Cysteine titrations can give results with a n error of about 1%, the results being better at p H 9 to 12. Because of the slowness of the reaction with PAIOH, the reproducibility of PRIOH titrations is generally poorer than t h a t of titrations n i t h EhIC. This is particularly pronounced in titrations of cj.stine in the presence of sulfite. R(X1iable results cannot be obtained a t cystine concentrations loner than 10-a-lI. An advantage of PRIOH is that pure commercial products are obtainable, n hile commercial ERIC must hc purified and is inore eapensive than PAIOH. ACKNOWLEDGMENT
This inwstigation n as supported by r r s z u c h grant from the Satioiial Sctirnce Foundation.
Table IV.
In presence of 4114 GHCl or 8.M urea and of 0.05M sodium sulfite nith 2 x l O - + J f EJIC a t titrating agent. All solutions allon-ed to stand 30 min. before titration Titration S o . of -S--SPotential, Supporting Electrolyte Denat. F’olt 1’s. Groups per Mole pH S.C.E. of .Ilbumin Buffer lgent 4 GHCl 6.5 -0 6 18.3 GHCl 6 5 -0 6 B 18.5 17.7 1 GHCl 6 5 -0 7 17.4 GHC‘1 6 5 -0 6 B 17.2 B Urea 6 5 -0 6 17.5 c GHCl 8 3 -0 8 17.0 Urea 9.7 -0.8 D -4. 0 023M SarHPO,, 0 0235i SaHzPOa. B. Same ab 9 plus 0 06 nil. octj 1 alcohol. P , 0 6M borau, 0 06 nil. octyl alcohol. D. Same as C, e\cq)t 0 01.2/ borax and 0 1Jf siilfite. B. H., J . .Inz. C h e m . Soc. 80, 3235
ii
(1958). 1.5) \ -
LITERATURE CITED
1’. D., J . -1iu. (’hem. Soc. 7 6 , 4331 (1954). ( 2 ) Edelhoch, H., Katchalski, E., LIajburg, R. H., Hughes, J. T., E d d l , J., 1 0 1 d . 75, ~ 5058 (1953). ( 3 ) H:ita. T.. J i e i r i . Research I n s t . Food (1) ,!o>.er,
Titration of 40 MI. of 3.56 X 10-6M Albumin Solution in Buffers a t Various pH Values
Kolthoff. I. 11..Stricks. IT.. ~~-
~ H E X23,’763 ( I 951).
>
I
-1s.4~.
(6) Icolthoff, I. JI., Strick., IV., J . A m . Chem. Sac. 72, 1932 (1950). ( 7 ) Kolthoff. I. 11..Stricks. IT., Morren, ’L., ASAL.’CHEM. 26, 366 ’( 195k). (8) IlacDonnell, I,. R.,dilva, R. B., FeeneJ-, R. E., Arch. Biochem. Biophus. 32, 288 (1951). (9) Matousek, L., Lancikova, O., Chem. list!/ 47, 1062 (1953).
f
10) Pihar, O., Ibitl., 47, 1617 (1053).
(12) ‘Sidg&ck; N. V., ments and Their Comoounds.” D. 298. Clarendon Press, Oxfoid, 1950: (13) Stricks, IT., Kolthoff, I. M., J . Am. Chenz. SOC.73, 4569 (1951). (141 I b i d . . 75. 5673 (19531. (15) Ibzd., 78; 2085 (1956). RECEIVED for revieTT- .lugust 15, 1960. Accepted Soveiiiber 9, 1960. Division of ilnalytical Chemistry, 138th Xeeting, AC8, S e n 1-ork, S . Y., September 1960.
Use of Four-Electrode Conductometry for the Automatic Determination of Carbon Dioxide and Ammonia in Concentrated Scrubbing Water of Coke Oven Gas EMBRECHT BARENDRECHT and NORBERT G. L. M. JANSSEN Centrad Laboraforium der Staafsmijnen in Limburg, Geleen, Holland
b Four-elect rod e cond uctometry has proved to be a very useful tool in analyzing solutions with good conductivity and solutions contaminated with oil, tar, and the like. Two electrodes carry a 50- or 60-count-persecond alternating current through Ihe solution electrolyte to be analyzed, and the potential difference produced i s measured with the two other electrodes in such a way that the conductivity can be recorded directly. The use of a cell of special construction renders the measurement remarkably independent of the flow rate of the scrubbing water through the cell. Moreover, the grounding problem could b e resolved. Application of this method for the automatic determina-
tion of carbon dioxide and ammonia in concentrated scrubbing water of coke oven gas i s described.
