Application of Colorimetry to the Analysis of Corrosion-Resistant Steels Determination of Lead LEWIS G. BRICKER
AND
KENNETH L. PROCTOR, Industrial Test Laboratory, United States Navy Yard, Philadelphia, Pa.
A procedure for the determination of small amounts of lead in corrosion-resistant steels i s suggested. The method i s rapid and the precision is greater than in any other procedure investigated. Lead is removed from the bulk of interfering elements by utilizing a seeding out procedure, employing ammonium hydroxide and hydrogen sulfide, and is finally isolated by selective extraction with a chloroform solution of dithizone. Final estimation may be made both visually and b y photoelectric colorimeter.
I
N CONNECTION .with an investigation of residual elements in corrosion-resistant steel (4, 7), a procedure for the quantitative determination of very small amounts of lead was required. The usual methods lacked the required sensitivity, especially for the complex material to be analyzed. The increasingly popular dithizone reaction, introduced by Fischer (2) in 1925, offered the sensitivity for the range of lead expected, 0.1 to 0.001 mg. All attempts a t a direct separation of lead by dithieone failed, primarily because of the oxidizing action of ferric iron upon dithizone in a basic solution (6),or the formation of insoluble lead ferrocyanide in the reduced medium. Although direct extraction of lead, by dithizone, from steel has been claimed by several labora-
tories (private communication), the only effective means found by the authors of separating small quantities (0.1 to 0.01 mg.) of lead from high nickel-high chromium corrosion-resistant steel was that suggested by Lundell and Hoffman (9) using hydrogen sulfide and "seeding out" with ammonium hydroxide. REAGENTS
STANDARD LEADSOLUTION (1 ml.
= 0.02 mg. of lead). Dissolve 3.197 grams of lead nitrate in a 1000-ml. volumetric flask with 1%nitric acid and dilute to the mark with the acid. Transfer 10 ml. of this solution, by means of a pipet, to another 1OOOml. volumetric flask and dilute to the mark with 1yonitric acid. DILUTENITRICACID (1%). Dilute 10 ml. of nitric acid, s p e cific gravity 1.42, to 1000 mi. with distilled water. CITRICACID (50%). Dissolve 500 grams of reagent citric acid crystals, CsHa0,.H20, in distilled water and dilute to 1000 ml. SODIUM CYANIDE(loyo). Dissolve 100 grams of reagent sodium cyanide crystals in 1000 ml. of distilled water. THYMOL BLUEINDICATOR. Dissolve 0.1 gram of thymolsulfonaphthalein in 21.5 ml. of 0.01 N sodium hydroxide and dilute t o 250 ml. with distilled water. AMMONIA-CYANIDE MIXTURE. To a 500-ml. volumetric flask add 100 ml. of 10% sodium cyanide and 75 ml. of 28YGammonium hydroxide. Dilute to the mark with distilled water. Filter the solution if it is not clear. DITHIZOKE SOLUTION (0.002%). Dissolve 0.020 pram of diphenylthiocarbazone in chloroform and dilute to 1000 ml. with chloroform, Store in a dark bottle and keep in a cool place. No purification of the Eastman Kodak product was found necessary.
PROCEDURE
Figure 1.
Weigh a 1.000-gram sample into a 300-ml. Erlenmeyer flask and dissolve with 25 ml. of a mixture of equal parts of hydrochloric and nitric acids. Add 25 ml. of 70% perchloric acid and a few drops of hydrofluoric acid, and heat to strong fumes of perchloric acid to oxidize all the chromium. Cool and dilute with 150 ml. of distilled water. Titrate the solution with 1 to 1 ammonium hydroxide until the iron precipitate just redissolves. Pass hydrogen sulfide into the solution through a three-hole stopper fitted with a 60-ml. long-stemmed separatory funnel, until the solution is saturated. Add 10 drops of 1 to 5 ammonium hydroxide, by means of the separatory funnel, and continue passing the hydrogen sulfide for 3 minutes more. Filter immediately through an 11-cm. No. 40 Whatman filter paper, rinsing the flask and washing the pa er with cold water saturated with hydrogen sulfide. Ignite the llter paper and contents in a KO.1 Coors porcelain crucihle. Cool the crucible and add 10 ml. of hot 1 to 1 nitric acid. Stir the solution, crush any large particles, and digest for a few minutes. Filter the contenb of the crucible through a 9-cm. No. 40 Whatman filter paper into a 125-ml. Squibb separatory funnel, wash with 1% nitric acid, and discard the paper. Add 5 ml. of citric acid solution to the filtrate, add 28% ammonium hydroxide until faintly ammoniacal and cool under a cold water tap. Add 10 ml. of sodium cyanide solution. Carefully add ammonium hydroxide until a drop of thymol blue indicator solution shows a blue color indicating a pH of 9.0 to 9.6. Extract repeatedly with 10-ml. portions of the dithizone solution by shaking vigorously for a t least 30 seconds. Continue the extractions until the color of the dithizone layer remains unchanged. Allow each successive dithizone layer to separate for 2 minutes and combine all the extractions in another clean 125-ml. separatory funnel. The contents of the first separatory funnel may be discarded. Add 25 ml. of 1%nitric acid to the combined extracts and shake well to free the lead from its complex. Allow the layers to separate. Drain off the dithizone layer and discard it. Add 10 ml. of chloroform to the acid portion and shake to remove the remaining dithizone. (If the lead content is indicative of more than 0.020%, as estimated from the dithizone ex. taraction-i.e.. using more than 30 ml. of the dithizone solution-
Lead-Dithizonc System
0.009% solution, using l-cm. cell B . 0.01 5 % iead, a i dithironate, with excess dithironr present, using a 1.m. celi C. 0.01 5 % lead, aa dithironak. with excess dithirone removed, urine l-cm. cell A.
