Polarographic Determination of Uranium D. I. LEGGE Anglo-Transvaal Consolidated Investment Co., Ltd., Johannesburg, South Africa
directly applicable to the materials being investigated. I t has been shown by Harris and Kolthoff ( 3 ) that moderately acid media are most suitable for accurate polarographic determination of uranium. The reduction is usually represented as a oneelectron reduction UO?++ e ----t UOz+ (1) followed, in the presence of complex-forming compounds such as oxalic acid ( 5 ) ,by
This work was undertaken as part of a program of research on the recovery of uranium from South African ores. Existing methods of uranium analysis had proved to be too time-consuming, insufficiently- sensitive, or lacking in accuracy for this purpose. The present method has been applied to ores and all types of process samples. I t involves separating the uranium from the bulk of the other metallic impurities by eluting a nitric acid-ether solution through a short column of cellulose pulp. The uranium is then determined polarographically in an electrolyte containing oxalic and sulfuric acids. Small quantities of a large number of impurities have no effect on the method. Low grade samples can be analyzed with greater accuracy than was previously possible. The time necessary to complete an analysis is about 3 hours, but on a routine basis aierages under half an hour.
A
+
2UOz+
+ H + --+
+ UO(OH)+
(2)
which represents in effect a two-electron reduction. The present method in its general form requires a purification step to remove gross impurities, but is not affected by small amounts of any other normally occurring elements. Purification is required only to decrease the concentration of these elements t o the same order as that of the uranium. By means of this method well over 2000 uranium analyses have already been made with a precision in almost every case of better than 3% of the actual value.
RAPID and accurate method was required for the dctermination of uranium in samples which in some instances might contain less than 0.00025% ( 2 . 5 y per gram) of uranium in the presence of over a thousand times this concentration of various other soluble elements, including, iron, manganese, magnesium, calcium, and aluminum with lesser amounts of other ~ dcounting i ~methods ~ i soluble impurities and much silica. ~ had not proved entirely satisfactory for these samples. Colorimetric methods are generally affected bv . very . small quantities of impurities and the sensitivity decreases a t low uranium concentrations ( 9 ) , since the transmittancy of a solution increases logarithmically with decreasing concentration. The microfluorometric method (IO)is very sensitive a t low uranium concentrations, but rigorous purification from most ions is required. Polarographic methods have found considerable application in uranium analysis ( 11) but none have been described which were
APPARATUS AhD RE4GENTS
A Sargent-HeyrovskG photographic recording polarograph Rlodel XII, was used in all this work. The maximum galvanometer was 0.00533 per mm. scale deflection, but it was found advisable in order to secure good wave definition, not to work below one half of this sensitivity. Scale deflections could ~ ~be read to the nearest 0.5 mm. The rate of flow of mercury as defined by the capillary characteristic mz’3t1’6 was 2.60 m g , ~ / 3 s e c . - l / ~a t -0.5 volt all determinations Tveremade at 25” + 0.1O C.
Figure 2.
Figure 1. Cellulose Column
UO?++
Hot Box
The cellulose column used for chromatographic purification is shown in Figure 1. The sintered disk was of coarse porosity. No silicone or other surface treatment was given to the glaw. The “hot box” (Figure 2) i n s insulated by a I-inch layer of exfoliated vermiculite and heated internally from all sides, but not from the bottom, by three 600-watt Calrod elements. I t is a great improvement over the ordinary open hot plate and enabled twelve samples a t one time to be prepared and fumed without danger of bumping. Polaritan reagents (Hopkins and Williams) were used 1% henever these were available. However, Baker’s Analyzed or Merck’s (Darmstadt) reagents gave satisfactory results in all cases. Nitric acid-ether (5%) refers to a 5% volume solution of concentrated nitric acid in peroxide-free ethyl ether. Cellulose powder was Whatman’s chromatographic cellulose powder (standard grade). Gelatin was Davis (Australian) edible gelatin, 1617
ANALYTICAL CHEMISTRY
1618 Table I. Electrolyte Acid tartrate
Comparison of Electrolytes Sensitivity, I Comments 1 .O?