u
non-, most industrial conductonietric ana1yst.s (batchn iscl or continuous) have bcrn ( arried out with the aid of audio-frequency alternating current antl t n o platinizrd electrodes. I n caws, honevcr. nhere rather concentrated solutions (specific conductance larger than lo-* o h n - l cni.-l) are to be measured. the main difficulties for setting up a n automatic analyzer are causrd by the rcquired geometric dimensions of the rrll, because the cell constant, expressed in cni.-’, must be large. Moreover, in industry, such concentrated solutions STIL
are oftc’n coiitaniinatd n ith orgaiiic suhstanws such as oil, tar, and the like, so that the platinized clectrodes nil1 soon be poisoned and the conducting liquid path betwcen these electrodes be blocked. There are two interesting methods to eliminate these difficulties. The first method was originally proposed a hundred years ago by Reetz (9) n here induction currents grnerated in a closed loop of the solution to be measured are practically proportional to thc conductivity of the solution. This electrodeless method has been rediscovered lately and seems to bf a proniisiiig tool (2, 4, 6, 10, 1 1 ) . Another nwthod that can be adapted is also rather old (Q), antl makes use of four electrodes as described herr. SwVOL. 33, NO. 2, FEBRUARY 1961
199
Figure 1. Principle of fourelectrode conductometry ( I , 3,
5,7) era1 investigators (1, 3, 5, 7 ) have applied this method as a laboratory tool, using direct current,. Recently, a modification was proposed by Spillner ( l a ) , which proved to be very useful. The direct current has been replaced b y a small alternating current of 50 c.P.s., which simplifies the measurement. The method is not limited to this frequency, but i t is possible to work with any value in the audiofrequency range (IS). Improvements of Spillner’s method are described in full detail in the following sections. They entail an alteration of the measuring cell, a resolution of the grounding problem encountered during analysis of industrial solutions in the plant, and an arrangement for recording the conductivity of the solution directly. The resulting measuring equipment has been successfully used for the continuous, automatic determination of carbon dioxide and ammonia in scrubbing water of coke oven gas by a sample-reagent delivering system. THEORY
Measurement of a potential difference -.v2, Figure 1) in an electrolytic solution caused by a direct current
2
Figure 2.
200
Improved conductance cell ANALYTICAL CHEMISTRY
passing through this solution (via the current electrodes 8, and 82) is significant only if no polarization occurs at the two measuring electrodes, M I and M2. Polarization does not occur if the current passing through these electrodes is zero. By preference, the measuring electrodes should be reversible and be placed in a part of the cell where the potential gradient is zero. These additional conditions are necessary to ensure good accuracy ( 3 ) . To fulfill the main condition. which is no polarization, E4f1-,vz is measured with an electronic voltmeter. Under these circumstances the law of Ohm is applicable and consequently EMM,-.u2is inversely proportional to the conductivity of the electrolyte, provided the current passing through the cell is constant. The measured value E,tfl.-J.2must be corrected if the potentials of lMl and iM2ZJS.the solution differ at zero current through the cell. Unlike the classical method, operating with an audio-frequency of 100 C.P.S. and with two platinized electrodes, the cell constant need not be large; this has the advantage that blocking of the conducting liquid path between the electrodes by foreign matter is avoided. I n the alternating current method of Spillner (fg), any correction of the measured value E v l -lfl is not necessary, because the direct current contribution to this value can be fully eliminated, for instance, by placing a condenser in series with M1 and fM2, respectively. Consequently, the material of the measuring electrodes must fulfill only the condition of being nonattackable, so reference-type electrodes are not necessary. Moreover, contamination of these electrodes, resulting in a direct current contribution, cannot influence the value of E M , - M ~ . The value of the cell current can be considered as constant since the resistance R in the cell circuit is much higher than the resistance of the cell itself, the latter being variable in principle because of changes in the concentration of the liquid to be measured, polarization, and contamination of the electrodes with dirt such as tar and oil. Finally, the current remaining constant, it is possible to evaluate the conductivity of the solution straightforwardly in the same manner as described when using a direct current.
