I N D U ST R I A L A N D E N G I N E E R I N G C H E M I S T R Y
Kovember 15, 1934
without use of a stirring rod or other means of vigorous agitation. If the liquid is covered with a layer of gasoline or the work is done in an atmosphere of carbon dioxide, the return of the blue color after completion of the titration is very much slower, supposedly because oxygen is nearly excluded. However since a titration may be completed within less than a minute, there is very little error from the oxygen of the air. Copper, iron, and manganese must be absent.
425
was 0.16 mg. in each of six tests. With reagents alone plus 20 mg. of lead, the zinc found was 0.03 mg., indicating a plus error of 0.02 mg. of zinc, due to added lead, which is almost within the limits of error of reading the turbidities. To &gram portions of pine sawdust were added varying amounts of zinc, the material was burned, and zinc estimated: ZINC ADDED Me. 0. 0.02 0.05 0.10
ZINCFOUND Me. 0.06 0.08,0.08 0.12, 0.12 0.15. 0.16
Estimation of the separated zinc by the colorimetric phosphate method (7‘) was not successful because of difficulty in removing all phosphate not combined with zinc-i. e., a sufficient amount of phosphate to vitiate the determination was always found in running a blank on reagents. The well-known method for macrotitration of zinc with ferrocyanide and uranium indicator may be used when the amount of zinc is more than 0.3 mg., though the iodometric method above described is probabIy preferable, being quicker and better adapted to measurement of very small amounts. The Eegriwe test for zinc (9) with diethylaniline and potassium ferricyanide is not much more sensitive than the cloud test with potassium ferrocyanide and is not adapted to quantitative estimation of zinc.
ZINC ESTIMATED BY IODOMETRIC TITRATION. To 5-gram samples of apple leaves containing much lead varying amounts of zinc were added. The mixtures were burned, the zinc was separated, and the amount measured by iodometric titration:
RESULTSOBTAINED To the solution of the ZINC ESTIMATED BY TURBIDITY. ash of several 5-gram portions of peach leaves 20 mg. of lead as acetate were added, and the zinc was separated by hydrogen sulfide and estimated by ferrocyanide turbidity as above described. Without added lead, the zinc found on two tests was 0.15 mg. each time. With 20 mg. of lead added, the zinc found
(1) Bodansky, M., J. IND. ENO.CHBM.,13, 696 (1921). (2) Eegriwe, E.,2. anal. Chem., 74,225 (1928). (3) Fairhall and Richardson, J . Am. Chem. Soc., 52, 938 (1930). (4) Fales and Ware, Ibid., 41,487 (1919). (5) Heyrovsky, J., Mikrochemie, 12,26-64 (1932). (6) Lang, R., 2. anal. Chem., 93, 21 (1933); C. A., 27, 3419 (1933). (7) Todd and Elvohjem, J . Bid. Chem., 94,609 (1932).
ZINC ADDED
ZINCFOUND Mg 0.015, 0.015, 0.015 0.055 0.26, 0.22
.
Me. 0. 0.05 0.20
LITERATURE CITED
RECEIVED March 14. 1934.
Quantitative Determination of Lead as Periodate HOBART H. WILLARD AND J. J. THOMPSON, University of Michigan, Ann Arbor, Mich.
I
N ANOTHER paper (3) the proper conditions have been
given for the precipitation of lead as pure triplumbic pareperiodate. It was found that this salt was sufficiently insoluble in very dilute acid solutions to make possible its use in the volumetric and gravimetric determination of lead.
