Determination of Some Heavy Metal Ions by Complexometric Titration with Sodium Azide Indicator F. G. SHERIF and 6. I. RAAFAT' Chemistry Department, Faculfy o f Science, University o f Alexandria, Alexandria, Egypf, U.A.R.
b Copper and iron can be determined in acid solution by titration (ethylenedinitri1o)tetraacetic against acid (EDTA) using sodium azide as an indicator. The end point depends upon the difference between the colors of each of the metal azide complexes and the corresponding chelate with EDTA, and upon the fact that the instability constants of the former are greater than that of the latter. Zinc, aluminum, and nickel can also be determined by backtitrating the excess EDTA added to each of these ions against a standard copper solution, using the azide as an indicator.
D
investigations of the orange uranyl (4) and green chromium(II1) (5) complex ions with the azide as a ligand, it was found that small amounts of (ethylenedinitril0)tetraacetic acid, EDTA, would reduce the interference of Fe(II1) and CulII) ions appreciably. The blood-red color of the ferric azide complex and the olive green color of the copper azide ion disappeared completely. These observations suggested the possible use of EDTA as a titrating agent for these ions, using the sodium azide as an indicator. The end point would be dependent upon the difference between the color of the metal azide complex ion and that of the E D T A metal chelate, and also upon the fact that the instability constants of the azide complexes (1, 2) are greater than that of the EDTA chelates ( 3 ) . Very few inorganic substances have been used as indicators in complexometric titration.. For example, iodide has been used in the titration of bismuth, and thiocyanate in the titration of iron. Sodium azide, besides being a simple inorganic reagent, has the advantage of offering a buffering medium in presence of suitable amounts of a strong acid such as HCl, and thus the method is comparatively insensitive to variations in the pH values of the solutions. In this work, the determination of Cu(I1) and Fe(II1) in small quantities in solution is presented. Conditionq 1
URING THE
Present address, Misr Rayon Co.,
Alexandria, Egypt, U. A. R.
2 1 16
ANALYTICAL CHEMISTRY
0.4
9
Q
0.3 W
3m
&
u1 0
0.2
m
a
0.1
0 0
0 2
O b
06
0 8
1
M L . Q O ~ M EDTA
Figure 1. Spectrophotometric titration of copper(l1) and iron(ll1) I. Cu(ll), 0.6354 mg.j wavelength, 400 rnp II. Fe(lll), 0.4468 mg.; wavelength, 450 rnp
for a visual end point, direct and reverse, against EDTA, together with a spectrophotometric end point, were studied. The possibility of determining metal ions such as Zn, Al, and Ni by back-titrating the excess EDTA with a standard solution in the presence of azide indicator is introduced. EXPERIMENTAL
Apparatus.
9 Unicam spectropho-
tometer, Model SP 500, with 1-cm. glass cells was used. The p H was measured using a llarconi p H meter, type TF 511C. Reagents. -1 stock solution of Cu(II), 0.1.M. as prepared from crystallized copper sulfate. The copper content was determined gravimetrically as the thiocyanate. A stock solution of Fe(II1). 0.08M, was prepared from -1nalaR ferric chloride. The iron content was determined gravimetrically as the oxide. Other concentrations were prepared from these by appropriate dilutions. AnalaR .11Clr.6Hz0 was used for the preparation of a solution 0.01.11 in Al(II1). The AI(II1) content \vas determined gravimetrically ab the oxide. The nickel solution was prepared from SiC12.6H,O and analyzed gravimetrically as the dimethylglyoxime complex. AnalaR zinc metal was dissolved in dilute hydrochloric acid and a solution 0.1148M in Zn was obtained. This zinc solution was utilized for the .tandardization of EDT.4, using Erio-