Figure 3. Electrical analog of cell with simplified measuring system
An electrical analog is given in Figure 3. This figure shows that the separate parts of the cell constitute a symmetrical bridge system. Consequently, the grounded flat rings A1 and A z are on the same potential. Therefore, after these electrodes have been grounded, current leakage is avoided. Variations in conductivity, caused by temperature differences, are corrected by a thermistor, T, Figure 2. The variable current passing through the cell is of the order of 1 ma. Measuring and Recording Conductivity. I n a limited range of
concentrations the conductivity is approximately a linear function of concentration. TO obtain a linear scale for indicating concentrations on t h e recorder, t h e output current I , of t h e measuring apparatus fed t o the recorder REC must be proportional t o the conductivity, Figure 4. T h e alternating current (50 t o 60 c.P.s.) passing through the cell via the electrodes S1 and S2is controlled so as to cause the resulting potential drop between the electrodes MI and X2-i.e., to be always constant and opposite to a fixed reference voltage, EV1, provided the temperature is constant. When the temperature varies, the constant reference voltage is modified by a thermistor, T , in such a way that the resulting current, I,, becomes independent of the temperature. ._
I
......
I I
EXPERIMENTAL
Figure 4. Block diagram of cell and measuring apparatus
Conductance Cell. I n most industrial applications the liquid input and output of the conductance cell are grounded b y the liquid t o be investigated. This means that the current leaks away through a n outer circuit (Figure 1, current from SI to SZ via grounding). To avoid this complication, a completely symmetrical cell was constructed ( 8 ) , Figure 2.
I = isolating amplifier and rectifier II = high-gain d. c. amplifler 111 = magnetic amplifier IV = matching transformer with harmonic filter V = stable d. c. amplifler with variable gain for full-scale setting VI = reference-voltage, at same time serving as function of temperature VI1 = current transformer with rectifier Vlll = variable voltage for zero-scale setting REC = recorder
t-----I 220
v
1
)V
output to recorder
Ix
'1
1 , Figure 5. = 2 7 0 Q-'/zW 12 M - 1 W R3 = 1 2 kQ-1W Rd = 270Q-'/zW R5 = 3,3 k Q - l W - l % R g = 2,2 kQ-1 W - 1 % R7 = 100R-1/2W-1% Rg = NTC-type B8.320.05P/l K (Philips) (T in Figure 4 ) Rg 1,9 kQ-WW Rlo 5 Q-WW R11 = 2 5 Q - W W R l z = 3 0 0 CLHelipot 3 X 3 6 0 ' R 1 3 = 6 0 n-170 R14 3 kn-WW Ris = 1 0 - W W R l g = 3 0 0 R-Helipot 3 x 360' R i 7 = 21,5 Q - W W R i a = 1,5 kQ-WW-5,5W R i g = 4,7 kR-WW-5,5W R i o = 4,7 k Q - W W - 5 , 5 W R21 = 1 0 0 kQ-l/qW Rzz = 1 0 0 kQ-'/pW R23 = copper wire resistor; volue dependent o f temp. coeff. from Zener diode 26 R 2 4 = W W resistor; volue dependent of Zener voltage difference between Z10 & 2 6 R95 1300 R - W W RI
RZI
Rz
R27
The total static transfer function of the circuit is:
I,
= py
[-1 + (PIPIWIIIPIV)/G EVl
x
PT'II
@V
volue dependent of Zener voltage difference between Z 1 0 & 26 R z s = copper wire resistor; value dependent o f temp. coeff. from Zener diode 2 6 R 2 9 - R 3 0 = 470 kQ CI = 47000 pF (Polyester) Cp = 4 7 0 0 0 pF (Polyester) C3 2 0 0 0 pF., 5 0 volts Cq = 2 5 pF,, 4 0 0 volts Cs = 2 5 pF., 4 0 0 volts Cg 100 pF., 5 0 volts C7 = 2 HF,, 500 volts GI = Gs = OA85 (selected for R i = 4 0 0 R a t 1 ma. and 30' C.; R i = 4 7 0 I? at 0.5 mo. and 30' C.) G3 to G6 = O Y 5 0 6 1 (Intermetall) G7 = Ga OA85 Go = Glo = B60C230 (Siemens) G11 = B300C70 (Siemens) Zg = Zener diode t y p e 2 6 (Intermetoll) ZIO = Zener diode t y p e Z10 (Intermetall) BI E88CC Bz = 150B2 T I = power transformer prim. 2 2 0 volts, 1 0 0 ma.
~ V I I
=
G
=
Er I
=
EVIII
=
- EVIII] ( 1 )
where fii.
