Direct Titration of Ferric Iron by Stannous Chloride S A standard stannous chloridc solution may be stored n i t h out difficulty, the reaction
2FeCI3
+ SnCIj = 2FeC12 + SnCl,
1) * used as the basis of a method for the determination of fcsriron. It is not only possible to carry out the reduction needctl foi this Zimmermann-Reinhardt method, but Fresenius ( 1 ) toutidcd upon this reaction a titriiiwtric determination, later im~)rovcdby other authors (2-4, 6 ) . However, the literature t1oc.h not show that procedures to facilitate, observing the end point of l l i i . t i t r:ttion have becn used i n pr:ictiw. niit\ I IC
Glass Wool
each liter of final solution, and several pieces of ma solved in it to expel the oxygen. Then 12 grams o stannous chloride are added per liter, and after it h the solution is made up to the required volunir. The solution ii stored in a bottle directly connected with the buret. (6). A carbon dioxide atmosphere is maintained over the solution by an automatic apparatus that fits into the stopper of the flitsk (Fi The solution may be standardized against 0.1 'V po bromate solution. Into a titration flask with a i?arro\v neck arc introduced 1 ml. of concentrated hydrochloric acid, several maible fragments, one or two crystals of potassium bromide, and 1 drops of a rubrophen solution or of methyl orange indicator. Then 20 ml. of stannous chloride are added and the sample is titrated rapidly until it becomes completely colorless. The titer does not change appreciably on storage: July 1, 0.8970 F ; October 2, 0.8850 F . Potassium thiocyanate solution, 4.85 grams per 100 grams of water. Ammonium molybdate solution, 1.96 grams per 100 grams of water. Sodium phosphate solution, 1.1 grams per 100 grams of water. Ammonium phosphomo1yt)d:ttr solution, 1.9 grams pcr 100 gr:ims o/' dilute aninintii:1.. ~
~~
Table 1. CaCOl
-~
~
~
~.
Titration of Ferric Iron
(Indicator: 4 drop. of a i n i i i o n i u m molybdate and 3 drops of rodiitni phos~ihate) I'c Taken
l.'r Fo1tnrl
Alg.
Slg.
I>ifference r'
.wg.
+ 0 . 15 - n 11
HCI
Table 11.
Titration of Ferric Iron
(Indicator: a t n n i ~ n i u i i iphosrihoinolybdate, 3 drop?) 1:r
Figure 1.
4pparatus
Taken .MQ.
Fc. I o i i n d Jig.
Diffcrenrc .110.
-n.oo
+0.08
Height of apparatus. 150 mm. Diameter, 40 nim.
--o.o> +O.O!J
Ihpcriments performed by thc authors indicate that the abovr i,eaction enables the elaboration of an accurate and mpid analytical method. Because the reaction progresscs a little slon-ly, rspccially toward its end, the sample should be heated to 60" to 70" ('. and the acid concciitration kept absut pH 0.6. A4ddedanimonium chloride also accelerates thc proems. T o cscludr oxygen, :LII atmosphere of carbon diosidt. i n tht, titration flask is intiispensable. A double indicator is used to show the end point. T h e reduction of most of the ferric ion is indicated by thiocyanatr, and thvn i f the solution is straw-yellow ammonium molybdate is added. 111 the presence of phosphate thc molybdate is reduced by stannous chloride to molybdenum blue. The same reaction t,akw place in the presence of silica, arsenate, or germanate, but these cannot be used as indicators. 011addition of molybdate the solution becomes green, changing to blue a t t,he end point. Thts retluction of molybdate requires phosphate ion in order to incrcitsis the oxidation potential of molybdatcx. Thus the phosphoniolybdate indicator can be used only at low ferric concentration; nthi.1,tho ferric ions bind the phosphoric acid as a complex and thfx niolybdcnum blue reaction citniiot take place. The reaction drscribc:d is reversible. REAGENTS
Stannous Chloride, 0.1 N . In order to avoid the oxidation of stannous chloride, in preparing the solution 80 ml. of concenI r;itvcl hydrochloric acid are poured into the storage hottlc for
-0.01 - 0 , 3(j --O.l-I -(l,25 - 1 . 00
I'(
-2.x3 i , 3.5
+
-0.38 .L 0 , 2!I -0.02 - 0,43 -0.1,; -0.24 - 0 . fit;
I'ROCEDUKI.:
T\\-ent>.milliliters of thc solution to be tested, rontaiiiing 3 to 130 nig. of iron, arc pipetted into :I 100-ml. flask x i t h B narrow ricsclt and acidified Ivith 1 to 2 ml. of concentrat,ed hydrochloric, :ic.itl. Then 1 gram of ammonium chloride is added, the misturr is Iio:ttcd to about GO" to 70" C., and 2 t o 3 drops of thiocynn:ttr ~ added. The liwly vfsolutinn :ind sevewl piecacs of n i : ~ r h l (arc' Table 111. l'e Found Mg.
Batch i n a l y s e s
(57.61 mg. of iron taken) Added rorcigrl Ion
+0.26 -0.29 +0.26
. i i .90
57.93 J 7 . 90
0.0
57.64
+n.o3 +0.05
5 7 . 67 37.69 27.61 .i7.66 57. 69 57.59
-0.03 +0.02 +0.05 -0.05
S7.69
57.61 57.74 57.67
ni ffercnrr Jlg.
