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
Table I.
Determination of Copper and Lead in Typical Naphthas from Middle East Crude Oils
Parts per Billion Sample
E F
Copper
2 1
1 1
1 0
Lead
1 0
1 1
1 1
2 1
2 2
2 0
2 2
2 2
3 2
work but special lead-free acids, containing less than 0.005 p.p.m. of lead, were found to give acceptably low blanks. Borosilicate glassware was used throughout and was thoroughly cleaned with a fresh hot chromic-sulfuric acid mixture before use. All water used in the work was oncedistilled water which had been passed through a mixed bed ion exchange column. EXPERIMENTAL
Table II.
Comparison of Square W a v e Polarographic and Colorimetric Methods on Concentralions of 10 to 100 P.P.B. of Added Metal
811 figures are p.p.b. Copper Found sw-P Colorimetric
Added 10 12 11 7 17 20 19 l8 25 26 26 38 50 48 49 100 104 107 72 a Not determined because of shortage of
12 19 24
5 23
0
the total peak height is about 70% for copper and 80% for lead when the sample contains 1 p.p.b. of the metal. This value of the blank is high but i t is consistent and decreases rapidly for higher concentrations-e.g., a t 20 p.p.b. it falls t o about 10%. METHOD OF ANALYSIS
To 100 ml. of sample add 15% v./v. bromine in CC14 with stirring until a dark brown color persists for 2 minutes. Allow to stand for a further 5 minutes. Place the beaker on a steam bath until the dark brown disappears, then transfer to an electric hot plate, and boil for 5 minutes. Cool to room temperature and transfer the contents of the beaker to a separating funnel with the aid of two 10-ml. portions of warm 4N HC1 and shake for 10 minutes. Separate the aqueous (bottom) layer into a n Erlenmeyer flask, add 0.5 ml. of H N 0 3and boil down to 1 ml. Repeat the beaker washing and extraction procedure, add the aqueous layer to the Erlenmeyer flask, and again boil down to 1 ml. Add 0.5 ml. of "01 and 1 ml. of 60% HC10, and evaporate to dryness t o destroy remaining traces of organic matter. Add 3 ml. of 4iv HC1, simmer, cool, and transfer to a IO-ml. volumetric flask and make up to volume with water Transfer 2 ml. of this solution t o a polarograph cell, deaerate, and record the polarogram between -0.1 and -0.6 volt applied direct current potential against the mercury pool anode. Measure the peak height in millimeters and calculate the concentration of each metal using the appropriate calibration factor or graph. Calibration may be achieved by either the standard addition technique or, since the peak height for both copper and lead is linearly related to concentration, by the use of a calibration curve.
204
ANALYTICAL CHEMISTRY
sWP
32 70 sample.
59
111
Lead Found Colorimetric 13 15 15 20 25 25 23 25 25 54 55 35 112 95 95
I n the calibration technique described by Barker ( 1 ) the peak height of the metal is compared with that given by a standard resistance when the same square wave voltage is appliedLe., the vertical scale of the graph is calibrated in reciprocal ohms and the peak height due to the metal is expressed in these units. When calibrated in this way and working with a fixed drop time, the instrument is compensated for day-today variations in sensitivity since any such variation would affect equally the peak height given by the metal and the standard resistance. For this reason and because of the additional advantages of speed and convenience, this method \vas used throughout the work. PRECAUTIONS AGAINST CONTAMINATION
The entire Ivork was done in a room separate from general laboratory work. Reagent grade hydrochloric and nitric acids contained too much lead for this
Table 111. Comparison of Square W a v e Polarographic and Colorimetric Methods on Typical Naphthas"
Found, P.P.B. CoDDer Lead ColorColorSWP imetric SWP imetric A
Sample
I
E 1 b 2