1300 Q-WW = W W resistor;
PIPIIpIIIPIV
x I O
Wiring diagram of measuring apparatus
output current fed to recorder . . PIV = static transfer (indices correspond to numbering of the blocks, Figure 4) = gain setting for full-scale in ohm-' =
In our case
amplification of transformer VI1 in ohms conductance of liquid in ohm-' reference voltage corrected for temperature, product G X El71 being independent of temperature zero-scale setting voltage
>>
~ I ~ I I ~ ~ ~ 1, I ~hence: I V
I o = rv(G X EVIX ~ V I I - EVIII) ( 2 ) Therefore, the current I , transferrpd to the recorder is proportional to the
r?
=
IV
=
sec. 2 8 0 volts, 2 5 ma. 2 X 3.15 volts, 0.6 amp. 2 0 volts, 2 5 ma. 5 0 volts, 4 0 ma. 10 volts, 0.5 amp. push-pull output transformer 2p-cores t y p e SZ55 (VacuumschmelzeHanau) prim. 2 X 1 3 5 0 W, diam., 0.1 2 mm. sec. 2 X 3 0 0 0 W, diam., 0.1 2 mm. R P + R, 5: 8 1 O Q matching transformer prim. 1 0 volts, 0.5 amp. sec. 1 1 0 volts, 0.05 amp. current transformer 2,u-cores t y p e SZ55 (VacuumschmelzeHanau) prim. 750 W, diam., 0.2 mm. sec. 2 X 7 5 0 W, diam., 0.2 mm. magnetic reactor t y p e Tur.52~ (Giesenhagen) d.c. amplifier type EV2 1 1 [Joens) 1 0 0 mv., 5 ma. d.c. amplifier t y p e EV21 1 (Joens) Modified for gain control
1-
VI1 =
Iii
=
II
=
V
=
conductance, G'; nioreovcr, it is corrected for changes in temperature. The zero of the scale can be set on a predetermined point by means of a variable, auxiliary voltage EVIII. The width of the scale range can be chosen independent of the voltage EvIIl by the gain setting PV. The static gain ~ I ~ I I ~ I I must I ~ I Vbe much larger than 1, otherwise the static difference between the two voltages to be compared-Le., E,M~ - M * and EvI-becomes too large. I n a dynamic sense, however, this static gain must not be too large, to avoid VOL. 33, NO. 2, FEBRUARY 1 9 6 1
201
1
/mainline
Figure 6. Apparatus for automatic determination of carbon dioxide in scrubbing water
&-
N2
"5
-
Table I. Determination of the Two Gases in Scrubbing Water with the Prototype
Date
Grams per Liter Content, recorder Chemical sealea analysis Ileviation~
1959 8/8
l2/8 14/8 17/8 31/8 1/9
%/9 3/9 1
/9
C ~ R B O NUIOXIDEC 26 5 $0 8 26 0 +1 9 26 8 +I 9 3 26 4 -1 1 1 0 27 9 +0 4 0 0 0 29 0 -0 7 1 29 3 -0 7 7 27 9 -1.7 9 29.4
26 7 26 5 27 26
28 29 29 27 28
Std. dev.
1.3%
A+MbIoNIAd
3/11 4/11 6/11 7/11 9/11 10/11 12/11 13/11 17/11 19/11 3/12 8/12 29/12 1960 1311
32 2 31 0 33.2 33.7 30.2 31.0 31 4 288 308 293 29 5 29 4 31 4
33.0 31.6 31.2 36.4 30.5 30.5 31 2 29 6
304
30 7
310
296 30 6 28 9 31 0
Std. dev.
-2 4 -1.9
$6.4 -4.8 -1.0 $1.6 +0 6 -2 7 -06 -1 0
-3 3
+1 7 +1 3 -1 0
2 8%
Established with aid of 10 solutions, analyzed chemically. * Chemical analysis as reference. Procedure of chemical analysis: Precipitate as CaC08, filter, dissolve in known amount acid, boil solution to remove CO,, and titrate excess acid with base, Procedure of chemical analysis: Distill NH, in a known excess of acid; titrate excess acid with base.
