951.
I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y
September, 1924
These results on commercial samples are seen to be practically as satisfactory as those obtained when mixtures made from pure compounds were used. There is a tendency toward slightly low values, which could probably be lessened by more violent agitation during the titrations. Error Per cent This method does not work SO smoothly in the presence of -0 04 magnesium compounds, and all the experiments outlined herein were made with materials that were practically free O4 from them. It is hoped that the method can be modified to i,"':," include products containing magnesium compounds, since - 0 07 commercial calcium arsenates sometimes contain a sufficient amount of the latter to affect the results. 1;E:
to see if the results on the mixtures agreed with the values calculated from the originals. The data obtained are given in Table 111. TABLE 111 REACTION TO ALCO- Ca(0H)z Ca(0H)z HOLIC PHENOLcalc found MATERIAL ANALYZED PHTHALEIN Per cent Per cent Sample A N o t alkaline 9 19 0 9 04 15 Sample A Ca(0H)z Sample B Faintly alkaline 0 20 4 60 Sample R Ca(0H)z Sample C Strongly alkaline 8 j6 60 Sample C iCa(0H)z 12 0 80 12 1: AtB 4 40 4 33 Equal mixture 4 32 (A+BJ-C 2 95
+ +
g: ii
-/ 2 $ s\
-'
Copper as Reducing Agent in Iron Determinations' By J. M. Hendel HUNTERCOLLEGR OF THE CITYOF N E W YORK,N E W YORK,N. Y.
OPPER gauze has been found very convenient in the reduction of ferric sulfate preparatory to titration by permanganate. It is rapid in action and requires no blanks, and is therefore of especial value to those who make iron determinations only occasionally. The reductor consists of ordinary window screening of 15-mesh cold-drawn copper gauze, cut into squares 4 cm. on a side and attached to a copper wire or a glass rod. It is immersed in the gently boiling ferric sulfate solution until the color of ferric iron has disappeared. After allowing 3 minutes more to insure complete reduction, the gauze is washed with a stream of water and removed. If a total area of 64 sq. cm. of gauze is used, 25 mg. of iron may be reduced in 8minutes and 150 mg. in 15 minutes; with 32 sq. cm. of gauze, 125 mg. of iron require 30 minutes for complete reduction. Commercial grades of copper gauze that are 99.8 per cent pure may be readily obtained. The gauze used in this work contained not more than 0.015 per cent iron. The 'reaction may be represented by the equation:
C ,
Cu
+ 2Fe+++
=
Cut+
+ 2Fe++
However, the solutions titrated immediately after reduction and cooling to room temperature gave results for iron 0.4 per cent too high, probably due to the presence of cuprous ion, because t h e substance causing the high results was not removed by a stream of hydrogen. Aeration, however, always removed the source of error. The stability of ferrous sulfate solutions toward oxidation by the air, even a t the boiling point, has been previously o b s e r ~ e d . ~I,n~ €he presence of small amounts of cupric ion, however, the aeration of hot ferrous sulfate solutions, normal in acidity, causes appreciable oxidation of ferrous ion. The rate of aeratioh was 1 liter of air in 5 minutes, the air being drawn from outside the laboratory, through absorbent cotton, and thence into the ferrous solution. To secure correct results for iron, then, all reduced solutions were cooled to room temperature, made normal in acidity, and aerated for from 1 to 10 minutes, after which they were titrated with standard permanganate solution. Table I shows the results obtained with the copper reductor with various amounts of ferric sulfate and varying aeration periods. The reduction was carried out in 0.1 N sulfuric acid, in a volume of 175 cc. The theoretical value for the iron was determined by careful reduction with the platinumhydrogen reductor2 and by the zinc reductor, followed by titration with permanganate. 1 2
3
Received June 2, 1924. Hendel, Dissertation, Columbia University, 1999. Baskerville and Stevenson, .r. A m . Chem. Sac., SS, 1104 (1911).
I TABLE No. 1 2
3 4 5
Aeration time Minutes 3 5
IO 3 5
6 7 8
10
9
10
10
1 1 3 3
11 12 13
3 5
Iron taken Mg. 24,65 24.65 24.65 101.0 101.0 101.0 126.1 126.1 126.1 150.7
150.7 150.7 160.7
Iron found Mg.
24.62 24.67 24.75
101.0 101,o
100.9 126.2 126.2 125.8 150.8 150.7
150.3 150.1
Difference Mg.
-0.03 +0.02 +0.10
.. ..
-0.1 +0.1
+o. 1 -0.3
+0.1 -0:4 -0.6
From the table it may be observed that for 25 to 125 mg. of iron, aeration from 3 to 10 minutes gives correct results. The acidity during the reduction must be no more than 0.25 N . Quantitatively complete reduction in 0.5 or 1 N sulfuric acid was never obtained, even after 1 hour. I n all experiments described herein, however, the acidity during aeration and titration was normal. The presence of titanium has no effect on the determination of iron. A hot solution of 56 mg. of titanium in 175 cc. of 0.1 N sulfuric acid was subjected to the reduction process for a half hour and titrated hot without previous aeration; 0.32 cc. of 0.07181 N permanganate was required, indicating, perhaps, traces of titanous ion, but more probably cuprous ion, formed by oxidation of copper metal in the hot acid. A hot solution of 5.5 mg. of titanium in 0.25 N acid was then reduced for 10 minutes, cooled to room temperature, aerated for 3 minutes, and tested with permanganate; none was required. Finally, two solutions were reduced, which contained 126.1 mg. of iron and 2.3 mg. of titanium in 175 cc. of 0.25 N acid. After reduction for 15 minutes and aeration of the cooled solution for 3 minutes, titration with permanganate showed, in one case, 126.1 mg., and in the other, 126.0 mg. of iron. Any titanous ion that may have been formed was therefore reoxidized in the aeration. This confirms the results of many previous experiments, which showed conclusively that aerating cold solutions containing ferrous and titanous ions results in oxidation of the titanous ion only.2 The copper reductor is being applied to the determination of iron in the presence of vanadium, and of vanadium in steels and other alloys. Preliminary experiments with molybdate solutions indicate other possible applications of copper as reducing agent. The author wishes to express his appreciation to Miss Clara Adlerblum for her careful performance of the experimental work herein described.
