1040
A N A L Y T I C A L CHEMISTRY
Figure 7 illustrates again the phenomenon which the rotary stirrer seems to detect far more rea'dily than the reciprocating stirrer-the appearance of two crystalline modifications. From the shape of the curve evidence for two modifications of crystals for 1,l-diphenylbutane is shown, one melting a t -25.05' C., and another a t -28.35' C. I n other runs only the higher or the lower melting form appeared. The apparatus described is used when the dompounds cool to a viscous liquid or to a glass, and when the conventional techniques will not yield satisfactory freezing curves. I n such cases the crystallization is so slow that the heat evolved by virtue of fusion and stirring exceeds the rate of heat transfer through the walls of the freezing tube. The rotary stirrer produces a lower heat of stirring than the reciprocating stirrer, which is shown by calibration curves. At the same time, the larger area of conducting metallic surfaces im-
mersed in the liquid provides a uniform temperature throughout the fluid in addition to adequate agitation for thermodynamic equilibrium. ACKNOWLEDGMENT
The author is grateful to John H. Lamneck for the sample of allylbenzene and to Kasper T. Serijan for the sample of 1,l-diphenylbu tane. LITERATURE CITED
(1) Glasgow, -4.R., Streiff, A. J., and Rossini, F. D., J. Research Natl. Bur. Standards, 35, 355 (1946). (2) Mair, B. J., Glasgow, A. R., and Rossini, F. D., Ibid., 26, 621 (1941). RECEIVED for review J u n e 6, 1951. Accepted January 4, 1952. Presented before t h e Division of Analytical Chemistry a t t h e 119th Meeting of t h e A M E R I C ~CHEVICAL U SOCIETY, Cleveland, Ohio.
lodometric Codetermination of Copper and Iron ROBERT C. BRASTED School of C h e m i s t r y , University of Minnesota, Minneapolis, M i n n .
HE iodometric titration for the estimation of copper is a well Testablished procedure. The corresponding titration for the iron(II1) ion, though known (a), has not received the attention it deserves. The usual procedure in the determination of copper in either an alloy (free of arsenic and antimony) or an ore is to buffer the iron interference with fluoride ion or to remove the copper by reduction to the metallic etatr by aluminum (6, 6). S o instances have yet been found in the literature in 1% hich the copper and iron have been codetermined iodometrically using aliquot portions of a single sample. There is a wide variety of proceduIes for the pretreatment of a nitric acid solution containing dissolved copper to eliminate nitrous acid prior to the iodometric titration. The nitric acid solution may be evaporated to fumeswith sulfuric acidand hydrochloric acid or evaporated to the appearance of copper oxide ( 4 , 6 , 7 , I O ) . The titration of a freshly boiled and cooled solution is also recommended. In any of these procedures errors are likely through the loss of solution by spattering. Urea is effective in removing nitrous acid from dilute warmed solutions. A precipitate of urea nitrate is formed, however, when urea is added to nitric acid solutions of about 1to 1 concentration. The waiting period involved in allowing a solution to assume room temperature after boiling is tedious. It is obvious that the iodometric titration of a solution containing even small amounts of nitrous acid will be in error. The presence of excessive quantities of mineral acids, especially nitric acid, has been reported by Khitehead and Miller ( I O ) as a source of m o r in the iodometric titration of copper. The purpose of this investigation is threefold:
prior to the preparation of a 1 to 1 nitric acid solution used in the experiments. EXPERIMENTAL
Copper Determination in Presence of Nitric Acid. Stock solutions of a brass sample were prepared by dissolving the alloy in 1 to 1 nitric acid. The solutions were not evaporated nor were any other mineral acids added. After complete reaction the solutions were boiled, cooled, transferred Tyith any precipitated metastannic acid to a volumetric flask, and diluted to volume. The concentration of nitric acid in the solutions was about 0.14 111. Aliquot portions of these stock solutions n-ere used for both the copper and iron determination. The amount of copper added (see Table I ) was computed from the thepretical copper content of the brass, 59.9070 by electrolytic analysis. The iodometric determinations of copper by the several procedures later described are in good agreement a-ith the electrolytic
Table I. Detn. So.