chrome black T as an indicator to prepare a 0.02111 solution of EDTA (6). A 1M sodium azide solution was prepared and analyzed by titration against standard silver nitrate, using potassium chromate as a n indicator. Procedure. Copper Determination. Ten milliliters of a 0.01M copper sulfate solution were mixed with 25 ml. of 0 . 0 1 9 HC1, and 2 ml. of 131 sodium azide indicator were added. The E D T A solution was added dropwise to the intense brown copper solution. This brown color is due to the copper azide complex formed in presence of excess azide. The color changed gradually to olive green and suddenly to blue at the equivalence point. The titration was reversible. The blue end point is due to the formation of copper EDTA complex. Backtitrations nere also carried out by adding 0.01.11 Cu(I1) from a buret to a solution containing 10 ml. of 0.01-5f Cu(II), 2 ml. of azide, 25 ml. of 0.OlK HC1, and 10 ml. of 0.02.V EDTA, The blue color of the mixture changed to green a t the end point. Spectrophotometric titrations B-ere carried out between EDTA added in 0.05-ml. portions and solution. containing 1 ml. of 0.01M Table I. Results of the Determination of Copper, Iron, Zinc, Aluminum, and Nickel
Element Cu(I1)
Fe(II1)
Zn(I1) Al(II1) XI(I1)
Taken. mg. 1.271° 6.354= 0.638& 6.354* 6.354c 0 63jd 2.234O 4.468" 6.7020 8.937* 0.44id 4.468O
Found, mg. 1.271 6.364 0.635 6.345 6.384 0.635 2.198 4.383 6.702 8.760 0.447 4.488
1 8 . 0 1 8 ~ 15.028 15.018" 15.018 7 . 5 0 9 ~ 7.809 4.046c 3.911 4 046C 3.992 0.269 0.261 8.804c 8 751 8.804c 8 810 8.804' 8 751
Relative error, c /O
0.00 $0.15 0.00 -0.14 0.00 0 00 -l,l2 -1.tIO 0.00 -l.!M t 0 45 00
+O 06 0 00 0 00 -3.33 - 1 3X - 2 !)6 -0 00
Direct titration. Reverse titration. c Back titration. d Spectrophotometric titration. a
+ O 07 -0 60
Cu(I1) and the same acid and azide mentioned before. Optical measurements were taken a t 400 mp, which is in the xicinity of the wivelength of the maximum absorption of the copper azide complex, 380 mp. Iron Determination. -4mixture of 5 ml. of 0.008X Fe(III), 2.5 ml. of O.1N HCl, and 0.2 ml. of 1J1 azide indicator, diluted to 25 ml. with distilled water, gave an intenqe red color. It is important to emphasize here that the amount of azide should not exceed the above limit, otherwis? the titration mixture would become turbid because of the formation of ;he basic ferric azide. Even in cases where more acid was added to prevent 1 he formation of the basic salt, increased azide concentration would render the end point obscure. The addition of 0.02V E D T I to the above mixture masks the red color and the end point is reached when the solution turns from orange-red to faint yellow. The pH of these titrations should be below 3. Back-titrations were carried out by adding exceis EDTA to the iron solution and backtitration with 0.008A11 Fe(II1). The end point in this case i.j sharper and is obtained when the solution turns from faint yellow to orange-red. Spectrophotometric titrations were also carried out using 1 ml. of 0.008X Fe(II1) and the color mas measured at 450 mp) which is the wavelength of maximum absorption for the ferril: azide complex.
Zinc Determination. A known excess of EDTA, 17.5 ml. of 0.02MJwas added to 2 ml. of 0.1148X Zn solution, and 2 ml. of 1N sodium azide was added. The mixture was titrated against 0.02M Cu(I1) solution. I s the copper ion was introduced, it reacted with the EDTA, forming the blue chelate. The blue color increased gradually, and when the end point was reached, the solution turned suddenly to green, the color of the copper azide complex. Aluminum and Sickel Determination. -1mixture containing 15 ml. of 0.01X hl(III), 17.5 ml. of 0.02Jf EDTA, and 1 ml. of 1-ll azide indicator, was titrated against 0.02-1.1 copper solution. The end point was detected as above. In a similar fashion, the known excess of EDT.1 added to the nickel solution and sodium azide indicator was backtitrated against the standard copper solution.
tity of the titrant at the end point. The zinc titrations could be carried out in solutions of pH ranging from 2 to 5, with no effect on the result,. The amount of azide indicator could also be varied widely. In fact, the larger the amount of indicator, the more obvious was the end point. However, it is adviqable not to increase the acid concentration, as volatile hydrazoic acid is irritating to the mucous membranes of the nobe and eyes. Attempts were made to determine Pd(II), Pt(IV), Cr(III), and U(V1) by titrating the excess EDTA against copper, but the color produced bj- these ions with the azide masked the end point.
