Spectrophotometric Determination of Iron in Acids and Acidic Solutions by an Extraction-Formation Reaction Involving 3-(2-Pyridyl)-5,6-diphenyl-1,2,4=triazineas the Chromogenic-Extraction Reagent C.
D. Chriswell and A. A. Schilt
Department of Chemistry, Northern lllinois University, DeKalb, 111. 607 1.5
A rapid, simple, and sensitive method has been developed for the spectrophotometric determination of iron in acids and acidic solutions based upon the extraction-formation of tris(3-(Z-pyridyl)-5,6-diphenyl-1,2,4-triazine]iron( 1 1 ) thiocyanate. The basis of the method and effect of variables have been investigated and elucidated. In general, the method is suitable for acid concentrations up to 4M, applicable to iron concentrations of parts per billion, and relatively free of interferences. The iron content of some reagent grade acids are reported.
tiveness decreases appreciably below a pH of 1. One of these is 3-(2-pyridyl)-5,6-diphenyl-1,2,4-triazine (hereafter referred to as PDT) (11, 12). We have found that this highly sensitive iron chromogen can be used in conjunction with an appropriate combination of counter ion and immiscible solvent to extract iron from relatively concentrated solutions of strong acids. The results of our studies of the extraction-formation reaction involved and its application for the, determination of iron in acids are reported here.
As a part of a continuing search for new and improved chromogenic reagents from among the many different possible compounds that contain the ferroin chromophoric group ( 1 4 , we have been particularly interested in finding or designing chromogens that are capable of complexing metal ions in either strongly basic or strongly acidic solutions. The pH range over which most metal ions are complexed by ferroin-type chromogens is approximately from 2 to 11. Below a pH of about 2, hydrogen ions compete effectively with metal ions for chromogen ligands and thus prevent metal ion complexation. Above a pH of about 10 or 11, many metal ions react preferentially with hydroxide ions to form stable hydroxy complexes or precipitates. Some notable exceptions have been found. Iron can be complexed and determined spectrophotometrically in strongly basic solutions with 4,7-dihydroxy-l,lO-phenanthroline ( 7 ) or with phenyl-2-pyridyl ketoxime (8). Copper can be similarly determined in alkaline solutions using either 4,4’-dihydroxy-2,2’-biquinoline (9) or 2,9-dimethyl-4,7-dihydroxy-l,lO-phenanthroline( I O ) , both of which are highly stereoselective in reaction with copper(1). These four different reagents afford the distinct advantage of direct use in strongly alkaline solutions without the necessity of neutralization and pH adjustment. In addition to simplicity and speed of application, spectrophotometric blanks are minimal because addition of acid or buffer is unnecessary. The search for a ferroin-type chromogen that will be effective in strongly acidic solution, and thus afford the same advantages mentioned above, has been much less successful. Only a few of the many compounds tested to date have proved capable of complexing iron(II), copper(I), or cobalt(I1) in dilute acid solution, and their effec-
Apparatus a n d Reagents. Absorbance measurements were made with a Cary Model 14 spectrophotometer and silica cells of 1.00-cm optical path. PDT ( C ~ O H I ~formula N ~ , weight 310.34) was obtained from the G. Frederick Smith Chemical Company. The PDT solution (0.008M) was prepared by dissolving 0.25 gram of P D T in 100 ml of chloroform and filtering to remove insoluble inorganic matter. Various grades of chloroform (spectral, reagent, and technical grades) were used without adverse effect, except when supplied in metal containers. Any iron contamination can be removed by distillation of the chloroform. The ascorbic acid-thiocyanate solution was prepared by dissolving 10 grams of ascorbic acid and 41 grams of sodium thiocyanate in 100 ml of distilled water; iron contamination was removed from the solution by extraction with 10 ml of the PDT solution followed by a second extraction with 10 ml of chloroform. Standard iron solution was prepared from pure iron wire and a slight excess of hydrochloric acid so that the final solution contained 0.1220 mg of iron per gram and was approximately 0.05M in acid. Standards of lower concentration were prepared by dilution. Other chemicals were reagent quality. Distribution of P D T between Chloroform a n d Various Acid Solutions. Equal volumes of 3.0 X 10-4M PDT in chloroform and aqueous acid solution were shaken together for 1 minute in a separatory funnel. After separation, both phases were analyzed