202
ANALYTICAL CHEMISTRY
II .1 to cetf Figure 7. Apparatus for alcfomafic determination of content of ammonia in scrubbing water
instability. Thr frequency characteristic of the open loop is such that the closed loop is stable. Germanium diodes are applied t o rectify the alternating voltage from transformer VI1 (Figure 4). Thc dependence of the temperaturc on the inner resistance of these diodcs is compensated for by the resistance variation of the copper wire of the above-mcntioned transformer lrith temperature. I n Figure 5 the wiring diagram is shown with additional information about the accessories. Conductometry of System C02-NH3H20. The scrubbing water to be analyzed contains 20 to 35 grams of carbon dioxide and 30 t o 35 grams of ammonia pcr liter. Consequently, ammonia is always in excess as compared with the carbon dioxide. The v d e r is contaminated with oil and tar; moreover, i t contains hydrogen sulfide up to 0.5 gram per liter. By comparing the pK values of carbon dioxide and aninloilia a t 25" C., 6.35 (for the first step) and 4.75, respectively, i t appears that the conductivity of the water is determined mainly by the rontrnt of carbon dio\ide, The excess ammonia scarcely contributes to the conductivity. Consequently, carbon dioxide can be detrrmined by a single conduc-tivity measurement. It is also possiblc t o establish a direct relation between the content of ammonia and the measured conductivity, provided there is added an excess of acid with such a pK that the dissociation of carbonic acid is largely suppressed (this is necessary when the ionic equisalent conductance of the carbonate ion differs much from that of the anion of the acid to be added), and that an
excess of this acid contributes only slightly to the total conductivity. Of course, the acid to be added must bear a constant volume ratio to the amount of water t o be determined. Application of acetic acid, p K = 4'76 a t 25" C., permits both conditions to be readily fulfilled. Determination of Carbon Dioxide. I n making a continuous and automatic analysis of carbon dioxide in t h e sciubbing n ater, the sample stream was passed through a bed of coke particles (diameter, 2 nim.) t o remove coarse dirt, and through a degasser for equilibration a t atmospheric pressure, Figure 6. T h e coke filter and degasser did not add interfercnccs or remove desired constituents to a noticeable extent. To reduce the time lag. filter bed, degasser, and conductance cell were located near the sampling point. The speed of the scrubbing water through the cell is about 100 ml. per minute. Variations in this speed ranging from -100 to +200y0 h a w no influence on the measurenirnt . Determination of Ammonia. Ammonia can also be determined continuously. However, this calls for an accurate control of the constancy of the feed ratio of the scrubbing water and the acetic acid solution. HOWever, continuous monitoring requires a considerable amount of acetic acid solution. As continuous information was not necessary, an automatic, batchwise procedure was set up, Figure 7. A process timer controls the operational sequence as follows. iifter opening the magnetically operated valves T'I
and V,, pipets PI and Ps are filled with the sample and the acetic acid ( 2 . 2 5 S ) , respectively. Any time lag in the measurement can be reduced to a large extent if pipet PI (10-ml.) is of the overflow type. However, t o save on reagent, pipet Ps (10-ml ) is of the communication type and connected with a hlariotte flask, R. After 1 minute, when the pipets are completely filled, valves Vi and V?arc closed, and valves VS and Vd are opened. In the mixing vessel, 31, carbon dio\ide is expelled with a nitrogen stream (valve V , open). After 2 minutes. removal of carbon dioxide is complete and the misture is admitted open) as described to thc cell (valve before. The conductivity can be measured n ith the apparatus described previously. The measured c7onductivity is a linear function of the ammonia concentration, and the n.hole cycle can be rqwated rvery 5 minutcs. RESULTS AND CONCLUSIONS
Under laboratory conditions, analyses can be carried out with artifacts, both for carbon dioxide and ammonia caoncentrations, varying from 20 to 40 grams per liter n i t h acmu-acics of more than 0.5%. I n industrial plants, honever, the accuracy is about 2 or 37, oning to m a l l and variable amounts of sub-
stances tt-ith an electrolytic nature, such as hydrogen sulfide, which increase the conductivity (maximum 0.5 gram per liter), and to nonconducting organic materials, such as tar and oil, n-hich block the conducting liquid path and therefore increase the cell constant if expressed in em.-’ These interfering factors are difficult to eliminate; however, in our particular case they largely compensate each other. Carbon monoxide and oxygen did not interferr. I n Table I some results obtained with a prototype are given, together with the results of the chemical analysis (for procedure, see remarks in table). To check the capacity of the scrubbing water to absorb carbon dioxide, a relative accuracy of about 5% is sufficient. Taking into account the interfering factors mentioned above, the accuracy a t which the carbon dioxide and the ammonia contents can be determined is greater than r e q u i r d . From the performance of the prototype mhich has now hcen norking under plant conditions for several months, i t appears that alternating four-electrode conduetometry is a very useful measuring tool, especially for liquids having a relatively large conductivity value and/or for liquids contaminated n i t h nonsoluble organic niaterial.