Oxalic acid, 0 5.M
2.73
Hydroxylamine hydrochloride, 2iM (13)
1.70
Oxalic acid ( 0 . 5 M ) hydroxylamine hydrochloride (0 2.11)
+
2.68
Freedom from interference (except iron) is very good. Sensitivity is low. T h e wave slopes and is difficult t o measure Excellent sensitivity and wave form. Low electrolyte line Eliminates iron interference, b u t sensitivity is only moderate and wave slopes Excellent sensitirity. Moderately good wave form and freedom from interference
CHOICE OF ELECTROLYTE
the lowest uranium concentrations (Table 11). The maximum is easily suppressed with gelatin. The wave form is good, the electrolyte line being low and the diffusion current constant over the range -0.4 to -0.6 volt. Thus a single reading a t -0.5 volt (corrected for electrolyteline) gives excellent results using a calibration line prepared from standard uranium solutions in most cases. The most satisfactory results for the lowest concentrations (less than 5 X 10-6M) are obtained by recording polarograms and using the method of standard addition or preferably by “spiking” as outlined below. Spiking consists of adding 10 y per ml. of uranium to the oxalic acid electrolyte. It has the great advantage that a whole day’s supply of electrolyte may be accurately spiked by a single addition of standard uranium solution and that all analyses, even at the lowest concentrations, may then be made by means of a single reading. A wave height of a t least 23 mm. a t half the maximum galvanometer sensitivity is assured, so that precision of wave height measurement can be a t least within =!=2%.
I t has been shown that even in moderately acid media uranyl solutions do not give a diffusion current which is proportional to uranium concentration over the whole range of concentrations (from 10-3 to 10-6M) in which the author was interested SEPARATION OF URANIUM ( 3 ) . At very low uranium concentrations (below 5 X 10-6M), there was no relationship between wave height and concentration A very convenient method for separating microgram quantities in electrolytes consisting of sulfuric or hydrochloric acid either of uranium from gross impurities is employed. It is a modification (described below) of the chromatographic method (1) using alone or containing added potassium chloride. In these experia cellulose adsorbent. A nitric acid solution of the sample is adments also a completely satisfactory maximum suppressor for use with these electrolytes could not be found. Gelatin, caffeine, sorbed on cellulose powder. This is placed in a short glass column thymol, peptone and methylcellulose either gave high electrolyte and 5% nitric acid-ether is added. The uranium is readily desorbed and passes down the column in ether solution. Most inlines in relation to the height of the uranium wave, or suppressed the wave height in the concentrations which were needed. terfering elements are retained by the cellulose. Methyl red was effective only in a critical concentration which The modification is more rapid and more economical in ether than the original method and the separation is sufficient for the varied with the concentration of uranium in solution. Salicylic present purpose. It gives an eluate solution which usually con( 6 ) , citric, and tartaric acids gave poorly defined waves with small tains no metallic impurities except possibly traces of iron or mandiffusion currents. In general the use of an acid salt electrolyte gave a lower diffusion current than did the corresponding acid. The electrolytes mentioned above (except acid tartrate) could be used only with Table 11. Relationship of Diffusion Current to Concensubstantially pure uranium solutions. This also applies to the tration of Uranyl Sulfate i n 0.5M Oxalic Acid, 0.9M Sulcatalytic nitrate wave (2, 4 ) . Acid tartrate (7’) was a fairly satisfuric Acid, 0.015% Gelatin a t 25” C. factory electrolyte for some unpurified samples, but failed comK = id/c, Uranyl Sulfate, Diffusion Current, Diffusion Current, pletely with samples containing very high proportions of other C , Mole/Liter i d , pa. ca. /hlillimole/Liter metals (especially ferric iron). Even when the solution was 7.03 10 - 6 0,0703 7.15 z x 10-5 0.1430 boikd with hydroxylamine, the ferric wave could not