EXPERIMENTAL Weighed samples of Mallinckrodt’s reagent-quality lead or Kahlbaum’s sheet lead were dissolved in nitric acid and evaporated t o dryness. The dry salt was then dissolved in 200 cc. of 0.025 N nitric acid and the lead precipitated at 100’ C. by the slow addition of 2 grams of sodium periodate, NaIOa, dissolved in 50 cc. of water. When the amount of lead was very small. no precipitate formed until after 1 CC. of the periodate solution had been added. When this occurred, it was necessary to decrease the acidity t o 0.006 N ; otherwise the precipitate did not have the theoretical composition. After the periodate was added, the solution was cooled in ice water and the cold solution stirred for 0.5 hour, because supersaturation was very pronounced. The lead periodate was filtered on a porcelain filterin crucible, washed with ice water, and dried for 2 hours a t 110’ It was then weighed as PbsH4(IO&. Results of analyses are shown in Table 1. To determine the lead periodate volumetrically the method of Andrews (1, 9) was used, because the best solvent for the salt is concentrated hydrochloric acid. The equation for the reaction can be expressed as follows: Pb8H~(IO~)~8HC1 6HsAsOs = 3PbC1.2 2IC1 6HsAsOd 6Hz0
8.
+
+
+
+
+
If arsenious acid is present in excess, all the chlorine will react with i t to form arsenic acid. The excess arsenious acid can then be titrated with iodate, using chloroform as indicator. TABLEI. GRAVIMETRIC DETERMINATION OF LEADAS PbaH4(IOeh L ~ A TAKEN D Gram 0.7001 0.6953 0.6028 0.5596 0.5591 0.5553 0.3853 0.1145 0.1121 0.0735
LEADFOUND Gram 0.7001 0.6954 0.6025 0.5596 0.5588 0.5552 0.3852 0.1147 0.1122 0.0734
ERROR
Me .
*o.o
$0.1 -0.3 fO.0 -0.3 -0.1 -0.1 +0.2 10.1 -0.1
TABLE11. VOLTJMETRIC DETERMINATION OF LEADAS Pbs%(IOa)n LEADTAKEN Gram 0.5920 0.4896 0.3069 0.2920 0.2060 0.1554 0.1103 0.1037 0.1001 0.0653 0.0496
LEADFOUND Gram 0.5921 0.4895 0.3067 0.2920 0.2063 0.1557 0.1101 0.1035 0,0999 0.0655 0.0494
ERROR
Me. +0.1 -0.1 -0.2 0.0 +0.3 4-0.3 -0.2 -0.2 -0.2 +0.2
-0.2
Am03 ADDBID
Gram 0.7111 0.7447 0.4006 0.4135 0.3297 0.2320 0.1299 0.1271 0.1125 0.1003 0.2116
426
ANALYTICAL EDITION
The recipitate was filtered into a Gooch crucible, the bottom of whicg was covered with a filter-paper disk, washed thoroughly, and transferred to a 150-cc. conical flask. An excess of arsenious oxide previously dissolved in a concentrated solution of sodium hydroxide or a suitable volume of standard arsenite solution was
drochloric acid containing an excess of standard arsenite, and titrating the excess with standard iodate, using chloroform as indicator. TABLE111. SEPARATION OF LEADAS PbaHe(IO&
added to the flask. Concentrated hydrochloric acid was slowly added to the cold solution, the flask being agitated until all the lead periodate dissolved, which required 35 to 40 cc., and the solution was titrated with 0.1 N potassium iodate until it was a light brown in color. A little chloroform was then added and the titration completed. The results of the volumetric determination of lead as periodate are shown in Table 11.
LEADTAKEN Gram 0.6771 0.5643 0.5163 0.5375 0.5161 0.5944 0.5570 0.5438 0.5567
SEPARATION OF LEADAS PERIODATE FROM OTHERMETALS The possibility of separating lead as PbsHd(IO& from other metals is rather limited, since most metals form periodates which are insoluble in low acid concentrations. The results of the separation of lead as periodate from other metals are shown in Table 111. The lead was determined volumetrically. SUMMARY Lead can be separated from nickel, copper, zinc, cadmium, aluminum, calcium, and magnesium by precipitating it as PbaHd(IO& from 0.025 N nitric acid solution by the addition of sodium periodate. The precipitate can be weighed or determined volumetrically by dissolving it in concentrated hy-
Vol. 6, No. 6
a
OTHERMETALS PRESENT MU. l7:mm. _ . -. . $0.7 0 . 4 N1 S6.6 0.26 Cua -0.3 0.05 Cu -0.1 0 . 2 1 A1 f0.2 0.22 Zn 0 . 3 7 Cda $2.7 +0.5 0.07 Cd +o. 2 0 . 2 Ca -0.4 0 . 2 MR obviously have been satisfactory.