Effect of Nitric Acid on Iodometric Titration of Copper
Copper Coppera Error, Added, G. Found, G. XIg.
1
0.0761
2
0.0761
0.0762
3
0.0761
0.0767
4
0.0761
0.0894
5
0.0844
0.0846
6
0.0844
0.0844
7
0 0761
0.0817
8
0.0761
0.0763
9 10
0.0844 0.0761
0,0847 0.0771
11
0.0761
0.0767
12
0.0844
0.0847
REAGENTS
13
0.0761
0.0771
Sodium thiosulfate, sodium hydroxide, acetic acid, potassium iodide,. potassium acid phthalate, and ammonium acid fluoride of analytical reagent grade were used without further purification. The thiosulfate solution was standardized against copper. Sulfamic acid \%-ascommercial grade, twice crystallized, sulfatefree. Nitric acid was Du Pont C.P. reagent, 70% by weight, specific gravity 1.42. N o attempt was made to remove oxides of nitrogen
14
0.0844
0.0850
1. To develop a simplified rocedure for the preparation of a n alloy sample (as brass free or antimony and arsenic) whereby neither evaporation, fuming, boiling, and cooling, nor elimination of nitric acid is necessary prior to an iodometric titration. 2. To develop a procedure whereby copper and iron may be COdetermined volumetrically on aliquot portions of a single sample using a single standard solution. 3. To determine the best buffer conditions using fluoride ion to eliminate iron interference, in the determination of copper.
0 0761
Remarksb
0.0 Freshly boiled solution, 0.14 M H s 0 3 , no sulfamic acid added +O.l 0.3 gram of sulfamic acid added hefore addition of K I + 0 . 6 Solution aged 24 hours, no sulfamic acid added +13.3 Solution aged 3 days, no sulfamic acid added + O 2 Solution aged 24 hours, 0.3 gram of sulfamic acid added 0 0 Solution buffered with NaOH and HOAc before titrationc, 0.3 gram of sulfamic acid added 4-5.6 6 ml. of 1 : l HSOa, no sulfamic acid added 0.3 gram of sul+ 0 . 2 5 ml. of 1 : 1 "03, famic acid added +0.3 Same detn. 8 +1.0 10 ml. of 1:l HPiOa, 0.3 gram of siilfamic acid added of "08, 0.3 gram of sulfamic +0.6 1 added , of 1:l "03, 2 grams of sul+0.3 I - --id added +1.0 : 1 HKoa, 0.3 gram of sulid added : 1 "03, 2 grams of sul+0.6 famic acid added ~
0 Calculated from total thiosulfate less t h a t attributed t o iron present in original brass sample. b Unless otherwise stated, no sodium hydroxide-acetic, acid or acid phthalate buffers used. Titrations made on nitric acid solut!ons. c NaOH. 4 A' added t o appearance of Cu(0H)z. Acetic acid added until Cu(OH)x just disappears.
1041
V O L U M E 2 4 , N O , 6, J U N E 1 9 5 2 analysis (see determinations 1 and 2, Table I, and determinations 1 through 8, 12, 15, and 16, Table 11). I t is noted in Table I that when aliquots are taken from the freshly boiled solution, the amount of copper found agrees with that added and that the difference between the buffered (No. 6 ) and the unbuffered determinations is no more than experimental error. An appreciable error is found on analysis of aged samples even in relatively low nitric acid concentrations (see Sos. 3 and 4, Table I). The rate of nitrous acid formation is rapid enough to necessitate some pretreatment prior to an iodometric titration (1, 2 ) . The elimination of nitrous acid by sulfamic acid is readily observed on aged solutions or solutions containing large quantities of nitric acid by nitrogen effervescence. The reaction is represented by
HSO?