always be 7.15 4 x 10-5 0.2860 eliminated because of the high concentration of iron which was 10 - 4 0.712 7.12 5 x 10-4 3.545 7.09 present. Likewise, stannous chloride did not effect complete reducI n - 87.’2 7.12 4 x 10-1 28.613 7.15 tion of ferric iron before an interfering stannous wave appeared. Four electrolytes were found to be useful in the presence of small quantities of normally occurring impurities. These are compared in Table I, and typical polarograms from a 5 X 10-4M solution of uranyl sulfate in 0.6 each electrolyte are shown in Figure 3. The electrolyte sensitivity, I , is given by the espression I =id/ 0.5 Crri 2 ’ 3 t 1’6 where id is the diffusion current in microamperes, C is the molar concentration of reducible 0.4 :ions in the solution, and the expression M * ’ 3 t ” 5 is W z the capillary characteristic as previously defined ( 1 2 ) . 0.3 The diffusion current which will be obtained from a given concentration of uranium is thus directly proO3 portional to I , irrespective of the capillary characteristics. 0.1 .I t was decided that some purification stage would be essential for any generally applicable method and the electrolyte finally selected after numerous trials was 0.5M oxalic acid. In use, sulfuric acid from the sample preparation is present and hydrochloric acid -0.1 -0.2 -0.3 -0.4 -0.5 is added to form a calomel electrode with the mercury ED.E., VOLTS VI. S.C.E., VOLTS pool in the cell. This electrolyte gives the greatest Figure 3. Comparison of Electrolytes uranium diffusion current of any which has been exPolarograms of 0.8 X lW‘M uranyl sulfate, 0.9M sulfuric acid, 0.015% gelatin amined. It also gives a constant value for the ratio at 25’ C. A. 0 . 5 M oxalic acid. B. 0.5M oxalic acid plus O.2M hydroxylamine hydrochloride. C. 2M hydroxylamine hydrochloride. D . Acid tartrate of diffusion current to concentration ( i d / C ) for even I”
-
3
-
d!i -
I
I.
V O L U M E 2 6 , NO. 10, O C T O B E R 1 9 5 4
1619
ganese. These do not interfere in the polarographic determination and there is no loss of uranium. It is important that enough calcium nitrate be added before this treatment to fix any soluble sulfate, because a large concentration of sulfate ion will prevent complete extraction of uranium by the ether. I n the presence of rhloride ion some ferric iron may be desorbed. This does not interfere in the determination if the quantity is small, but chloride should be avoided if possible a t this stage. The eluate (in 5% nitric acid-ether) is evaporated with a measured small quantity of sulfuric and perchloric acids and brought just to fumes of sulfuric acid to ensure the absence of reducible organic material. -4 measured volume of electrolyte is then added and the polaro- ' graphic diffusion current is observed a t -0.5 volt or meawred from a recording. 4\10UNT O F URAIIU31 DETERMIYABLE
The diffusion current is 7.12 pa. per millimole of uranium per liter (238 y per mi.). The maximum concentration of uranium which can be determined depends only on considerations common to all polarographic work and should not be greater than 5 X lO-3M. The instrument sensitivity should be set to give a scale deflection of i o to 100 nim., which can be measured to h0.5 mm. or to better than O.i5% of the true value. This measurement usually determines the over-all precision of the analvsis. The minimum concentration of uranium which can be determined directly is about 10-6JI (2.38 y per ml.). This gives a scale deflection of 6
o.6
examination of unknown ore samples where some may prow to be almost or completely barren, but it is recommended as standard procedure for all analyses on samples of low value. The uranium content of any sample can be concentrated into a final volume of not more than 3 ml. Hence at least 3.5 y of uranium should be present in the original sample. With less than 3.5 y of uranium determinations can still be made, but the precision will not be within *3%. The accuracy and precision are reduced in all determinations if unsatisfactory grades of cellulose pulp are used in the purification stage. Coarse grades of cellulose pulp may allow channeling of the solution to occur, with consequent appearance in some cases of interfering cations in the percolate.