MQ.
Sh"
IO i
io
+O.OJ -0.03
+0.10 +0.03
ANALYTICAL CHEMISTRY
362 fervescing solution is titrated by stannous chloride until its color pales to straw-yellow. Then 4 drops of molybdate and 3 drops of phosphate solution are added and the titration is continued until the color changes from green to blue. The last few drops are added slowly, to allow time for the reaction to take place. The procedure, however, does not last more than 1.5minutes.
The suggested method has the advantage that the standard Can be readily prepared stored and no special WUiPmerit is required for Observation Of theend Point.
The accuracy of the method is illustratedbythedataof Tables I and 11. Because aluminum, manganese, Einc, l e d , tungsten, and msenic do not interfere, the procedure is especklly efficient for the determination of iron in ores. A small amount of copper does not interfere, but if there is much more copper than iron, the end point is uncertain. Antimony if oxidized to the pentavalent state causes no interference. The procedure cannot be used in the presence of vanadium. Table 111gives dat.a obtained in batch andylysos.
(1) Fresenius, R..Z. ami. Cfiem., I . 26 (1862). (2) Jellinek, K., m d Winogradoff,L., Z. anorg. u. albem. Chem., 129, 26 (1923); 138, 101 (1924). (3) Muller, E..and Gbrne, J., 2. anal. Cfiem., 73,385(1928). (4) Scott, -standard Methods of chemical ADS~YS~S,"VOI. 1, p. 484, New York, D. Van Nostrand Co., 1939. (5) Sutton. I., "Volumetric Analysis," 12th ed.. p. 262, London. J. & A. Churchill. 1935. ( 6 ) Zengelis, C., B e . , 34, 2046 (1901).
LITERATURE ClTED
Recmveo April 4, 1949.
Assembly for Positioning Cuvettes Used for Microanalysis with Beckman Spectrophotometer V. EVERETT KINSEY Massachusetts Eye and Ear Infirmary and Harvard Medi
HE new cuvette holder discussed, with carriage and mechaTnism for positioning it, is especially suitable for the micro adaptation of the Beckman spectrophotometer as described by Lowry and Bessey (I). These workers adapted the Beokmen spectrophotometer for the colorimetric estimation of extremely small quantities of material. They substituted cuvettes (Pyrocell Manufacturing Company, 207 East 84th St., New York, N. Y.) whose widths are two tenths that of ordinary cuvettes without d e creasing the length of the light path (1 cm.), and reduced the depth of the solution necessary for analysis to several millimeters by placing a diaphragm having a hole 1.5 mm. in diameter in front of the exit slit of the instrument. These modifications make i t possible to carry out an analysis on 50 cu. mm. of fluid with approximately the same accuracy as the macromethod. It is essential, however, when using the micro adaptation that the cuvettes be reproducibly positioned within a limit of 0.25 mm. on either side of their centers, because the pencil of light emerging from the hole in the diaphragm is but 0.5 mm. less than the width of the cuvettes. The operator is handicapped in carrying out this exact positioning by two structural elements of the spectrophotometer. First, the construction of the carriage which carries the cuvettes isof light weight and is supported only centrally, so that considerable care must be exercised to prevent minor fluctuations in the position of the cuvettes. Secondly, the cuvettes cannot be positioned emilily in the individual compartments of the cuvette holder. This makes it difficult to bring the separate cuvettes into juxtaposition with the light beam when the carriaee is moved from one detent to another. In practice, the positioning has to be made frequently by fixing the cuvettes tightly against the spring of the individual cuvette compartments by means of wooden shims. In the author's experience it has been necessary to place shims on both sides of some of the cuvettes to accomplish the necessary positioning. I n other words, this positioning has to be done in part by moving the cuvettes independently from the cuvette holder. Although good analytical results can be obtained despite this difficulty, so much experience and patience are required that in many laboratories i t is customary to leave the cuvettes k e d in position throughout a long series of analyses., This is sometimes a disadvantage, as ~
itbecomesdiffib-. "" l.".l.. l..l ru..vre in case of spillage or other accident. To correct these shortcomings 8 new type of holder for the cuvettes, 8 new carriage, and a new mechanism for moving the carriage have been constructed (Figure 1). uY.IUUI
The cuvette holder is constructed by milling a trough from a solid piece of brass, the width of the trough being about 1 mm. more than that of tho cuvettes, and the depth being such that when the cuvettes are in place the light beam will just clear the lower surface. This makes it possible to use a minimum quantity of fluid for analysis. The extra millimeter of space is provided to hold a phosphor bronze spring, which is split into four leaves, so that the cuvettes are separately held tightly against the front of the cuvette holder. The spring extends about halfway down the surface of the cuvette. A seoond spring is placed near the end of the cuvette bolder to press the four cuvettes tightly against each other. Holes are drilled a t suitahle levels in the face of the cell holder to permit passage of the light. A handle is attached to the holder to facilitate placing i t in the carriage. The carriage consists of a brass base, on either end of which upright strips of spring phosphor bronze m e soldered. The cuvette holder fits snugly between the upright strips and rests solidly on the base. The outside dimensions of the holder are
