EDTA Titration of Total Iron in Iron(l1) and Iron(ll1) Mixtures Application to Iron Driers CLAUDE A. LUCCHESI and CLYDE F. HlRN Analytical Research Department, The Sherwin-Williams Co., Chicago 28, Ill.
,An (ethylenedinitri1o)tetraacetic acid (EDTA) titrimetric method for the determination of both iron(ll) and iron(II1) gives results in close agreement with those obtained by the Zimmermann-Reinhardt permanganate method. An aqueous solution contoining about 10 mg. of iron i s diluted with alcohol and the pH i s adjusted to 2 or lower. Excess EDTA i s added and the solution is neutralized. Then buffer and Eriochrome Black T are added, and the unchelated EDTA i s back-titrated immediately with zinc. The titration must be carried out in a solution consisting of at least soy0 alcohol in order to prevent indicator blocking. The accuracy of the method as estimated from the over-all bias of 18 determinations i s t0.002 0.02 mg. where 0.02 mg. i s the standard deviation of the bias. The precision, as estimated from the total standard deviation, i s 0.082 mg. The method has been applied to the determination of iron in iron driers.
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VI
have been reported for the E D T A titration of iron(II1j in aqueous solutions. To :ipply the EDTA method to systems containing both ferric and ferrous iron for the determination of total iron, the ferrous iron present had to be oxidized. KinIiurieii and Winnerstrand ( 7 ) reported a method which does not specify that the iron must be in the ferric state. However, thc method did not work hecausr upon tlie addition of the Erioi.hronie Black T to the buffered solution Ibontaining the excess EDTA, the indi(*atorimmediately assumed its end point color, red. Apparently in aqueouy *olutioii the iron forms so strong a complex with thl: Eriochrome Black T that it leaves the EDTA complex to form a complex with the indicator, even in t,he presence of a large excess of EDTA (12). The method reported here is similar to that of Kinnunen and Winner3trand ( 7 ) ,with the important difference that the end point of the back-titratioii is reached in an alcohol-water solution ANY PROCEDURES
i
containing 50 to 60% alcohol b y volunie. In the presence of the alcohol, the reaction between the iron-EDTA chelate ~ and Eriochrome Black T is s l o enough to permit a n accurate determination if tlie back-titration is carried out within 2 to 3 minutes. The alcohol method has been applied tcJ the determination of the total iron content of iron drier>. The same advantages reported previously for other paint drier5 also hold in the enbe ( ~ iiron driers (5, 8). X single drier determination can be made in about 30 minute\. EXPERIMENTAL
Reagents. The 0.01X EDTA, 0.01-If zinc, buffer, and indicator solutions are given by Lucchesi and Hirn ( 8 ) . Chemicals. 1 to 3 hydrochloric acid, 1 to 3 ammonium hydroxide, and 95% ethyl alcohol. Procedure. Standardization of t h e E D T A solution is given by Lucchesi and Hirn (8). IRONSALTS. Place an aqueous ~ o l u tion of iron salts containing about 10.0 ing. of iron in a volunie of 50 ml. or less in a 400-nil. beaker. Add 150 nil. of alcohol and adjust the p H to 2 or lower with 1 to 3 hydrochloric acid if necessary. From a buret add 40.00 nil. of 0.01.1.1 EDT.1 and neutralize n ith 1 to 3 ammonia. Seutralization is indicated by a change in tlie color of the solution from yellow to reddish. Add 10 nil. of the buffer solution and 0.3 gram of the Eriochrorne Black T indicator mixture. Immediately backtitrate the excess EDTA n-ith the 0.01M zinc chloride. The first appearance of the red color as seen in the standardization procedure marks the end point. The back-titration must be completed \\ithin 2 minutes. Calculate the iron Inoncentration with the expression: CC
Fe =
From a buret add 40.00 ml. of 0.01h~ E D T A solution and follow the above procedure. RESULTS
Results obtained with the iron salts procedure for the total iron content of aqueous solutions of the iron(I1) and iron(II1) chlorides, sulfates, and a m monium sulfates are shown in Table I. The results for solutiolls containing bob11 iron(I1) and iron(II1) animoiiium sulfates are shown in Table 11. I n hot'h tables the amounts of iron taken are based upon aliquots of standard solutions standardized by the Zininiermann-Reinhardt method ( 4 ) . The Zimmermann-Reinhardt results given in the tables are the averages of three determinations. The salts used are Baker C.P. ferrous chloride, crystal, FeCl?.4Hz0; llatheeon reagent grade ferric chloride, anhydrous poivdw sublimed; Baker C.P. ferrous sulfate, crystal; Baker C.P. ferric sulfate, powder; Merck reagent grade ferrous aninioiiiuni sulfate, Nohr's salt; Fisher reagent grade ferric ammonium sulfat'e, carystal. Results obtained with the iron drier procedure for iron iiuphthenutt~ and iron octoate are shown in Table 111. These results are compared with those obtained wit,h .%3'T;2I Method D 564-47 (1). DISCUSSION
I n completely aqueous media iruii(II1) is generally chelated with EDT.1 a t p H 2 to 3. At a higher p H the ferric ion hydrolyzes to form appreciable concentrations of hydroxy complexes which are more stable than the ferricE D T A chelate (10, 14). On the other
[(ml. EDTA) (molarity EDTA) - (ml. Zn) (molarity Zn)] (55.85) (100, weight of sample, mg.