r
0.5
-
0.4
-
4'0'3 -
5
p 0.9
:
0.1
1
0
ate the galvanometer at more than half its maxiI'olaroerams of 0.6 X 10-4.Muranyl sulfate, 0.9.W sulfuric acid, 0.015% gelatin at 25" C. A . Normal wave mum sensitivity, because this gives too much B. With 0.06% gelatin slope to the electrolyte line and causes a loss of acC. In presence of reducible organic matter curacy on single reading determinations. The complete removal of traces of reducible organic matter and of osygen is of vital importance a t such low The constant relationship of diffusion current to uranium conconcentrations of uranium, in order to obtain a good base to the centrations is shown in Table 11. wave. However, when the concentration of uranium is greater Table 111 gives the results of analyses on synthetic solutions than about 4 X 10-631 (10 y per ml.) such traces do not intercontaining up to 10% each of ferrous and ferric sulfates and fere, and a well shaped wave is readily obtained with a height of smaller amounts of many other ions to which known amounts of 24 i 0.5 mm. which can be measured to a precision within &2%, uranyl nitrate or acetate had bcen added. These mixtures were Thus by adding 10 y of uranium per ml. to the electrolyte, urasubjected to the entire purification and polarographic procedure. nium in a sample can be determined in concentrations as low as 4 x Table IV gives a comparison of yome figures obtained by this 10-6M (1.19 y per ml.) with an over-all precision of within &3%. method with independent analyses carried out by other labora(The inherent accuracy of the instrument is h l % . ) For routine to]ies using a chemical volumetric method, the peroxide coloriwork an oxalic acid electrolyte containing 10 y of uranium per metric method in conjunction with a photoelectric colorimeter, or ml. can be used in order to avoid the need for excessive care in a fluorometric method. sample preparation. iZ correction for this amount is then applied EFFECT OF OX4LIC 4CID COVCENTRATIO\ to the final analysis. This procedure is especially useful for the Variation of oxalic acid concentration from 0 1J-I to 0.5-11 caused no change in the diffusion current or in the general nature of the polarographic wave. The reduction was irreversible at all Table 111. .4ccuracy of Uraniuni Determinations concentrations of oxalic acid, as shown by the value 0.070 a liich Standard US08 Deviation was obtained for the slope of the wave when the log of the function Solution Added. Y UaOs Found. y r % z (zd-2) was plotted against the potential ( E d E ) of thc dropping 1 95s 964, 956, 976, 9.50 1.1 1.1 2 103 103, 103, 101, 102 1.0 1.0 mercury electrode (8). However, the waves were better formed 3 66 6.5, 66. fi7, 6.5 1.0 1.5 and more easily measurable a t the higher concentrations of oxalic 4 39 39.1.40.3,39.1,38.0 0.9 2.7 .5 8 7.4. 6 . 8 , 7 . 4 . 8 . 0 0.5 (j.3 acid. I
Table IV. Comparison with Other Methods Solution
Reference Method
Polarographic Method
-4
0.2640 0.0536 0.0138 0.0092 0.0045 0.0031
0.2600 0.0548 0,0134 0,0093 0 0042 0.0032
B C D E F
Difference
- 0.0040
+0.0008 - 0.0004 -0.0001 -0.0003
+0.0001
EFFECT O F GELATIN COYCEITRATIOY
The most suitable concentration of gelatin was found to be 0.015% in the cell solution. .4t this concentration maxima are effectively eliminated, the wave is horizontal and, therefore, the height is easily measurable by means of a single reading and the diffusion current is proportional to concentration of uranium down to a drop time of 1.5 seconds. When the concentration is much
1620
ANALYTICAL CHEMISTRY
larger than this value, the wave height is decreased and the wave becomes more rounded. The effect is shown in Figure 4. The diffusion current was halved when gelatin was replaced by methylcellulose. EFFECT OF SULFURIC ACID
The sample is heated to fumes in the presence of sulfuric acid, which forms a nonvolatile salt of uranium and oxidizes organic matter. It is convenient to use 1 ml. of 3M sulfuric acid. This gives a concentration in the cell solution of 1.OM sulfuric acid. There is normally a very slight loss of acid due to fuming, but this has no effect on the determination, as the diffusion current is not affected by changes in sulfuric acid concentration in the electrolyte down to below 0.5M. The effect of variations in sulfuric acid content' of the electrolyte is shown in Table V. The halfwave potential remains almost constant at -0.3T volt over this whole range. EFFECT OF REDUCIBLE ORG4NIC MATTER