spectrophotometrically to determine the concentration of PDT in each. The distribution ratio was calculated by dividing the concentration found for PDT in chloroform by that found in the aqueous phase. No loss of P D T by precipitation occurred, since the sum of the two concentrations was equal within experimental error to the original concentration in the chloroform. Spectrophotometric determination of the PDT concentration was based upon measurement of absorbance of 330 nm. When nitric acid or thiocyanate solutions were used, the determination was made by converting the PDT to its iron(I1) chelate and measuring its absorbance at 555 nm. Distribution of Acid between Chloroform a n d Various Acid Solutions. Equal volumes of chloroform and acid solution were shaken together for 1 minute. The chloroform layer was filtered through dry filter paper to remove any suspended droplets of aqueous phase and titrated to a phenolphthalein end point with standard potassium hydroxide in ethanol to determine its acid concentration. The distribution ratio was calculated by dividing the acid concentration found in the chloroform layer by that in the aqueous (the difference between the known initial concentration and that extracted). Identification of the Extracted Complex. The mole ratio method (13) was employed using a relatively concentrated solu-
EXPERIMENTAL
Schilt, W E. Dunbar, 6 .W. Gandrud, and S. E. Warren, Ta/anta. 17, 649 (1970). A . A . Schilt and W . E Dunbar, Taianta, 16, 519 (1969). A . A . SchHt and P. J. Taylor, Taianta, 16, 448 (1969). A A Schilt and K . R . Kluge, Jalanta. 15, 1055 (1968) A . A . Schilt and W . C Hoyle, Taianta, 15, 852 (1968) A . A Schilt and K R. Kluge, Jalanta. 1 5 , 475 (1968) A A . Schilt, G . F. Smith, and A Heimbuch. Ana/. Chem., 26, 809
(1) A A .
(2) (3) (4) (5) (6) (7)
(1956) (8) F. Trusell and H Diehl, A m i . Chem., 31, 1978 (1959) (9) A . A Schilt and W. C Hoyie, Anal. Chem.. 41, 344 (1969) (10) W . E. Dunbar and A . A Schilt. Taianta, 19. 1025 (1972)
992
A N A L Y T I C A L C H E M I S T R Y , V O L . 46, N O . 8, J U L Y 1974
(11) A A . Schilt, Jaianta, 13, 895 (1966) (12) A A . Schilt and P. J. Taylor. Anal. Chem.. 4 2 , 220 (19701 (13) J. H . Yoe and A E Harvey, J . Amer. Chem. Soc.. 70, 648 (1948)
Table I. Distribution of PDT between Chloroform and Various Solutions as a Function of Acid Concentration Distribution ratio
Acid
HClOi
larity
HC104
0.5 1.0 2.0
110 40 13 6.8
3.0
4.0 5.0 6.0 8.0
+
+ NaSCN
mo-
3.4
1.6 0.75 0.04
H&OI
150 150 28
7.4 0.85 0.20 0.07 0.05
HNOa
HCI
120
190 110 16 13 7.0 4.2 2.7 0.66
60 13
6.9 5.2 3.2
2.8 1.5
(0.5M)
120 70 26 23
20 10
3 M3L6 X A - I C
9
1.4
tion of iron(I1) to favor more complete extraction-formation of the complex, thereby minimizing curvature in the absorbance us. mole ratio plot. Exactly 10-ml samples of aqueous phase (4.48 X 10- 4M in Fe(II), 0.5M in NaSCN, 1M in HzS04, and containing 0.1 gram of ascorbic acid) were each extracted with exactly 15 ml of chloroform phase, each containing different known amounts of .PDT. After separation of the layers, 5.00 ml of the chloroform phase containing the complex was diluted to 25.0 ml with chloroform, and the absorbance was measured a t 555 nm. Recommended Procedure for Determination of Iron in Acids. The amount of sample to be taken for analysis depends on the type and concentration of acid, as well as the concentration of iron. The final concentration of acid in the treated sample prior to extraction should be a t least 1M but should not exceed 4M if the acid is perchloric or sulfuric, 3M if hydrochloric, or 2M if nitric. Acetic or phosphoric acid can be 4M or greater. Transfer a measured sample of the acid of appropriate size to a 100-ml volumetric flask, add distilled water to adjust the volume to approximately 80 ml (if heat is liberated, cool the flask and contents to room temperature), add 10 ml of the ascorbic acidthiocyanate solution, and dilute to volume with distilled water. Transfer a measured aliquot (containing approximately 2 to 20 p g of iron) of this solution to a separatory funnel, and add by pipet exactly 10 ml of the P D T solution. Shake for 30 to 50 seconds, allow adequate time for layers to separate and clear, withdraw a portion of the lower chloroform layer into a 1.00-cm absorption cell, and measure its absorbance a t 555 us. an identical cell filled with chloroform. A reagent blank is unnecessary since the reagents were treated previously to remove any traces of iron. Refer to a suitably prepared calibration curve to convert the absorbance to iron content, or calculate the concentration from the molar absorptivity (24,500 a t 556 n m ) .