IRON DRIERS. Weigh 0.2 gram of the drier to the nearest 0.1 Ing. h t o a 400ml' beaker' Add loo ml* Of and 3 ml. of 1 to 3 hydrochloric acid. Heat just to boiling on a hot plate: remoye and cool to room temperjture:
hand, based U F ) O I ~the criteria used by ill&, and v a ~ o u l i s(11),the minimum pH required for a satisfactory titration of iron(I1) with E D T A is about 7. This means that a back-titration proVOL. 32, NO. 9, AUGUST 1960
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Table 1.
Iron Content of Iron Salt Solutions
Salt in Solution Ferrous chloride
Ferric chloride
Iron, hIg. Taken Found 10.42 10.34 10.28 10.34 Av. 10.32 10.84 10.75 10.82
Ferrous sulfate
Ferric sulfate
Ferrous ammonium sulfate Ferric ammonium sulfate
10.96 Av. 10.84 13.62 13.69 13.74 13.69 Av. 13.71 10.33 10.28 10.28 10.28 Av. 10.28 11.04 11.13 11.13 11.13 Av. 11.13 11.18 11.19 11.16 11.14 Av. 11.16
detecting the end point is available. The two metal indicators commonly used a t pH 9, murexide and Eriochrome Black T, are not suitable for the E D T A titration of iron in water. The complex formed with murexide is too neak, and the complex formed with Eriochrome Black T is too strong. Effect of Solvent. T h e concentration of alcohol in t h e alcohol-n ater solution was found to affect the rate of reaction between the iron-EDTA chelate and Eriochrome Black T. As the ratio of alcohol to viater is increased, a correspondingly longer time elapses before t h e appearance of t h e irreversible red end point color. Under the conditions of the recommended procedure, the alcohol concentration is high enough to allow a back-titration time of 2 to 3 minute-, which is sufficient time under ordinary circumstances. A similar situation exists in the titration of cobalt (8). Similar experiments were performed with other solvents: dioxane, acetone, acetonitrile, methanol, propyl alcohol,
Table
cedure must be used. Excess E D T A must be added to the solution of iron(I1) and iron(II1) a t pH 2 to 3 so that the ferric chelate can be formed quantitatively. Then the pH must be raised to over 7 so that the ferrous chelate can form quantitatively. Because the effective stability constant for the ferrous chelate is a t a maximum a t pH 9, the back-titration should be carried out in a buffer of this pH. Thus, even in 100% water solution, backtitration a t pH 9 should make possible the determination of the sum of iron(I1) and iron(II1) salts, provided a means for
Table II. Total Iron in Solution Containing Iron(l1) and Iron(ll1) Ammonium Sulfate
Iron Taken, Mg. Iron Iron (11) (111) Total 2.228 8.940 11.17
4.455
6.705
Found 11.13 11.14 11.16 Av. 11.14 11.16 11.13 11.13 11.11
6.683
4.470
8.910
2.235
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Av. 11.12 11.15 11.09 11.09 10.96 Av. 11.05 11.15 11.09 11.08 11.10 Av. 11.09
ANALYTICAL CHEMISTRY
111. ASThl
Comparison Methods
of
Drier
EDTA
yo Iron in Iron Xaphthenate 6.14 6.14 6.09
Av. 6.12
6.09 6.04 6.01 6.05 6.07 6.08 6.05
yo Iron in Iron Octoate 6.15 6.01 6.20 6.07 6.13 6.03 6.11 6.12 6.16 Av. 6.05 6.14
