Use of Silicomolvbdic Acid Indicator before Volumetric Oxidation of Iron ALBERT C. TITUS AND CLAUDE W. SILL University of Utah, Salt Lake City, Utah
I
density of exactly 1.10 (25” C.) in the method below, since variations were made in the addition of the acid to fit the particular type of run being made. The stannous chloride was about 2 N and contained less tha,n 100 ml. per liter of hydrochloric acid expressed as the 1 to 1 solution. The slowly dissolving tin metal used t o keep it reduced did not cause appreciable introduction of impurities. SILICOMOLYBDIC ACID INDICATOR. To 13.2 grams of Baker’s reagent sodium silicate pentahydrate dissolved in water W R P added a solution made by dissolving 35 grams of Sterling “analysip certified” ammonium molybdate tetrahydrate and adding 49 ml. of concentrated sulfuric acid. The whole was made to B volume of 2 liters. (Molybdate is in less than equivalent amount, since the ratio of molybdenum to silicon is about 3 to 1 in place of the theoretical 12 to 1.) N-PHEKYLANTHRANILIC ACID INDICATOR. Eastman “white label” material (1.20 grams) was dissolved in a little warm water containing 0.72 gram of technical sodium carbonate, and the solution was made to 1200 ml. (1). BLUEENDPOINT. The disappearance of the blue color at the first end point was determined by the following definition: Looking through the solution onto the surface of a horizontal daylight fluorescent lamp placed at the same height, the end point was considered as the last major chan e in shade, upon swirling after each drop of dichromate was adfed With small samples this is synonymous with a change from blue to yellow or light reen, and even with larger samples, at least during the day, a fairly good blue to green end point can be detected without the I m p . The last drops were added from 0.5 t.o 1 minute apart.
N THE most generally used volumetric determinations of
iron an excess of stannous chloride must be destroyed with mercuric chloride before the standard oxidizing agent reacts with the ferrous ion. However, unless care is exercised in limiting the amount of the reducing agent the appearance of a gray colloidal mercury precipitate will serve as a signal for discarding the sample. In the electrometric determination an inflection point denotes the end of oxidation of the stannous chloride and the beginning of oxidation of the ferrous ion, while a second inflection point a t the end of the titration of the ferrous ion takes the place of a color change in the usual titration. Up to the present no method has employed an inside color indicator where the first inflection point should appear, although many indicators of the diphenylamine and other types have been used at the second inflection point. In the present method silicomolybdic acid indicator, which is reduced t o molybdenum blue by stannous chloride, replaces the troublesome mercuric chloride, since the blue color disappears just before the oxidation of ferrous ion begins. The volume of standard dichromate used between this “blue end point’’ and the appearance of the red of N-phenylanthranilic acid indicator is equivalent to the iron being determined. Although molybdenum blue formed under the existing conditions disappears just after the oxidation of the excess of stannous chloride and thus before oxidation of the ferrous ion begins, it was found b y W u (2) that with a mixed phosphomolybdic-phosphotungstic acid indicator the disappearance of molybdenum blue could be used to detect the end of oxidation of ferrous ion with either hydrogen peroxide or weak dichromate solutions. Were this the case under the conditions that obtain here, the method of the authors would not have been possible. Careful titrations of solutions made from pure electrolytic iron with those made from pure potassium dichromate have shown that with the new method equivalency is maintained within one part in a thousand.