51 1
INDUSTRIAL AND ENGINEERING CHEMISTRY
512 ~
Table 1. Sample
~~
Analytical Data Total Lead Piesent
%
T1755 0.05 gram N.B.S. 130 0.95 gram T1755 0.10 gram N.B.S. 130 0.9 gram T1755 0.15 gram N.B.S. 130 0.85 gram T1755 T1755 0.01 mg. leadb
+ +
0.0112
+
0.D315a
+ TI755 + 0.03 rng. leadh T1755 + 0.05 rng. leadb T1765 + 0.10 mg. leada 71755 + 0.15 mg. leadb T1755 + 0.20 mg. leadb
5
... 0.0213'
0.0020 0.0040 0 . 0060
0 0110 0.0160 0.0210
Lead Found
BY
By visual photoelectric comparison colorimeter
%, 0.001 0.001 0.011 0.011 0.020 0,022 0.030 0.032 0,001 0,002 0.004 0.004 0.005 0.005 0.011 0.012 0.014 0.015 0.020
0.021
One-half aliquot taken. Added as standard lead solution t o dry sample in flask.
% 0.0010 0.0010 0.0115 0.0112 0.0214 0.0215 0.0314 0.0318 0.0021 0.0024 0.0043 0.0040 0.0060 0.0064 0.0115 0.0114 0.0158 0.0158 0.0204 0.0210
wash the acid solution into a volumetric flask and dilute to volume with 1% nitric acid. Take a suitable aliquot and, if necessary, make up to 25 ml. with 1% nitric acid.) Add 5 ml. of ammoniacyanide mixture to t,he 25 ml. of 1% nitric acid containing the Lead. Measure exactly 30 ml. of the dithizone solution into the separatory funnel and mix by shaking for 1 minute. Allow the layers to separate for a t least 2 minutes and draw off the lower layer into a high-form 50-ml. Nessler tube. Stopper tightly and rompare with standards. Carry a blank on reagents through the entire procedure and subtract any correction from the result obt,ained by visual comparison. PREP.4R.4TION OF STANDARDS. T o a series of 11 clean separatory funnels add from 0.0 to 10.0 ml. of the standard lead solution, in increments of 0.02 mg. (0.002a/0), and dilute to 25 ml. with 1% nitric acid. Add 5 ml. of ammonia-cyanide mixture and exactly 30 ml. of the dithizone solution. hIix by shaking for 1 minute and allow the layers to separate for at lerst 2 minutes. Transfer the dithizone layers to 50-ml. high-form Nessler tubes, stopper, and place in a color tube support. Compare the unknown with the standards by viewing transversely, using a titration lamp or other similar source of light. The distinction of 0.0017, is easily discernible. PHOTOELECTRIC COLORIMETER ESTIMATION. After the final shakc-out, instead of transferring the lead-dithizone to a Sessler t,ube, siphon off the aqueous layer. Add 20 ml. of 2% ammonium hydroxide and 5 ml. of 10% sodium cyanide, shake vigorously, and allow the layers to separate. Siphon off the ammonia layer and repeat the shake-out with the same solutions. The excess dithizone is uwnlly removed by two shake-outs, but a third may be necessary. Drain the lead dithizonate into a 50-ml. Nessler tube and rinse the separatory funnel n i t h 10 ml. of chloroform. Dilute to 50 ml. with chloroform. Read the solution in a Klett-Summerson photoelectric colorimeter, using a S o . 52 filter after setting the instrument a t zero with chloroform. Determine the percentage of lead present by reference to a calibration curve prepared by treating a set of standard solutions in the manner described. DISCUSSION
The usual methods for determining lead were investigated: gravimetrically as the chromate, molybdate, and sulfate; electrolytically as the peroxide. A purification of the sulfate by extraction with ammonium acetate was also attempted. The initial hydrogen sulfide separation, common to all the above procedures, presented difficulties. The careful adjustment of hydrogen-ion concentration and the large bulk of precipitated impurities made the subsequent separations entirely unreliable. On many occasions no lead was found spectrographically in the precipitate from any of the gravimetric methods when known amounts of lead had been added. The electrolytic determination was not considered adaptable to microquantities of lead. r'olumetric methods Tvere rxpected to prove equally inadequate for w e on illit crileli. Seeding out of basic iron sulfide, with
Vol. 17, No. 8