isopropyl alcohol, and ethylene glycol. I n general, the results indicate that any of the above solvents, except perhaps methanol, may be used in place of ethyl alcohol. I n dioxane, propyl alcohol, and isopropyl alcohol, Eriochrome Black T assumes a yellowish or green rolor and the end point color change is not easily detected. The role of the organic solvent in retarding the reaction betn een the Eriochrome Black T and the iron-EDTA chelates is not known. Apparently more than a dielectric constant effect is involved because solvents of very low dielectric constant (az dioxane) are no more effective in slon ing the reaction rate than solvents of high dielectric constant (as acetonitrile). The functional group of the solvent does not seem to be of primary importance becawe
retardation was observed with a number of classes of solvents. However, each solvent tested contained functional groups theoretically capable of coordinating with iron, and it may be that these solvent molecules actually enter the coordination sphere of the iron in the iron-EDTA chelate to produce a species with different reaction kinetics. According to Schwarzenbach and Heller ( I S ) , in aqueous solutions of low pH, E D T A acts as a hexadentate group and occupies all the coordination positions of the iron. As the pH of the solution containing the hexadentate chelate is raised, OH- groups enter the coordination sphere of the iron and carboxylate groups of the EDTA are freed. At pH 9 to 10 the main iron-EDTA species present in water are FeY(OH)-2, FeY(OH)z-3, F e y + , and FeY(OH)-3, where Y-4 is the E D T A anion. As the pH is raised in the alcohol-n ater solution the hydroxy groups of the alcohol may enter the coordination sphere of the iron in place of the OH- groups, and thus, form a species more stable to substitution by Eriochrome Black T. Chelating agents with the structure of EDTA, except that one or more of the acetic acid groups have been replaced by ethanolic groups [as hydroxy(ethylenedinitril0) triacetic acid], are known to have improved alkaline stability ( 3 ) . Iron Salts Procedure. The procedure for iron salts is expected to work for simple salts of iron(I1) and iron(II1). All cations which can be titrated with E D T A in alkaline media will interfere and must not be present in the sample or must be masked; anions which form strong complexes with iron also interfere ( I d , 1 4 ) . The chief advantage of the recommended procedurp over the Tiron method for total iron is that the sample need not be oxidized prior to the titration. Thus, special precautions need not be taken to prevent the photosensitized reduction of the iron(II1)-EDTA chelate (6). The Eriochrome Black T end point is also sharper than that of Tiron. The iron salts method was applied t o the determination of iron in National Bureau of Standards Sample 27b, Sibley iron ore, to estimate the precision and accuracy of the method. Samples containing about 120 mg. of iron were treated with concentrated hydrochloric acid and diluted to 260 ml. The iron in three separate 25.00-ml. aliquots was than determined according to the iron salts procedure. The averages of the three titrations for four samples are 68.39, 68.43. 68.48, and 68.03%. The standard deviation of the four results is 0.21’7& The average value of all EDTA results is 68.33%; the NBS value is 68.23%. The precision and accuracy mere also estimated from the data in Table 1
by a procedure described by Strouts, Gilfillan, and Wilson (14). The accuracy of this method is given by the over-all bias, which was found t o be 0.002 i 0.02 mg., where the 0.02 mg. is the standard deviation of the bias. The precision of the method, as estimated from the total standard deviation of the E D T A results, is 0.082 mg. The standard deviation of the E D T A results within samples is 0.049 mg.