RESULTS AND DISCUSSION Study of Extraction-Formation Reaction. Although formation of iron(11) complexes of ferroin-type chromogens (Equation 1) is exceedingly unfavorable and incomplete in strongly acidic solutions, it was found that extraction with an appropriate immiscible solvent (Equation 2) can provide the condition necessary for more complete overall formation of the complexes in certain cases. Fe2+
+ 3LH+ = +
[FeL,]"
+
3H+
(1)
[FeL,]" 2X- = ([FeLJX,), Of the various extraction solvents (0),ligands (L), and counter ions (X- ) tested, the most effective combination found was chloroform, PDT, and thiocyanate ions, respectively. Other, less effective, ligands tested included 1 , l O phenanthroline, 4,7-diphenyl-l,lO-phenanthroline,2,2'bipyridine, and 2,4,6-tripyridyl-1,3,5-triazine. Solvents tested were chloroform, methylene chloride, ethylene chloride, nitrobenzene, isoamyl alcohol, methyl isobutyl ketone, and various combinations of these with each other or with acetone, ethyl alcohol, or dimethyl formamide. None of the more common anions proved more effective than thiocyanate. Detailed studies were made of the chloroform-PDT-
PCT/'ROh
Figure 1. M o l e ratio plot
Absorbance at 555 n m vs. mole ratio of PDT to i r o n ( l l ) obtained by extracting 10.0 ml of 4.48 X 10-4M iron(ll) solution ( 1 M in H2S04,0 . 5 M in NaSCN, and containing 0.1 g ascorbic acid) with exactly 15 ml of PDT in chloroform, followed by an exact dilution of 5 X of the extract with chloroform
thiocyanate combination to learn the identity of the extraction product and more about the nature, possibly the mechanism, of the extraction-formation reaction. The results obtained are consistent with the following overall extraction-formation reaction: Fee+
+ 3(LH+X-),
= ([FeLJX,),
+
3H+
+ X-
(3) where species in the chloroform phase are indicated in parentheses with subscripts 0 , L is PDT, LH+ is monoprotonated PDT, and X- is either thiocyanate or the anion from the strong acid. Equation 3 is formulated in part on the basis of the following observed and measured equilibria in 1 to 4 M strong acid:
+ H+ = LH+ LH+ + x- = (LH+x-), L
(4)
(5 ) The equilibrium constant for Equation 4 (formation of monoprotonated PDT) is equal to 9.0 X 102 a t 25 "C ( 1 4 ) , so PDT exists predominately in its monoprotonated form in 1 to 4M strong acid. The data compiled in Table I further indicate that PDT predominates in the chloroform rather than the aqueous phase. Thus Equation 3 is formulated accordingly. Attempts to measure the distribution ratio of [FeLsIXz, a measure of the equilibrium constant for reaction 2 , gave results in excess of IO5, too large to measure precisely but conclusive evidence that [FeL3I2+ is essentially absent from the aqueous phase a t equilibrium, as implied by Equation 3. The equilibrium constant for Equation 1 could not be measured, because no detectable concentration of [FeL#+ forms in 1to to 4M strong acid. The validity of Equation 3 was further tested spectrophotometrically by the mole ratio technique which confirmed the reaction stoichiometry and identity of the extraction product. Data and mole ratio plot are shown in Figure 1. The equilibrium constant expression for the heterogeneous reaction, assuming that the p l y species of iron or PDT present in the two phases are those indicated in Equation 3, is the following:
The conditional constant K' can be defined as follows:
(14) K Schuett and A A Schilt, 111 , unpublished data, 1973
Northern
Illinois
Unlverslty, DeKalb,
A N A L Y T I C A L C H E M I S T R Y , VOL. 46, NO. 8, J U L Y 1974
993
1
I
1
1.c W
c 0
a
+-
I
\
bX
'i
0.6
z 0 k
0 Q
rY LL
I.---
,
\
0.2 I
,
1.0 MOLARlTY SCN0.5
3 ACID MOLARITY
Extraction of iron(ll) from perchloric acid (with and without added NaSCN) with 0.01M solutions of PDT in various organic solvents Figure 2.
