the sign of the emitter-base yoltage the transistor changes to the "on!' position. If the switching has to be performed many times per second, an electric signal is required, making the emitter alternately 1 volt positive and 1 volt negative v i t h respect to the base and with abrupt' changes from one into the ot'her condition. The necessary electric signal is a square wave. K h e n two square waves are used 180' out of phase, it is possible to "open" one transistor and to "close" the other one simultaneously, because
,
Current paths
. .... ..-..
.
One current path is realized when switches 1 and 2 (connections e-a and c-d, Figure 3 ) are closed. The other current path i s realized when switches 3 and 4 (connections d-f and a-b, Figure 3) are closed
+
coll tor
Figure 5. A. E.
Transistor positions
p-n-p transistor in conducting "on" position. p-n-p transistor in nonconducting "off" position.
Figure 6. Circuit of Figure 4 with switches replaced by transistors Transistors: OC 71, R:lOO-1000 ohms. IO-mv. Brown recorder (pen speed, 2 seconds)
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VOL. 31, NO. 3, MARCH 1 9 5 9
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I
Figure 7. genera tor
t
0
Square
wave
In combination with circuit of Figure 6 an electronic switch i s formed which fully supersedes mechanical switch of Figure 3. Frequency, about 20 cycles per second. Transistors. O C 71
vhen one square wave changes froni positive to negative the other changes from negatiye to positive. In Figure 6 the switches of Figure 4 are replaced b y transistor switches. The three limiting resistors are connected to the emitters, and a t the other side, are tied together. This is the common point for the two square waves. The base connections are connected two by two. This forms the other two connections for the two square waves. If C is negative with respect to A , current will be passed by transistors 1 and 2. The current path is printed in solid lines. At the same time B is positive with respect to A . This means that transistors 3 and 4 do not conduct the current. The path represented by the broken line is forbidden. During the next half cycle the reverse holds true. This circuit switches the current in exactly the same way as the mechanical switch. The frequency of this electronic switch is determined by that of the square wave generator. The circuit of this generator is given in Figure 7 . Transistors are also used in this part. It is a conventional bistable multivibrator circuit. The square waves appearing across the resistors of 1500 ohms are 180" out of phase, permitting their use for driving the transistor sn-itches of Figure 6. However, the voltages of these square waves change from 0 to 2 volts. The changes in voltage have to be between +1 and - 1 volt. Therefore a t a p of a voltage-divider is used as a common point (connection A). The voltage a t points B and C are alternately 1 volt higher and loner than a t point A. The six transistors used in this circuit are mounted in a small box nhich is thermostated a t 25" C. with a simple temperature-sensitive relay. This is important for the generator, the frequency of which is temperature-dependent. The height of the peak in the i - E curve is slightly dependent on the
446
ANALYTICAL CHEMISTRY
Figure 8,A. Symmetric square wave produced by oscillator of Figure 7 Resistors in two collector circuits are both 1500-ohm
6. Asymmetric square wave of same oscillator One of 1500-ohm resistors i s replaced by one of 150 ohms
switching frequency. For highly accurate a ork the frequency has to be constant, thus requiring better thermostating. This will be obtained by mounting the two transistors of the generator on a heat sink. MODIFICATIONS OF CIRCUIT
The frequency of the generator is determined b y the two capacitors and the two resistors of 36 kiloohms. When OC 71's are used, the frequency is about 20 cycles per second. I n actual practice this has been proved the best frequency. It is possible to change the frequency. When the capacitors are changed from 16 to 0.03 pf. the frequency changes from about 1 to 600 cycles per second. The resistors niay be changed from 30 to 60 kiloohms. \Then the d u e s of the resistors are outside these limits, the square 11-ave !\-ill be deformed. However, the capacitors niay be changed \Tithin wider limits than mentioned above. For derivative polarography this is not required. If the capacitors and resistors are equal, the square wave is entirely symmetrical. For theoretical considerations it may be necessary to change the synimetrv of the square wave, b y diminishing one of the 1500-ohm resistors. The safe lower h i t is 150 ohms. With a still lower resistance the current through the transistor may become too large. Figure 8,B, gives the pattern of the square wave nhen the resistor is diminished to a value of 150 ohms. The pattern of the square wave in the symmetrical case is given in Figure 8.A.
0
Figure 9. Influence of frequency on i-E curves Polorograms made with Brown recorder as indicated in Figure 6. Solution 3.75 X 1 0-4M CdCh per 0 . 2 N KCI. A€ = 100 mv. € varies from right to left from 0.5 to 1.O volt. Frequencies from top downwards. 317.4, 66.4, 31.2, 14.8, 0.73 cycles per second
Figure 10. Influence of frequency on peak height Polarograms made with Brown recorder. Solutions: 5 X 10-4M ZnSO? per 0.2N KCI and 3.75 X 1 O-4M CdCll per 0.2N KCI. AE = 100 mv.