LEADFOUND Gram 0.6778 0.5709 0.5160 0.5374 0.5163 0.6971 0.5575 0.5440 0.5563
Double precipitation would
ERROR ~~
I-
LITERATURE CITED (1) Andrews, L. W., J. Am. Chem.. Soc., 25, 756 (1903). (2) Jamieson, G. S., “Volumetric Iodate Methods,” Chemical Catalog Co., Inc., N. Y., 1926. (3) Willard and Thompson, J . Am. Chem. Soc., 56, 1828 (1934). RE~CEIVED July 16, 1934. Presented before the Division of Physical and Inorganic Chemistry at the 81st Meeting of the American Chemical Society, Indianapolis, Ind., March 30 to April 3, 1931. From a dissertation submitted by J. J. Thompson to the Graduate School of the University of Michigan in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Chemistry.
Electrometric Determination of Chlorides in the Ash and Sap of Plants and in Ground Waters J. R. NELLER, University of Florida Everglades Experiment Station, Belle Glade, Fla.
B
EST (8) has described the use of a silver-silver chloride electrode for the titration of chloride ions in a suspen’ sion of soil and water. He reported that the method was sensitive to within one drop of a 0.028 N silver nitrate solution and that it was applicable for various types of soils. After being used for several years with entire satisfaction for both mineral and organic soils, the method was adapted to determine chlorides in the ash of plants and in the expressed saps. In those cases where it can be employed the method is particularly desirable for direct use upon liquid plant sap, where the presence of coloring matter generally prohibits the use of a color indicator. Furthermore, it permits a measurement of the actual chloride-ion content of the unaltered sap. This paper deals with comparative analyses of the chlorine content of plants and of their saps by the electrometric method and by the standard method (1) in which chlorides are determined in an acid extract of the ash of the material, and discusses the limitations relative to the direct estimation of chlorides in liquid saps and juices.
ELECTROMETRIC METHOD Briefly, the electrometric titration of chloride ions is based upon the change in polarity of a silver-silver chloride electrode when the chloride-ion concentration of the liquid in which it is immersed reaches a minimum when titrated with a solution of silver nitrate. The method requires the use of a reference electrode of the same potential-O.521 volt a t 25” C.-as that of the silver-silver chloride electrode a t the point where it changes in polarity. This reference electrode consists of a platinum wire dipping in a solution of p H 3.1 to
3.3 made up according to the method of Clark (3) with the substitution of sulfuric for hydrochloric acid. Enough quinhydrone is added to saturate the solution. The silver-silver chloride electrode is prepared by coating 3 to 5 cm. of a silver wire with silver chloride by electrolysis in a sodium chloride solution about 0.1 N . The current from a partially exhausted dry-cell battery is sufficient and the rate of deposition may be partially controlled by dilution of the sodium chloride solution. The coating should be thick enough to produce a light brown color on the wire and should be deposited slowly enough to extend over a period of 15 to 30 minutes. If wire about 0.8 mm. in diameter is used it is sufficiently rigid to be carried directly to the binding post of the galvanometer. Electrodes prepared in this way have retained their sensitivity for months if stored in the dark when not in use, with the silver chloride coated tips immersed in distilled water. Best made this system suitable for the determination of chloride ions in solutions of varied reactions and buffer capacities by connecting the two electrodes by means of a potassium nitrate bridge, made by filling a glass U-tube with a heated mixture consisting of 5 per cent of purified agar agar in a saturated solution of potassium nitrate. A galvanometer of a sensitivity of 0.125 microampere per millimeter was used for most of the measurements, although one of 0.5-microampere sensitivity will do very well.
EFFECTS OF ACIDS,SALTS,AND BASES In order to ascertain the feasibility of the use of the electrometric method for chlorides in the acid extracts of ashed