Curves A , 6 ,and C are for nitrobenzene, ethylene chloride, and chloroform, respectively, with thiocyanate present in aqueous phase. Curves A ' , B', and C' are the same solvents but without thiocyanate
~
~~
~~
Table 11. Distribution of Acid between Chloroform and Aqueous Phases Distribution ratio X 103 Acid
HClOd HzSOa HC1 HBr "01 Hd'04
Acetic Chloroacetic Dichloroacetic Trichloroacetic
For M acid
0.004 0.14 0.01
0.04 3.1 0,007 550 50 130 340
For 4 M acid
0.002 0.14 0.01 0.03 1.8 0.005 670 72 110 250
From the data of Figure 1, the calculated K' (for conditions 1M H2S04 and 0.5M NaSCN) is (9.0 A 0.4) X loll. Simple calculation shows that the extraction-formation of the iron complex is 99.9% complete from a solution 1M in and 0.5M in NaSCN if an excess of PDT is employed such that [LH'X- lo is 10-3M. Complete elucidation of the mechanism for the extraction-formation of the complex will require more rigorous kinetic studies; however, certain observations clearly indicate one reasonable possibility. The rate of attainment of equilibrium for Equation 3 is greatly dependent on the type of counter ion X- present, with relative rates observed as follows: SCN- >> C1- > Br-, S042-, c104-, NO3-. Since thiocyanate and chloride are known to form extractable iron complexes, these ions can act as carriers to transfer iron to the chloroform phase more efficiently than the others. It thus appears that the iron must first be extracted from the acid solution in the form of a complex with X- before it can react in a favorable environment ( i e . , one of low acid concentration) with PDT to form the final product. The chloroform phase provides a favorable environment because it is relatively free of acid, a s evidenced by the data in Table 11. Tests confirm that the extraction-formation reaction occurs readily in the presence of thiocyanate from 2M acidic solutions of all the acids in Table I1 except trichloroacetic acid. The latter is both sufficiently strong and extractable that its presence in the chloroform phase prevents formation of the iron PDT complex. Effects of Variables on Extraction of Iron. The efficiencies of several different extraction solvents, both with (0.5M) and without sodium thiocyanate in the aque994
ANALYTICAL CHEMISTRY, VOL. 46, NO. 8, J U L Y 1974
Effect of thiocyanate concentration on the extraction of iron(l I ) from 2M perchloric acid by 0.01M PDT in chloroform. Figure 3.
