A More Environmentally Acceptable Determination of Iron in An Ore Jeffrey Byars and Terry W. ~ c ~ r e a r ~ ' Murray State University, Murray, KY 42071 The method described by Kaufman and DeVoe (1)for titrimetric determination of iron, in which zinc is used a s the reductant in situ rather than s' a Jones reductor, has several advantages over conventional determination of iron. It disposes ofthe use of mercury(I1) compounds, which are environmentally problematic, it avoids the preparation reductors, and the redu&iou~reaction and upkeep of is easy for the student to understand. Consequently, we have adanted this method to the classical determination of iron in ore samples in our quantitative analysis course. A desirable feature of our modification is the use of standard permanganate in place of dichromate; the residues from the titration (containing manganese (11). iron (111). zinc (111, sodium ion, chloride, phosphate, and sulfate) are relativelv innocuous once the excess acid is neutralized. A minor problem with the reduction of iron by zinc is the large excess of HCI required to dissolve the ore sample. To minimize the amount of zinc consumed by the HCI, some of the excess HCl is removed by boiling briefly after dissolution of the ore is complete.
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Procedure 1. In a hood, add 15 mL of concentrated HCI to weighed samples of ore ('%on Oxide", Thorn Smith Chemist, Tmy, MI) in 500-mL Erlenmeyer flasks.' Cover with a ribbed watch glass (or an ordinary watch glass with glass hooks). Heat nearly to boiling in the haad (a large hotplate is convenient), swirling occasionally until the ore is dissolved, about 5-15 min. Some white residue of silica may remain and can be ignored. Add additional HCl only if evaporation reduces the volume significantly;that is, iron oxide is reprecipitated. Fallowing dissolution, boil the solution gent13 for a few minutes to expel as much HCKg) as possible. 2. Cool briefly, and add about 1 g of zinc metal,4replacing the watch glass immediately. Heat m cool the flask as necessary while swirling it gently, to maintain a vigorous but not violent reaction. until the zinc is consumed. A few flecks of nonreartwe black restdue may be ignored Add a ~ c c o n dl-g port~nno f r m c and heat or cwd the aulutlun aa necemary. 3. Add about 0.5 e of zinc and heat or coal as necessam. Repeat addluon of these smaller ponions unril reduction is complete When the solution appears to be complelrly rcduced, addanothcr0.5 gof7incand hcat nenrlg to hoxling for about 5 min." 4. If any zinc remains undissolved, add 5 mL of concentrated H2S04in 20 mL of water, and swirl the flask until the remaining zinc is dissolved. Add 15 mL of ZimmermannReinhardt reagent (79g MnSO,, 100 mL H,S04, and 200 mL H3P04,diluted to 1L)and dilute the solution to about 150 mL with water. Titrate immediately with standard 0.02 M K ~ n 0 ~ . ~ . ~ completing ARer the titration, carefully neutralize the contents of the flask (we use 50% NaOH) before discarding into waste containers or as directed by the instructor.
To minimize the time required for thedetermination,our students prepare the standard K.Mn04 by volumetric dilution of 0.2 M stock solutiou. The latter & prepared a s follows: dissolve 3 3 2 3 g of reagent KMn04 and dilute to a Iltcr: hcat to near-boiCng for i n hour, c o o ~ ~ o v e r n iin ~ hthe t dark; filter through a fritted-glass filter, discarding the first 50 mL of filtrate: store in the dark in a brown elass bottle. I t has been our experience that this stock solution, and standard solutions prepared from it, are stable for a t least a month if stored a s noted above. The standard solution is DreDared bv tenfold dilution of the stock solution. standardization i"s performed by the McBride method, against sodium oxalate. Alteruativelv. the standard could be prepared by precise volumetric diiition of a stock solution of known concentration. I n our laboratory, the students prepare the solutiou by volumetric dilution, then standardize the solution. This permits assignment of a grade for the accuracy of standardization and of the iron determination. Results of this analysis tend to be 0.5-1% low, and precision is about 0.7% relative standard deviation for very good work. Although this is not as good as the more convenlonal methods, mlmnuzatlon of waste generated can make this modified method desirable when moderate accuracy and precision are acceptable. Literature Cited 1.Kaufman,S: DeVoe, K J Chpm Edue. 1988,65,1&184.
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For 2WnL burets and ore that is about 35% Fe, samples of about 0.3 g are appropriate. The HCI concentration cannot drop much below 6 M, but that does reduce somewhat the amount of zinc required for reduction. We found that zinc ribbon (J.T. Baker) reacts more readily and is easier to dispense than granular zinc, 1 g being about 3 4 cm of 0.005-in. ribbon. The blue-green hue of iron(ll)will be apparent in a solution that is completely reduced; a solution that appears green with a trace of yellow will require an additional portion of zinc. 6Wefound that 3 g of zinc will suffice for reduction of 0.3 g samples dissolved as described above. The final portion of zinc should not be added until one is ready to complete that particular titration, since iron(ll)undergoes air oxidation. If no water has been added to the solution, reduction is rapid, taking a total of 1&15 min, and the higher concentration of iron makes it easier to see the color change that indicates complete reduction. A solution which has been diluted with water may take considerably longer to reduce. The greenish tint of the solution, which intensifies as the titration progresses, makes endpoint detection a bit troublesome but not particular!~ difficult.Dilution with water minimizes this side reaction and makesthe endwint much easier - ~ - to -~ see. - - The - - oermanoanate -~ =- - endooint - -rt h ~ appears s as a cnange frompae green to tan. An absolute y white baCKgrO~n0for tne flask :s of consioerabe nelp in pnpolntlng the enopo nl, and an inilia lr8al"tllrat on 1s InvalJaole for most st~dents.
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Volume 69 Number 11 November 1992
935