ACKNOWLEDGMENT
The authors thank Wilhelmus Martens for his aid in carrying out the experiments. They also express their thanks to Cornelk Bokhoven for his stimulating interest. LITERATURE CITED
(1) Benson, G. C., Gordon, A. R., J. Chenz. Phys. 13,470 (1945). (2) Calvert, R., Cornelius, J. A., Griffiths,
V. S., Stock, D. J., Ibid., 62, 47 (1958). (3) Elias. L.. Schiff, H. I., Ibid., 60. 595
(1956): (4) Griffiths, V. S., Anal. Chim. Acta 18, 174 (1958). (5) Gunning, H. E., Gordon, A. R., J. Chern. Phys. 10, 126 (1942). (6) Gupta, 8. R., Hills, G. J., J . Sci. Instr. 33,313 (1956). ( 7 ) Ives, I).J. G., Swaroopa, S., Trans. Faraday. Soc. 49,788 (1953). (8) Janssen, N. G. L. M. (to Stamicarbon, S . Y.), Setherlands Patent 94955 (June 15, 1960). (9) Kohlrausch, F., Holborn, L., “Das Leitvermogen der Elektrolyte,” S. 5-8, Teubner-Verlag, Leipxig-Berlin, 1898. (10) Myers, I\-. R., J . Sci. Instr. 35, 173 (1958). (11) Salomon, 11.. Chenz. Techn. 10, 207 ~
I
.
(1933). (12) Spillner, F., Chenz. Iny. ~
Technik
1957, 24. (13) Spillner, F., Gummersbach, Rhineland, private communication.
RECEIVED for review August, 1, 1960. Accepted September 29, 1960.
Polarographic Method for Parts per Billion of Copper and Lead in Catalytic Reformer Feedstocks B. W. SAMUEL and J. V. BRUNNOCK BP Research Centre, Petroleum Division, The British Petroleum
b The square wave polarograph has been applied to the determination of trace amounts of copper and lead in catalytic reforming feedstocks. Both elements may b e determined simultaneously in concentrations below 20 p.p.b. with a precision such that duplicate determinations should not differ by more than 2 p.p.b. for lead and 3 p.p.b. for copper. Duplicate determinations for both metals can b e made by one analyst within 4 hours, of which only two are manipulative time. ODCRN
CATALYTIC
REFORJIISG
processes require that the feedstocks be virtually free of those elements which give rise t o catalyst poisoning; copper and lead are two such elements. The commonly used colorimetric methods for the determination of these metals a t low concentrations involve the formation of the colored complex of copper
Co., ltd., Sunbury-on-Thames, Middlesex, England
ith sodium diethyldithiocarbamate and of lead with dithizone. Both these colorimetric procedures rcquire substantially full time nianipulativc effort by the analyst and the time requirement for duplicate determinations of each metal is about 1 hour. The mcthods have a preciqion of 1 2 0 11.p.b. for copper arid k 1 0 p.1i.b. for lead. One of the main characteristics of the square wave polarograph ( 2 ) is its very high sensitivity. Follon ing th(, installation of the llcrqm-Harwell instrument in these laboratories, a program n a s undertaken to assess its suitability for the determination of copper and lead in reformer feedstocks n i t h a greater degree of precision than those afforded b y the colorimetric procedures. APPARATUS
The Mervyn-Harwell square wave polarograph, manufactured by Nervyn
Instruments Ltd., Koking, Surrey, England, is the commercial form of the original square wave polarograph designed by Barker at Harwell (1, 2 ) and employs a conventional dropping mercury electrode system. At maximum sensitivity the instrument, at a drop time of 5 seconds, gives peak heights of the order of 2.2 mm. and 1.2 mm. per 0.001 mg. per liter of copper and lead, respectively, in the solution being polarographed. REAGENTS
Nitric Acid, sp. gr. 1.42. Hydrochloric Acid, sp. gr. 1.18. Both acids were reagent grade for foodstuffs analysis: P b < 0.005 p.1i.m. The same batch of reagents and deionized water was used throughout the work and blanks were run a t intervals during the program. When taking 100 ml. of sample and a final volume of solution of 10 ml., the contribution of the blank toward VOL. 33, NO. 2, FEBRUARY 1961
203