Table 111. Recovery of Iron from 2 M Hydrochloric Acid Solutions Vol taken for extraction, ml Acid solution"
P D T in CHCh
Net absorbance
10.0 25.0 25.0 50.0 100.0 100.0 150 250 250
10.0 10.0 10.0 10.0 10.0
0.346 1.005 0.069 0.134 0.266 0.710
10.0 10.0 5.00 5.00
0.400 0.142 1.32
Fe Concn, p M Added
Found
13.6 16.4 1.09 1.09 1.09 2.72 1.09 0.109 1.09
14.0 16.2 1.11 1.08 1.07 2.87 1.08 0.115 1.07
a Solution 2 M in HCI, 0.5M in NaSCN, and containing 0.01 g/ml ascorbic acid and known added concentration of iron.
ous phase, are compared in Figure 2 . Extraction of iron is seen to be most efficient and least dependent on acid concentration when chloroform and thiocyanate are employed. Extraction of iron from 2M perchloric acid as a function of thiocyanate concentration is shown in Figure 3. Similar results for 4M but consistently lower results for 5M perchloric acid were found. The effectiveness of thiocyanate in promoting complete extraction-formation of the iron(I1)PDT complex arises from its influence on the distribution ratio of PDT between chloroform and strongly acidic aqueous solutions (see Table I). In the presence of increasing concentrations of thiocyanate (or HSCN), a greater fraction of PDT will reside in the chloroform phase, causing the equilibrium for Equation 3 to shift in favor of increased complex formation. Thiocyanate not only plays an important role in the overall equilibrium; it also acts as a vehicle or carrier for iron, as discussed in the preceding section, to enable its rapid transfer into the chloroform phase for subsequent reaction with PDT. A third role for thiocyanate is as a reductant, because tests show that no special reductant is necessary to produce the iron(I1) complex from ferric iron. Use of ascorbic acid as an auxillary reductant is advocated to protect the complex in the chloroform phase from atmospheric oxidation and loss of color. The minimum concentration of PDT in chloroform necessary for quantitative extraction of iron from perchloric acid is dependent on the concentration of the acid, as shown in Figure 4. Higher acid concentrations require higher PDT concentrations. A chloroform solution, 8.0 X 10-3M in PDT, can quantitatively extract iron from 4M perchloric acid, 0.5M in sodium thiocyanate and containing 0.1 g/ml ascorbic acid. For 5M or greater perchloric acid concentrations, extraction is inefficient regardless of
1
I
I 2
2
4
6
8
PDT MOLARITY
1
0
ACID MOLARITY
x io3
Figure 4. Extraction of iron(l1) from perchloric acid B, C, D: 1 , 2, 4, and 5M HC104, respectively) as a PDT concentration in chloroform
(curves A, function of
Table IV. Determination of Iron Added to Various Acidic Solutions
Final sample acid concna
2 M HzSOa
2M HClOa 4M HC104 1 M HC1 2 M HCl 1M HNOa 2 M "01 9 M HOAc
+ +
1MHClO4 1M HNOs lMHCl0, 1M Hk30, 1 M H C l + 1M HNOa
Figure 5. Extraction of iron(l1) with PDT in chloroform as a function of acid and acid concentration
Table V. Determination of Iron i n Concentrated Reagent Grade Acids by Recommended Procedure Sample size Acid
Acid blank
Sample
Net
Concn found of added Fe, FM
0.041 0.024 0.026 0.031 0.042 0.031 0.033 0.021
0.888 0.880 0.831 0.901 0.907 0.901 0.894 0.881
0.847 0.856 0.805 0.870 0.865 0.870 0.861 0.860
13.7 13.9 13.1 14.1 14.0 14.1 13.9 13.9
0,029
0.894
0.865
14.0
Acetic
0.032
0 881
0.849
13.8
C hloroacetic
0.037
0,902
0.865
14.0 13.9
Sulfamic
__-
6
4
Absa n m us. air
Mean = Re1 std dev 0.6%
a Concentration of the final solution after diluting the measured sample with water, ascorbic acid thiocyanate solution, etc. as per recommended procedure. Each sample was prepared to contain 13.9 micromoles of added iron per liter, and a 25-ml aliquot was taken for the extraction.