INDUSTRIAL AND ENGINEERING CHEMISTRY
952
Vol. 16, No. 9
Continuous Conductivity Method of Measuring Small Concentration of Chlorine in Air' By T. B. Him CHEMICAL WARFARE SSRVICE, WASHINGTON, D. C.
HE method here described is one of several investigated by the chlorine, the flowmeter reading gave the rate of flow in the attempt to develop an instrument for continu- of chlorine irrespective of the actual purity of the material in ous measurement of small concentrations of chlorine in the cylinder. The concentrations of chlorine in the chlorineair, to be used in field experimentation. The method soon air mixtures were calculated from the flowmeter readings, and appeared to be unsuited to field use and was, therefore, not the milligrams of chlorine per liter as reported are estimated investigated in much detail, but as it may have value in not to be in error by more than 3 per cent. Although it is desirable to use water of very low conduclaboratory or plant control, especially as it can be used with other gases than chlorine, it seems worth while to present the tivity since the results show that the sensitivity is greater with such water, it is not possible to use very pure conducresults of the experiments. Electrical conductivity methods of following concentra- tivity water as the water must be in contact with air. Several tion changes of salts in liquids are well known, but the only grades of distilled water were used in the course of the work. extension of this method of measurement to gas analysis of The best water used had a specific conductivity of 1 x 10-6 which the author is aware is the determination of carbon di- reciprocal ohms a t 25" C., which compares with Kohlrausch's oxide in air by passing it into barium hydroxide solution and best water in equilibrium with air of specific conductivity measuring the decrease in conductivity of this solution. I n 0.8 X reciprocal ohms a t 25" C. The electrical resistance was measured with standard the method investigated, air containing small amounts of chlorine is continuously and rapidly scrubbed with distilled equipment of high quality, but no effort was made to attain water, which then flows through a conductivity cell. The extreme precision, since in the low concentration range of electrical resistance of the cell is a function of the chlorine interest here the precision was easily greater than the accuconcentration in the air and serves as a measure of this racy of the gas concentration measurements required. The cell constant of the cell employed was 0.327. Concentration. With a fixed rate of gas and water flow and water of defi-From the beginning it was realized that the success of this msthod depended upon obtaining a rapid, uniform, and fairly nite purity, measurements are regularly reproducible. The efficient method of scrubbing the air, since low concentrations effect of varying these conditions is shown in Fig. 1, where the and rapid fluctuations in concentration were desired. Several familiar types of scrubbers were tried, but most of them require too large a volume of liquid in the scrubber. For instance, with a small tower filled with glass beads, if the water is run through rapidly enough to wash it out thoroughly every few seconds, the ratio of the volume of air to the volumeof liquid is too small to permit accurate determination of very small concentrations of chlorine in the air, but if the sensitivity is increased by running the water more slowly, the rapidity of response to concentration changes is lost. For the present purpose very rapid response to concentration changes was necessary, but for many cases of laboratory or plant control this is not necessary and consequently tower-type scrubbers might well be employed. In all the experiments reported here a glass scrubber operating on the principle of the Ceco sprayer was used. I n this scrubber the water falling upon the center of a rapidly revolving horizontal disk is thrown off to the sides in a thin sheet through which the air to be scrubbed must pass. It is quite an efficient scrubber and has the especial advantage that the FIQ.1 volume of liquid in the scrubber at any time is small and there electrical resistance of the solution flowing from the scrubber is almost no resistance to gas flow. is plotted against the concentration of chlorine in the air being EXPERIMENTAL^ scrubbed for the conditions tabulated below: The chlorine and air were run through separate flowmeters Air flow Water flow Resistance Cc. per Cc. per Temperature of water and then mixed and led through a long coil of glass in a therCurve minute minute ' C. Ohms X 10' mostat to the scrubber. The water was also passed through a long glass coil in the thermostat before entering the scrubber. The chlorine was obtained from a cylinder of liquid chlorine. No check on its purity was made but as the chlorine flowmeter was calibrated with the chlorine from the cylinder by The influence of the purity of the water used is very great, titrating the iodine liberated from a potassium iodide solution as may be seen by comparing Curves I and I V or Curves I1 1 Received July 9, 1924. and 111. The greater the initial resistance of the water the 1 The experimental work was begun under the supervision of the author more rauidlv its resistance decreases with increasing amounts by D. H. Parker and completed b y E. P. Cox with the coaperation of Simon of chlorhe "in the air, but it is not possible to use water of Klosky.
T