RESULTS AND DISCUSSION
Chem. 8,346 (1958). (3) Schwarzenback, G. S., Analyst 80, 713 (1955). (4)Sherif, F. G., Awad, Aida, A m / . Chim. Acta 26, 235 (1962). (5) Sherir. F. G., Oraby, W. M., Sadek, H., J . Inorq. iYucl. Chem. 24, 1373
The data are depicted in Figure 1 and compared with LTolumetric results in Table I. In the determination of copper, changes of pH within a range of 4 to 6 were permissible in both forward and back E D T d titrations. Slight variations in either the acid or the azide concentration were found to have no effect on the sharpness and/or quan-
LITERATURE CITED
(1) Elshamy, H. K., Sherif, F. G., E g y p t . J . Chem. 1, 257 (1958), (2) Saini, G., Ostacoli, G., J . Inorg. Nucl.
(1962).
( 6 ) Welcher, F. J., "The Analytical Uses of Ethylenediaminetetraacetic Acid," p. 140, Van Kostrand, Princeton, 1957.
RECEIVEDfor review May 16, 1963. Acrepted September 4, 1963.
Extraction of Submicrogram Amounts of Molybdenum with Cu pfe rro n-Chlorof o rm Using Molybde num-99 W. B. HEALY' and W. J. McCABE Soil Bureau, and Institute of Nuclear Sciences, Department o f Scientific and Industrial Research, Wellington, N. Z.
b Extraction of Mo a t 0.01 - to 0.1 -pg. levels with cupferron-chloroform has been followed using Mogg. The extraction is most efficient, giving a distribution ratio of over 200 between the chloroform and ciqueous phases, Over 90% of Mo can be separated in one extraction when 'only 0.1 pg. is present in 1 liter of aqueous phase. Phases must b e separated within an hour of extraction, ovherwise breakdown of cupferron results in return of Mo to the aqueous phase. Bone samples in preparatioii for extraction can be ashed a t temperatures up to 850" C. without loss of Mo.
iatry," 11. 169, 1987, Wiley, New York, N. Y.). I n this laboratory, analysis of various materials, including teeth, bone, soft animal tissues, urine, and water, for submicrogram amounts of molybdenum has necessitated a preliminary extraction of molybdenum from acid solutions into an organic phase. This paper reports the use of Mog9to check on the effectiveness of cupferron in complexing molybdenum and facilitating its extraction into a chloroform phase when molybdenum is present in the range 0.013 to 0.13 pg. EXPERIMENTAL
T
cupferron (ammonium salt of N-nitrosophenylhydroxylamine) t o extract a number of elements, including molvbdenum. from acid and neutral solutims into organic phases has been recently reviewed (Morrison, G. H., Freiser, Henry, "Solvent Extraction in Abialytical ChemHE USE OF
and Materials. CUPFreshly prepared 6% aqueous solution of analytical grade cupferron (ammonium salt of N nitrosophenylhydroxylamine) , Mo9'. Solution of sodium molybdate containing 1.3 pg. of molybdenum for each microcurie of MoS9. Apparatus
FERRON.
CHLOROFORM. C.P. COUNTINGAPPARATUS. A Philips (iModel PW.4032) Scalar was used with a Geiger-Muller Liquid Counting Tube (20th Century Electronics Type M.6H). Four solutions were used for these extraction studies: (1) 2N HC1. (2) T550 Solution. Twenty grams of ovendried human teeth were ashed in a platinum crucible a t 550" C. for about 6 hours. The ash was dissolved in concentrated HC1, diluted, and filtered through a ?io. 542 paper into a 250-ml. flask. The re4due and filter paper mas re-ashed for 4 hours a t 580" C,, dissolved in a few milliliters of concentrated HC1 and added to the volumetric flask, which was made up to volume. The final solution was approximately 2 N in HC1. (3) B550 Solution. Seventeen grams of oven-dried bone (femur of sheep) were ashed at 550" C. for 6 hours, dissolved in concentrated HC1, Present address, Analytical Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tenn.
VOL. 35, NO. 13, DECEMBER 1963
21 17