the PDT concentration. These observations are consistent qualitatively with the equilibrium expressed by Equation 3 and the dependence of the concentration of PDT in chloroform on the concentration of acid in the aqueous phase (see Table I). The concentration range of acid over which extraction of iron is quantitative depends on the nature of the acid, as shown in Figure 5. This dependence is probably related in part to the extractability and strength of the acid (see Table 11). Consistent with Equation 3, high acid concentrations discourage extraction-formation of the complex. Spectral Characteristics of the Complex. Visible absorption spectra of the [Fe(PDT)3]X2 complex salt in solution with different solvents and anions were found to exhibit small differences in molar absorptivities but otherwise were essentially identical. At 555 nm, the wavelength of maximum absorbance, the molar absorptivity of the complex is 24,000 in ethanol-water mixtures (x = perchlorate or acetate), 23,500 is isoamyl alcohol (complex extracted from aqueous solution of p H 7 with x = perchlorate, thiocyanate, or acetate), 23,800 in chloroform (x = perchlorate or acetate), and 24,500 in chloroform (x = thiocyanate). Although the spectra show slight dependencies on both solvent and anion, they support the conclusion that the complex which is produced from strong acids
HC10:
H?SOI HC1 HBr HNOj Hap04
-
ml
g
25.0 25.0 20.0 20.0 25.0 25.0 25.0 25.0 10.0 10.0 10.0 10.0 25.0 25.0
41.39 41.40 36.10 36.16 29.23 29.33 36.92 36.71 14.02 14.02 16.56 16.58 26.14 26.10 4.89 10.09 5.03 9.97
... ..
,
...
Aliquot Absorbance extracted, a t 555 ml= nmb
100 100 100 100 100 100 100 100 100 100 50
50 50 50 10G 100 100 100
0.715 0.729 0,505 0,505 0,072 0.079 0.141 0.148 0,067 0.068 0.673 0.668 0.322 0.316 0,372 0.722 0.586 1.14
F e concn, mg/Kg
0.38 0.39 0.30 0.30 0.057 0.052 0.084 0.089 0.103 0.103 1.79 1.77 0.56 0.54 1.68 1.58 2.53 2.60
a The volume taken of t h e total 1OG ml of solution obtained on diluting the measured acid sample with water, 10 ml of the ascorbic acid-thiocyanate solution, and additional water to volume. Absorbance corrected for cell and solvent.
*
only by extraction is the same as that which forms readily and directly in aqueous solutions under more favorable conditions of pH. Determination of Iron. The recommended procedure for the determination of iron was tested and applied in a variety of situations to evaluate its effectiveness. Data compiled in Table 111verify that recovery of iron added in various amounts to hydrochloric acid is quantitative within expected spectrophotometric precision. The results also demonstrate that extraction recovery is complete using only 5.00 ml of the chloroform-PDT solution to extract 250 ml of the acid solution. An increase in detection sensitivity of 50X is effected by such an extraction. Results in Table IV show that iron can be accurately determined in various acids, mixtures of acids, and of different concentrations. Excellent precision and accuracy are indicated for the concentration found of known added iron (13.9pM or 0.776 pg/ml). The relative error for any one of the eleven results did not exceed 1.570, well within 3-sigma limits of reliable spectrophotometric procedures. Measurements conform to Beer's law up to absorbances well in excess of unity, provided that sufficient PDT is present in the chloroform to complex the iron. The recommended procedure is best suited for measurement of 1 to 20 f i g of iron, which produce absorbance between 0.05 and 1.00. If large samples of up to 500 ml are extracted with 10 A N A L Y T I C A L CHEMISTRY, V O L . 46, N O . 8, J U L Y 1974
995
Table VI. Effect of Foreign Ions Ions tolerated at 100 ppm:
Aluminum Ammonium
Arsenic Barium Bismuth Calcium Lithium Magnesium Potassium
Strontium Tin Zinc Acetate Bromide Bromate Chloride Citrate Fluoride
Iodide Nitrate Perchlorate Periodate Phosphate Pyrophosphate Sulfate Tartrate Tetraborate
Ions toIerated at lower concentrations (ppm tolerated):
Chromium (10) Cobalt (2) Copper (50)
Manganese (50) Nickel (2)
Oxalate (50) Vanadium (2)