+ HSOjSHi +KA+ H2S04 + Hi0
The error in terms of copper found is increased when more than 5 ml. of 1 to 1 nitric acid are added, but is decreased when sulfamic acid is added in a sufficiently large quantity (2 to 3 grams instead of 0.3 gram), so that crystals are present throughout the iodometric titration. This fact is proved by determinations 7 through 14. Previous investigators ( I O ) have reported the following errors in the iodometric titration of copper in the presence of 1 to 1 nitric a c i 8 2.3 mg. with 5 ml.; 6.6 mg. with 10 ml.; 20.3 mg. with 25 ml.; and no detectable end point with 50 ml. Determinations 13 and 14 prove that titrations with reasonable accuiacy are possible even in the presence of 50 ml. of 1 to 1 nitric ' acid if the solution is pretreated with 2 to 3 grams of sulfamic acid. The starch-iodine end point is stable for a sufficiently long peiiod of time to permit accurate titrations.
the addition of acid phthalate, as the acid fluoride furnishes sufficient hydrogen fluoride and fluoride ions ( 3 , 4). One gram of potassium iodide dissolved in 10 ml. of water was added and the solution was titrated with standard thiosulfate using st'arch as an indicator, The iron determinations were made by direct t.itration of solutions pretreated with sulfamic acid, unbuffered and no fluoride added. The ercentage of iron was calculated by subtracting the volume o r thiosulfate due to copper alone from the total volume of thiosulfate due to co per and iron. The effectiveness of the sulfamic acid is shown g y a determination in which this reagent was omitted. The amount of iron added was 0.0075 gram, the amount found was 0.0121 or an error of 4.6 mg. Determinations 1 through 16 represent a range of iron percentage from 0.35 to 87.43. I n all cases the amount of copper and of iron found agrees within experimental error with that added. The excess quantity of sulfamic acid used when large amounts of nitric acid are present (Table I ) was not needed for the determinations recorded in Table 11, as the concentration of nitric acid never was greater than about 0.2 AI. Bufier and Complexing Systems. Satisfactory determinations of copper were accomplished in sulfamic acid-pretreated solutions to which 4 M sodium hydroxide was added until copper hydroxide precipitated, for low iron content, or iron(II1) hydroxide for high iron content. Potassium acid phthalate and ammonium acid fluoride in about. equal quantities (2 grams) were' then added. Identical results ryere obtained in these analyses, whebher acetic acid or potassium acid phthalate was added aft,er neutralization with sodium hydroxide. An attempt JTas made t,o use phosphoric acid as a complexing agent for t,he iron; however, unsatisfactory results were found in terms of Ion- iron cont,ent due to the instability of the iron phosphate complex. Sufficient. iron(II1) ion is present, even aftrr phosphate complexing to oxidize iodide to iodine.
Table 11. Codetermination of Copper a n d Iron Ironb Irona Error, \Ig. Added, G. Found, G. 0 0844 0,0005 0 0006 0.0 +O.IC 0 0005 0 0006 0 0844 0 0 +O.IC 0.0076 0 0761 0.0000 -0.1 .. d 0.0075 0 0761 -0 2 0.0000 .... d 0.0075 0 0761 0 0000 +0.3 0 0761 '0'6 0.0075 0 0075 0.0 0 0781 0 0 0.0075 0 0075 0.0 0 0761 0 0000 8 +o 0 0.0217 .... 0.0217 0 0761 0 021Y .... f0.2 0.0217 to 0 0761 .. 0 0220 +0.3 I1 0 0761 -0.1 0.0712 0 0711 0.0712 12 0 0761 o:oia1 0 0000 0 0 . . . . d 0.0708 13 0 0061 0 0708 ... 0.0708 14 0 0061 0 0709 CO 01 0:Ooil 15 0 0061 0 0 0 0708 0 0000 . . 16 0 0061 0.0063 f0.1 0 0708 0 0000 . . .d " \Vhere both copper and iron determination are recorded, different alicluots were used for calculations. \T-hen only copper or iron determinations are recorded. copper is calculated f r o m total roliune of thiosulfate less t h a t attributed t o iron present. b Includes 0.35'7, iron f r o m original brass amyl le. .. Solutions buffered n-it11 sodiuru hydroxide a n d acetic acid (see Table I , 1lPtn.
No.
C.iliiier .4dded, ( 2 ,