Determination of Iron The determination of iron is described for application t o volumetrically measured solutions of ferric chloride. To 14 ml. of hydrochloric acid (density 1.10 at 25’ C.) in a 500ml. Erlenmeyer flask add 25 ml. of m t e r . Boil until 5 to 7 ml. have been converted to steam, driving the air from the space above the solution past the rvater seal formed under a small watch glass placed over the mouth of the flask. Use a small sintered quartz boiling chip to obtain even boiling. (If much acid is present in the ferric chloride, proportionately less acid can be measured into the Erlenmeyer flask.) Transfer quickly from the burner first used to a hot plate regulated so that the solution boils only slo~vly,and add 45 ml. of the ferric chloride solution being analyzed. Upon resumption of boiling add 2 N stannous chloride drop by drop until the yellow color of the ferric ion disappears, and then add 5 drops of the silicomolybdic acid indicator to cause an intense blue in the solution. The xatch glass is kept in place on the gently boiling solution as much as possible until the solution is cooled. Thus air is kept from re-enterin the flask until cooling renders the solution practically invulnera%le to air oxidation. For the 45 ml. of 0.1 ,V ferric chloride used by the authors, 2.75 ml. of stannous chloride were used, and at the blue end point, the volume was close to 80 ml. If less than this amount of solution is to be analyzed, the volume at the first end point must be adjusted t o 80 ml. by prior addition of water along with the ferric chloride. Immediately titrate with standard potassium dichromate to the disappearance of the blue color. At once add a mixture of 85 ml. of water and 40 ml. of 1 t o 1 sulfuric acid (density 1.49 a t 25’ C.), which has just been boiled for at least 3 minutes and is still hot. Boiling will start again almost at once. Heat for a total of 20 minutes to destroy the silicomolybdic acid, so it will not cause R muddy brown in place of a clear dark red color at the second end point. Remove from the hot plate and cool under running water without swirling (6 minutes were used in cooling). Titrate with the dichromate with constant swirling, using a total of 8 drops of the N-phenylanthranilic acid indicator and observing the end point without use of the lamp mentioned above. The total volume at the end point 778s close t o 200 ml. in the work of the authors, some evaporation occurring. The indicator goes
Reagents and Apparatus In all work check runs were made with separately distilled water and are reported separately. Appropriate blank runs were made with about 0.50 ml. of the iron solutions to furnish data for calculation of the actual blanks, which were found to be about 0.06 ml. in terms of 0.1 AT dichromate. The blank runs, using slightly divergent volumes from 0.50 ml. of iron, were corrected by simple proportion to fit exactly that volume of iron. In the same way in the regular runs the volume of dichromate for exactly 45 ml. of iron was calculated whenever the amount of the iron measured out varied slightly from that value. Potassium dichromate was recrystallized, and part of this again recrystallized, to give the materials from which the A and B series of solutions were volumetrically prepared. The A and B ferrous chloride solutions were prepared from about 2.8 grams of pure electrolytic iron (preserved in nitrogen). 25 ml. of distilled hydrochloric acid (density 1.10 at 25” C.), and water to bring to a volume of about 500 ml. Stock solutions of about 20 liters each were made with ferric chloride and with potassium dichromate and were used in showing that no change occurred in the reagents, and what modifications caused error. In the former the hydrochloric acid content was equivalent to about 25 ml. of 1 to 1 acid per liter. However, the total acid present at the blue end point from all sources, and in all types of runs, varied no more than 10 per cent from the equivalent of the addition of exactly 14 ml. of hydrochloric acid with a 416
lune 15, 1941
ANALYTICAL EDITION
TABLEI. TITRATIONS O F IROK
Iron Sampir
WITH PURE
Dichromatea
TABLE 11. TITRATIOSS OF STOCK FeClp DICHROMATE
DICHROM-4TE
Blank t o Be Subtracted
Calculated Xormality of FeCl2 01 FeCh
417
Type of Titration Normal(bydropsdwith swirling) Fast additi( ' mate without swirli' 3 hours' boiling w i l sulfuric acid (watch glass in place) Same (no watch glasb) ~~
-----Stock Dichromateb-nksaB -l----, 1A 1B 43 55 0.56 0.54 43.5.1: 0.54 0.55
WITH
STOCK
Calculated Dichromate NormalitvC 0.55 0.10071 0.53 0,10076 1C
"
Stock FeCla Pure FeCh, lh, 0 09942 h.
37.32 38.14 44 1 s c 0.58'1
0:os 0 09
0 09718h 0 09932 I).09929 u'09g31
MI. per 45.00 mi. of iron (per 0.50 ml. of iron in blanks). Normalities of dichromate by weight. b Calculated from weight normality of pure iron, using ratio 37.32/:38.11 (blanks canceling), C Average of only 3 runs. d Includes r u n with 131 (0.47 ml.) equivalent t o 0.55 ml. as B2 as calculated from B I / B ~normality ratios. a
out of action rather easily, so that the 8 drops are added in installments. It seems likely that it precipitates, since the sodium salt of the indicator is used in the acidic solution. If the last addition of the indicator is close to the end point it must be followed by at least 2 separate drops of dichromate, the first of which does not cause appearance of the permanent red color.