An increased diffusion current occurs in the presence of reducible organic matter. Often there is a rounding of the base of the wave. These effects ruin the uranium determination. By heating with sulfuric acid and nitric acid until sulfuric acid fumes just appear and completing the oxidation with 2 drops of perchloric acid, interference due to organic matter is completely eliminated. EFFECT OF OXYGEV
The presence of oxygen in the sample causes a current to flow before the reduction potential of uranium is reached, thus destroying the flat base of the wave. While the total diffusion current is increased in proportion to the oxygen concentration, uranium values obtained graphically may be low, as shown in Figure 5. A slow stream of nitrogen bubbled through the solution for 15 minutes removes oxygen adequately. EFFECT O F IONIC IMPURITIES ON ANALYSIS
sample contains any of these ions, their complete removal is easily achieved by a slight modification of the cellulose treatment described below. The presence of chloride, sulfate, nitrate, or perchlorate ions within the limits here considered has no effect on the wave, but phosphate ion reduces the wave height by about 2%. PROCEDURE
Solid Samples. Prepare a solution of the sample by any suitable means, preferably containing between 20 y and I mg. of uranium, and without separating the insolubles evaporate the solution to dryness and proceed exactly as outlined below for ' liquid samples.. Liquid Samples. Evaporate a measured volume of solution just to dryness in a 400-ml. beaker. Add 3 ml. of 3-V nitric acid and 3 grams of calcium nitrate. Warm and swirl the beaker gently to redissolve the soluble material. To the solution 'together with any insolubles, add sufficient cellulose powder (with plastic spoon) to adsorb all the solution. From 5 to 10 grams is usually required. Place 5 grams of cellulose powder on the sintered disk in the cellulose column and add 5% nitric acid-ether to cover the cellulose. Stir with a glass rod and allow the ether to drain. If the sample contains large amounts of any of the interfering elements mentioned above, use 10 grams of cellulose powder. A good method of ensuring a compact wad with retention of little ether is to close the top of the column with the palm of the hand for a few moments.
Table
\-. Relation between Diffusion
Current of Uranyl Sulfate and Sulfuric Acid Concentration
(For IO-4.M uranyl sulfate in 0 5.M oxalic acid at 26" C ) 3M His01 Approximate Molarity, id. pa Used, Mi. H2S01 in Cell x 10 0.3 0.3 6.87 0.5 7.10 0.5 0.8 0.8 7.09 1.0 1.0 7.12 1.2 1.2 7.10 1.6 1.6 6.96 6.01 2.0 2.0
Only trace quantities of impurities are left after the cellulose adsorption treatment when it is carried out as described below Transfer the contents of the beaker to the column by means and the following discussion applies only to concentrations of ions of a glass rod and tamp lightly t o ensure a continuous column. of about the same order as the uranium-Le., a maximum conWash the beaker with 25 ml. of 5% nitric acid-ether and pour centration of 10-3.U. Tests were made by adding 0.2 ml. of a the washings through the cellulose column. Percolation is rapid. Collect the percolate in a 100-ml. flat-bottomed boiling flask 0 . l M solution of the sulfate, chloride, or nitrate of the individual which has been cleaned with nitric acid-ether. Wash the colmetals to a polarographic cell containing 2 ml. of approximately umn with four 15-ml. portions of nitric acid-ether and collect 5 X 10-4Jf uranyl sulfate solution. The uranium wave height this in the same flask. ildd 10 ml. of water to the percolate, was then measured and after a correction for dilution of the soluthen distill off the ether on a steam bath and boil the residue over tion by the added impurity, the concentration of uranium was again determined. In general, cations 15-hich form even slightly 0.6 L soluble oxalates reduce the wave height and may -. I in addition cause a slight shift in the half-wave 0.5 potential, This