ml of the chloroform-PDT solution, the concentration of iron can be as low as 2pg/l. in the acid solution. On applying the recommended procedure to determine iron in reagent grade acids, the results in Table V were obtained. Duplicate determinations show good agreement. Recovery of iron, within 2% experimental error, was confirmed in each case by use of the method of standard additions. Acids not listed for which recovery of iron was unsatisfactory are dichloro- and trichloroacetic acids. These acids inhibit complete formation of the iron(I1)-PDT complex in chloroform because of their extractability and strength. Results of interference studies are given in Table VI. An error of 3% or greater occurs in the presence of 100 ppm of any of the seven ions listed in the bottom section of the
table; however, lower concentrations (noted in parentheses in ppm) are tolerated. The presence of these ions a t their respective interference levels is not ordinarily expected for reagent grade acids. The most severely offending ions are cobalt(II), nickel(II), and vanadium(IV); 5 ppm of these cause positive errors for iron of 5, 19, and 1070, respectively. Interference by 100 ppm of oxalate is unusual in that it is time-dependent and can be avoided by using longer shaking times of 60 sec or more. Studies of the effects of time revealed no serious sources of error. Sample solutions treated with the ascorbic acidthiocyanate solution could stand for a t least 1 hour prior to extraction without adverse effect. Extraction was complete for a shaking time of 15 sec, but a minimum of 30 sec is recommended. No change in absorbance was detectable on storing the final chloroform extracts in stoppered flasks for 3 days. The usual safety precautions should be observed when handling concentrated acids, particularly to guard against excessive heat and splattering on diluting them with water to the concentrations recommended for this procedure. It is also important to cool the diluted acid solution prior to adding the ascorbic acid-thiocyanate solution, because hot solutions of strong acids promote decomposition of thiocyanate to carbonyl sulfide and ammonia. Noteworthy advantages of the recommended procedure are its speed, simplicity, and sensitivity. Moreover, there is no necessity for a reagent blank determination because the reagents are easily freed of iron contamination prior to USe.
Received for review January 11, 1974. Accepted March 7, 1974.
Analytical Applications of X-Ray Excited 0ptical Luminescence Direct Determination of Rare Earth Nuclear Poisons in Zirconia Arthur
P. D’Silva and Velmer A. Fassel
Ames Laboratory-USAEC and Department of Chemistry, l o w a State University, Ames, lowa 50010
An X-ray excited optical luminescence technique for the direct quantitative determination of fractional ppm levels of rare earth “neutron poisons” in ZrOz and Zircaloys is described. A blend of Zr02, KzCO3, Sr(N03)2, and WOa is heated to 1050 “C for 2 hours to yield a quaternary oxide phosphor host with a composition of KzOs2SrO2Zr02.3W03. The irradiation of this pho.sphor by Xrays causes the emission of optical line luminescence of the rare earth impurities. When the phosphor sample is irradiated at 1 5 0 “C, the intense host band luminescence is quenched yielding improved signal to noise ratios for Sm. Dy. Eu, Pr, and Tb, which is the internal reference element. The Gd luminescence, which is quenched at 1 5 0 “C, is observed at ambient temperatures. The detection limits under these conditions are: 0.05 ppm for Gd, Sm, Pr; 0.1 for Eu; and 0.02 pprn for Dy. 996
A N A L Y T I C A L C H E M I S T R Y , VOL. 46, NO.
8, JULY 1974
Zircaloy 2 and 4 are preferred alloy structural materials in nuclear power and breeder reactors. These alloys meet the criteria of low absorption coefficients for thermal neutrons as well as strength and corrosion resistance a t elevated temperatures ( I ) . The rare earths Gd, Sm, Eu, and Dy are the most potent “neutron poisons” that may occur in these alloys as residual impurities derived from ZrOz, the base material for the preparation of these alloys. As a consequence, the presence of the above rare earth impurities in ZrOz a t levels greater than 1 ppm is considered undesirable (2). The processing of ZrOz to Zr metal and the H . H. Klepfer. “Proceedings of the USAEC Symposium on Zirconium Alloy Development,” GEAP-4089, Vallecitos Atomic Laboratory, General Electric Company, San Jose, Calif. 1962. L. G. Wisnyi, KAPL-3322, Koolls Atomic Power Laboratory, General Electric Company, Schenectady, N . Y . . 1967.