It would seem wise to filter out any cloudy material, such as silica, during application of the method. Should the blue end point be passed, another drop of stannous chloride can be added and the end point again approached. Results Solutions made from pure electrolytic iron whose normalities were 0.09942 and 0.09960 by weight were analyzed according t o the method, using pure dichromate solutions. The normalities found were 0.09931 and 0.09953 after application of the blanks. Had the blanks not been applied the uncorrected volumes of dichromate would have been such that the higher values for the iron solutions would still check the known normalities to one part in a thousand. The use of blank runs is considered unnecessary unless the presence of appreciable impurities is suspected in the reagents. The results with pure ferrous chloride, B1, solution are tabulated in Table I ; similar results with pure ferrous chloride, Ai, pure dichromate, A,, and pure dichromate, Az, gave the second value discussed above. I n Tables I and 11each figure represents a n average of four runs, deviating from t h a t average by not over 0.04 ml. except where otherwise noted. There is a 0.06-ml. deviation from the average in the case of the 42.45-m1. value for the "3-hour, no watch glass" work in Table 11. I n Table I the stock ferric chloride v a s titrated with pure dichromate, Bi, which was also used to titrate pure iron, Bi. The normality of the former was thus found to be 0.09728 by using the pure iron, B1, as the ultimate standard and employing the volume ratios of the dichromate. The normalities of the dichromate solutions, the impurities which might be present in the reagents, and any errors in the method all cancelled out in the above and in the similar untabulated comparison of the stock ferric chloride and the pure iron, AI, solution. The latter gave 0.09738 N for the stock ferric chloride. I n Table I1 the value 0.09733 was used for the latter solution and was obtained by averaging the two values given above. The normality of the stock dichromate was obtained from that of the stock ferric chloride (Table 11). Careful checks at the end of the work showed no appreciable change in the reagents and stock solutions during the course of the work. I n Table I1 the results with the stock solutions of dichromate and ferric chloride are arranged chronologically. The
43.47e
.,
....
0.10090
.. ..
.. ..
0.10332 0.10298 FeC13 0 09733 N . b 111. pkr 45.00 ml. of FeCla (per 0.50 ml. in blanks). c -0.06-ml. blank on dichromate. d Later it was considered normal procedure t o add faster t h a n b y drops, b u t with continuous swirling, between two indicator changes. 0 Represents 2 runs, averaged. 42.4je 42.69C
0
runs and check runs give a n average of 0.10073 N for the stock dichromate for normal titrations against the 0.09733 N ferric chloride. The fast addition of nearly all the dichromate, in the presence of 4 of the 8 drops of indicator, did not materially alter the results when the titrations were finished with the remaining indicator in the normal manner (third and fourth lines of Table 11). Table I1 shows t h a t there was very little error upon heating for 3 hours with the sulfuric acid unless the watch glass was not in place, when the error due to air oxidation I was such that even in 20 minutes i t would have amounted to over 0.1 in1 I n these experiments boiling was a t a lower rate than usual in order to conserve solution volume.
Blank Runs An elaborate system of blanks was used in the work tabulated in Table 11, in order to test for a n y impurities present in the reagents. It was later concluded that blanks need not be made in routine application of the method. The 1B blank runs actually used were made with but 3 drops of stannous chloride and with about 0.50 ml. of ferric chloride. This same type of blank was the only one needed in the work reported in Table I, since the ferrous chloride used there had oxidized but slightly and so needed but 3 drops of stannous chloride in the regular runs. In the regular runs in Table 11, 2.75 ml. of stannous chloride were used to reduce the ferric chloride, necessitating a blank with that amount of stannous chloride. These 1A blanks used up over 35 ml. of dichromate before the blue end point. Because of the large amount of green chromic ion present at this stage the total volume was made 130 ml. in place of the usual 80 ml. To test the effect of this volume change the 1C blanks were made in the same way as the 1B blanks, except that the volume at the blue end point \vas 130 ml., less water being used later, so as not to change the volume at the final end point from that in the method. In the 1A and 1C blanks the extra water was added with the approximately 0.50 ml. of iron solution. The volume effect wai; negligible, and the 1A blanks checked the other types. In making the calculat,ion of the actual correction to be made on the dichromate a case is selected from the first line of Table 11, where 0.54 ml. of dichromate n-as needed for 0.50 ml. of ferric chloride in a 1B type blank. However, 45.00 ml. of ferric chloride required 43.55 ml. of dichromate, so 0.50 ml. should have required only 0.48 ml. of dichromate. The difference of 0.06 ml. is the blank to be subtracted from the volume of dichromate t o give the 43.49 ml. actually used in titrating the reduced iron. The second line of Table I1 gives 0.07 ml. as the blank and the average is taken to be 0.06 ml. The blanks applied in Table I (next t o last column) were similarly calculated and used.