accompanying coprecipitation of lead, by hydrogen sulfide was the only method of separation giving consistent, satisfactory results The dithizone separation and estimation of lead adopted were essentially the same as those found in many methods for determination of lead in organic and biological materials recorded in the literature during the past 12 years ( 1 , 5 ,8, IO). Stannous tin, bismuth, and thallium are the only interfering elements, Tin is readily oxidized by the nitric acid used for solution of the lead after ignition; bismuth and thallium rarely occur in a corrosionresistant steel except when deliberately added. If the presence of bismuth is suspected, i t can be extracted from the nitric acid solution at pH 2.0 with dithizone in chloroform (9). For routine work the mixed colors, of the dithizone-lead dithizonate, were compared visually with standards. The standards were prepared weekly and no appreciable change was noticed during this time, provided the tubes were kept well stoppered and at a temperature below 80" F. Results were reproducible and the accuracy was consistently within +=0.0020/, (Table I). A spectrophotometric investigation was made for the purpose of estimating lead with a photoelectric colorimeter. Transmission curves were made for the single and mixed-color dithizone solutions using a General Electric recording spectrophotometer with a slit width of 10 millimicrons. A study of the transmission curves in Figure 1 discloses that the lead dithizonate shows an absorption peak a t approximately 520 millimicrons. The dithieone reagent shows two absorption peaks, a t approximately 440 and 600 millimicrons. A Klett-Summerson No. 52 filter, with a mean transmission of 520 millimicrons, was chosen for the KlettSummerson colorimeter used by the authors. I n addition to some interference by the reagent a t the absorption maximum for the lead dithizonate, there was also some interference from the reagent due to the wide transmission ranee of the filter. Readings on mixed-color, dithizone-lead dithizonate solutions were irregular and no reasonable curve could be drawn. After removing the excess dithizone with an ammoniacal solntion and reading the lead dithizonate, using the No. 52 filter in the colorimeter, very satisfactory results were obtained which gave a maximum deviation of *O.OOl% (Table I). Lead dithizonate is appreciably soluble in the excess dithizone extracting solution (IO). However, if the volume and the waqhes are kept uniform no serious error results from this solubility factor. The curve is made from a set of standards carried out in the prescribed manner. It follom Beer's law for all practical purposes. ACKNOWLEDGMENT
The authors desire to thank H. A. Sloviter and R. T. Cook for preparation of the spectrophotometric transmission curves. LITERATURE CITED
(1) Allport, S . L., and Skrimshire, G. H., Analyst, 57, 440-9 (1932). (2) Fisher, H., Wiss. Verofent. Siemens-Konzern, 4 , 158 (1925). (3) Lundell, G. E. F., and Hoffman, J. I., "Outlines of Methods of Chemical Analysis", p. 51, New York, John Wiley & Sons, 1938.
(4) Milner, O., Proctor, K. L., and Weinberg, S., IND. ENQ.CHEM., (5)
&JAL. ED., 17, 142-5 (1945). Ross, J. R., and Lucas, C. C., Can. M e d . Assoc. J.,29, 649-50
(1933).
(6) Sandell, E. B., "Colorimetric Determination of Traces of Metals", p. 76, Xew York, Interscience Publishers, 1944. (7)
Weinberg, S.,Milner, 0. and Proctor, K., IND.ENG. CHEM.,
ANAL. ED.,17, 142-5 (1945). (8) Wichmann, H. J., Murray, C. TV., Hariis, M., Clifford, P. A., Loughrey, J. H., and Worhes, F. .4., Jr., J . Assoc. Oficial Agr. Chem., 17, 108-35 (1934). (9) Willoughby, C. E., Wilkins, E. S., Jr., and Kraemer, E. O., IND.ENG.CHEM..AKAL.ED..7 , 285 11935). (10) Winter, 0. B., Robinson, H. M., Lamb, F. W., a n d Miller, E. J.. Ibid., 7, 265-71 (1935).
THEopinions expressed are those of the authors and are n o t to he construed as reflecting the official views of the S a v y Department, through whose permission this article 18 published.