The use of alcohol in E D T A titrations for other metals may be used to prevent traces of iron from blocking the Eriochrome Black T indicator. The end point in the titration of zinc solutions containing a trace of iron was sharpened by the addition of alcohol to the sample. Iron-Drier Procedure. T h e iron drier procedure is patterned after a n earlier iron salts procedure for t h e other paint driers a n d differs only in t h a t t h e iron drieis niust be acidified and heated t o make t h e iron available for chelation with E D T A (8). Although it has been tested for only the iron octoate and naphthenate driers, i t should be applicable to other iron carboxj late compounds, provided solution can be effeclted. The heating makes the benzene used in the earlier method unnecesm-y. The addition of the iron drier to the alcohol-water results in a turbid solution, nhich can be clarified with hydrochloric acid. The turbidity formed b y the naphthenate drier can be cleared up I!ith any acid, but only hydrochloric
acid is able t o clear the octoate solution. When the clear hydrochloric acidtreated solutions are titrated, the results are slightly low if the solution is not heated. Even rvith heating, titration of naphthenate samples acidified n ith acids other than hydrochloricLe., sulfuric, nitric, perchloric-yield low results. This indicates that the chloride has a special function, and consequently, hydrochloric acid is specified in the procedure. Direct titration of the iron driers with E D T A a t p H 3 to the Tiron end point indicates that the iron in the driers is almost all in the ferric state. This leads one to believe that the turbidity which forms in the alcohol-mter solvent is probably a mixture of an iron hydroxy complex of the drier acids and ferric hydroxide. Very stable carboxylate complexes of the type Fe,(OH)2(RC00)6+are known for a number of aliphatic acids, and ferric hydroxide is known to cause low result3 in the E D T A titration of iron a t pH values of greater than 3 ( 2 , IO). Thus, the addition of any acid would rapidly neutralize the hydroxides and raise the value obtained by the EDTh titration. The chloride, however, is apparently necessary t o decompoqe the carboxylate complexes because chloride forms itable complexes \vit h iron (I11).
Research Department with the statistical calculations. LITERATURE CITED
(1) Am. Soc. Testing Materials, Phil-
ACKNOWLEDGMENT
adelphia, Pa., Designation D 564-47, “Stand2rd Methods of Testing Liquid Driers, 1947. ( 2 ) Cheng, K. L., Bray, R. H., Kurtz, T., ANAL.CHEM.25,347 (1953). (3) Hampshire Chemical Co., Nashua, N. H., Hampshire Chemical Bull., “General Information on Chelation,” 1959. (4)Hillebrand, W. F., Lundell, G. E. F., Bright, H. A, Hoffman, J. I., “Applied Inorganic iinalysis,” p. 396, \%ley, T e a York, 1953. (5) Hirn, C. F., Lucchesi, C. A., ASAL. CHEN.31,1117 (1959). (6) Jones, S. S.,Long, F. A., J . Phys. Chem. 56, 25 (1952). (7) Kinnunen, J., Winnerstrand, B , Chemist Analyst 4 4 3 3 (1955). (8) Lucchesi, C. A , , Hirn, C. F., ASAL CHEJI.30, 1877 (1958). (9) Nnlay, L. S.,Selmood, P. W.,,J. A m Chenz. SOC.77, 2693 (1955). (10) Perrin, D. D., J . Chem. Soc. 1959, 1710. (11) Reilley, C. S . . Vavoulis, V.,ASAI.. CHEX31,243 (1959). (12) Schwarzenbach, G., “Complexometric Titrations,” Interscience, Kew York, 1957. (13) Schwarzenbach, G., Heller, J., H r h . Chzrn. Acta. 30, 576 (1951,). (14) Strouts, C. R. N., Gilfillan, J. H., Wilson, H. S., “,lnalptical Chemistry, the Working Tools,” Vol. 11, p. 1018, Oxford at Clarendon Press, London, 1955.
The authors acknowledge the assistance of G. G. Schurr of the Paint
RECEIVED for review February 15, 1960. hccepted May 9, 1960.
Analysis of Slag from the Manufacture of Uranium Metal Determination of Magnes um Oxide and Magnesium Meta JOHN McKEND’ Eldorado Mining and Refining, Itd., Tunneys Pasture, Ottawa, Ontario
b A complexometric method for the quantitative determination of magnesium metal and magnesium oxide in magnesium fluoride slag is described. The magnesium metal and magnesium oxide are leached from the slag with an ammoniacal solution (pH 10) of (ethylenedinitri1o)tetraacetic acid (EDTA) and the total magnesium in the extract is determined by a complexometric procedure. The magne-
sium metal is determined in a similar manner after the magnesium oxide has been extracted by leaching the slag with dilute acetic acid in the presence of potassium dichromate. The magnesium oxide is determined by difference. Provision is made for the removal of ferric, aluminum, uranium, and fluoride ions which interfere in the determination. Precision data from the analysis of samples are presented.
I
pi THE manufacture of uranium metal by the reduction of uranium tetrafluoride with magnesium, considerable magnesium fluoride slag is produced. Although magnesium fluoride is the main by-product, numerous side reactions occur to give slag con-
1 Present address, DLIPont of Canada, Ltd., Maitland, Ontario.
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