Summary
A new method for the volumetric determination of iron uses silicomolybdic acid indicator in showing the beginning of ferrous ion oxidation. N-phenylanthranilic acid indicator 1
Any loss of spray from uncovered Rsak is also included under this t e r m
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
418
shows the end of t h a t oxidation with standard potassium dichromate solution. The volume of solution used between the two color changes is equivalent to the iron being determined.
The authors are indebted to H. H. Willard of the University
of Michigan for the electrolytic iron.
Literature Cited (1) Syrokomsky and Stiepin, J. Am. Chem. Soc., 58, 928 (1936). (2) Wu, J. Bid. Chcm., 43, 189 (1920).
Acknowledgment J*
Vol. 13, No. 6
aided the senior author during the
preliminary work with the method.
C ~ N T R I ~ ~ 6.3 T I from ~ N the Chemioal Laboratories of the University of Utah.
Pressure-RegulatingApparatus for Vacuum Systems FREDERICK M. LEWIS United States Rubber Company, Passaic, N. J.
F
REQUENTLY in practical laboratory work i t is advantageous to employ a regulating device for vacuum systems in cases which do not warrant the expense or complication of a precision apparatus. Several vacuum regulators are described in the literature, representing a considerable range of complexity and precision. So far as is known none of these operate on a principle similar to that indicated below. The instrument described here is intended to combine simplicity with a moderate degree of precision. It is not difficult to construct or operate, is self-contained and positive in action, and will maintain a desired pressure in a reasonably tight system within k0.2 mm. in the range 5 to 100 mm. Higher operating limits require modifications. Two types are illustrated; these differ only in the manner of controlling the operation point. In operation, a high vacuum, chamber A , is allowed to act periodically through a sintered-glass disk, on a system, B, in which it is desired to control the pressure. The operation is controlled by means of the mercury column in the open arm of a manostat, chambers A and B being alternately connected and sealed aa the manostat fluctuates in response to pressure differences in B. The level difference, h (Figure I), or the pressure, p (Figure 2), is then the operating pressure of the system.
n SIU TERED-
GLASS DISK
FIGURE 1
Figure 1 illustrates a unit in which the desired pressure is maintained by rotatin the entire apparatus on a pivot until the porous disk is just sealecfby one level of the mercury, as the difference in the levels between the two mercury columns equals the operating pressure. In practice this is not measured on the apparatus but is read on a manometer connected to B. An increase in pressure in B will result in a movement of mercury in the manostat tube, so as to open the disk between A and B. The stopcock, D,is not essential, but facilitates rapid evacuation of the system at the start. It is closed during operation.
h
ONE /NcH
FIGURE 2 Figure 2 indicates a device employing a residual pressure-in chamber C, rather than a balancin mercury column, to set the oint at which the porous disk wilf be sealed by the manostat %he pro er pressure is obtained in C by leaving open sto cocks D and Zuntil the auxiliary manometer connected to B inxicates the desired operation point, and then closingthem. It isadvisable to insert a stopcock (not shown) between the apparatus and pump ,to aid in this last adjustment; it should be closed as the pressure approaches the desired point to allow the pressure to become equalized. Further drop in pressure in A cannot affect the pressure in B unless a leak or gas evolution in B raises the pressure so as to open the sealed porous disk. Stopcock F is provided only to prevent rapid surges of mercury when air is allowed to enter the a paratus at the completion of an operation. Fine adjustments o f the operating pressure are best made by slightly tilting the unit. A 0.75-inch disk constructed of 60-mesh glass or a commercial G-3 disk appears satisfactory for operation in the range 5 to 100 mm. Hi her working pressures may be attained by using a h e r sintered-gyms partition than specified. Lower maximum operation ressure allows coarser disks, because of lower pressure differentiaracross the disk. A satisfactory artition may be constructed of other porous materials cementexin position. Smooth operation is facilitated by mounting the disk in a slanting position. The mercury placed in the manostat should be boiled out under vacuum, so that bubbles of gas will not rise in the closed arm of the manostat tube and cause a drift of operation pressure. The unit should be protected by a dry ice trap to avoid clogging of the disk. If uncondensed vapors pass through the trap an absorption tube may be added to the assembly.
Acknowledgment The author wishes to thank W. A. Gibbons and Clyde Coleman of the U. Rubber Company for their interest and permission to publish t,his work.
s.