A very useful niodification 1- a s ~ i t c h n hich short-circuits the emitter-base connection of one of the generator transistors. The generator 12 bloched in one of its stable positions On11 one current path is no\\ possible, g i ~ i n ga classic polarogram. For absolute measurenients it is necessary to kno\$ the cell voltage euactly. I n this case the measured cell voltage
Figure 1 1 . Influence of asymmetry of square wave Solution 3.75 X 1 0-4M CdCI? per 0 . 2 N KCI. 3
3
AF.100 mv. Frequency 500 cycles per second. E varies from right to left from 0.5 to 1.0 volt. Upper curve corresponds to square wave of Figure 8,A. tower curve to Figure 8,B
'
3
has to be corrected for the voltage drop across the transistors. t-sing transistors, with a loner resistance in a conducting condition (such as GE Type ZKl88A) will reduce this correction. The current losses b y the off resistance of the transistors may be too large in absolute measurements and in determinations where the currents are very small (belon 1 l a . ) , as in trace analysis. By uqiiig silicon transistors with a much higher off resistance, this difficulty may bc overcome. RESULTS
!
-1'
.
n
1
-
Varying the frequency \\ill vary the polarogranis obtained. As shoun in Figure 9, the oscillations caused by the dropping mercury electrode become more pronounced a t higher frequencies. The peak height is smaller a t higher and a t very lon frequencies. A frequency of about 20 cycles per second is the best compromise. Although a slightly higher frequency may be chosen. the influence
of the dropping electrode may 1,cconie too large, requiring a better ciainpirig of the recording system (a 2-second pen ,speed IO-mv. Brown recorder). The influence of the frequenct. on the peak height is given in Figure 10. For cadmium the peak height is faid>- constant between 30 and 70 cycles per second. For zinc this holds true between 15 and 80 cycles pcr second. K h e n t'he symmetry of the square wave is changed! the polarogram of Figure 11 are obtained. If the square wave is very asymmetric and the frequency is high (500 cycles per second) the duration of t'he shorter part of the whole cycle is so sniall that the reaction cannot take place and the i-E purl-e becomes similar to that in classic polarography. Consequently the tn.0 types of curve of Figure 11 are not obtained a t low frequencies. ACKNOWLEDGMENT
The author thanks Bot) Tuning for performing the measurcmeiits necessary. LITERATURE CITED
(1) Ishibashi, lI., Fujinaga, T., Bull. Chena. SOC.J a p a n 2 5 , 68 (193.)). ( 2 ) Krugers, J., Chem. W e e k h l a d 53, 672 (1957). RECEIVEDfor review LIay 28, 1038. A4cceptedOctober 14, 1958.
Porous Cup Technique in the Determination of Magnesium in Cast Iron A Statistical Study A. C. OTTOLlNl General Motors Corp., Defroif, Mich.
,A rapid and accurate solution method has been developed for the spectrochemical determination of magnesium in cast iron in the concentration A thorough range 0.01 to 0.16%. statistical study was carried out to evaluate instrumental errors, chemical preparation errors, and day-to-day reproducibility. The standard deviation of the method is 1.8370 ai the 0.075% concentration level; the accuracy is of the same order. Porous cup electrodes are used to introduce the sample into the excitation gap. Synthetically prepared standards facilitate calibration. The method has replaced chemical methods for magnesium in this laboratory.
R
a program was initiated for the routine control of magnesium in nodular cast iron. The requirements were that the method be rapid and capable of chemical accuracy. The best available chemical methods had a standard deviation of the order of 6.0%. They required about 2 days of elapsed time for analysis and were complex. Bryan, Sahstoll. and T'eldhuis ( 3 ) reported a spectrochemical solution method in 1949. Their reported average errors of &lo% or lcss indicated that i t might meet requirements. Prior to this program, three techniques for introducing solutions into the analytical gap had been studied in this ECENTLP
laboratory. Evaporation of the solution on the surface of a preheated electrode ( 7 ) had been evaluated for the analysis of steels. ;1 rotating disk electrode (6) had been found successful for the analysis of both new and used lubricating oils. Porous cup electrodes (5) had been used for the anal!-sis of steels, netv oils. lead-base alloys, and for the determination of hisniuth in cast iron. They n-ere selected for this investigation because of their inherent uniformity and case and simplicity in handling. Using this techniqur and a series of synthetic standards, a rapid, precise, and accurate method was developed for the determination of magneqium in cast VOL. 31,
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