effect has been observed with the alkali and alkaline earth metals, and with alumi0.4 num, magnesium, cadmium, nickel, ferrous iron, W W L* arsenic, magnesium, and chromium. Lead, zinc, 0.3 and thorium have no effect on the sensitivity, but colbalt causes a slight exaltation of the wave height. fj0.2 I n no case was the error caused by the presence of i any of these metals greater than 1.5% of the actual value. Vanadium and ferric iron give waves from zero volts which can be compensated if not too large, but no waves result until the solution is approximately 5 X l O - 4 M in these ions and there is no 0 1 P 3 4 5 6 1 8 9 10 11 effect on the uranium wave height. ED.E., VOLTS VI. S.C.E. VOLTS Copper and bismuth give waves that coalesce Figure 5. Effect of Oxygen additively with the uranium wave. Antimony X lO-4M uranyl sulfate, 0.09M sulfuric acid, 0.01570 gelatin Polarograms of 0.4 and molybdenum give waves which interfere with a t 25' C. A . No deaeration the uranium wave, while titanium(II1) and tin(I1) B. 5-minute deaeration reduce the uranyl ion and eliminate the wave. If a C. lo-, 15-, and 20-minute deaeration
-
5
-
d -
1621
V O L U M E 2 6 , NO. 10, O C T O B E R 1 9 5 4 a hot plate with constant swirling for one minute. This oxidizes a large proportion of the organic matter in the solution and prevents too vigorous a reaction when sulfuric acid is added. Add 1 ml of 31.1.7 sulfuric acid and place the flask in the hot box, until fumes of sulfuric acid are seen. Cool slightly, add 0.5 ml. of concentrated nitric acid, and heat until the nitric acid is evaporated Cool and wash down with 2 ml. of water, then add 2 drops of perchloric acid, and heat again ,just to fumes of sulfuric acid to ensure complete oxidation of all organic matter. To the flask add 2 85 ml. of supporting electrolyte consisting of 0 5W oxalic acid containing 0 1% volume hydrochloric acid and, 0.015% gelatin (and 0 001% uranium as uranyl sulfate-the spike-if a very low value is expected) and mix well. This addition may be made from an automatic buret calibrated to delivei 2.85 ml. The total volume of the sample is now 3 ml. The electrolyte must be prepared fresh each day, since decomposed gelatin n ill ruin the estimation. Transfer about 2 ml. of:olution t o the polarographic cell and place in a nater bath a t 20 rk 0.1’ C. An internal mercury pool anode is satiqfactory and convenient and gives better results than a silver nire anode -4ny type of cell may be used. The type employed in this work \\-a? a 65-mm. length of 15-mm. glass tubing with a sealed-in glass bubbler which did not quite touch the mercury surface Deaerate the solution by bubbling n i t h tank nitrogen, which should be first passed through alkaline pyrogallol solution and then through water also kept a t 25” =k 0.1” C. Record the polarogram from 0 to -0.5 volt. Results of satibfactory accuracy can be obtained on samples containing at least 24 y per ml. X ) by means of a single reading at -0.5 volt, which must be corrected for the electrolyte line. The method of standard addition gives the most accurate results, and is especially recommended for samples containing less than 20 y per ml. Hoxever, results are obtained more quickly by the use of a calihration curve and the precision is within 5 ~ 5 % .
ACKNOW LEDGM E N 1
The author is indebted to Maxwell Lipworth and H. T. Tucker for their assistance in developing the uranium separation techniqueand in carrying out the polarographic v,-ork,respectively. Informat,ion regarding the behavior of locally important impurities, especially sulfate, in the cellulose column treatment was obtained from reports of the Sout,h African Government Metallurgical Laboratory. Acknowledgment is made to the consulting chemical and metallurgical engineer, F. L. Melvill, and to the board of Anglo-Transvaal Consolidated Investment Co., Ltd., for permission to publish this method. LITERATURE CITED
(1) Hurstall, F. H., and Wells, R. A,, Analyst, 76, 396 (1951).
(2) Crompton, C. E., Tichenor, R. L., and Young, H. A , , U. S. Atomic Energy Commission, CD-GS-35 (1945). (3) Harris, W. E., and Kolthoff, I. M.,J . Am. Chem. SOC..6 7 , 1484 (1945). (4) Ibid., p. 1488. (5) Kolthoff, I. AI., and Harris, W. E.. Ibid., 68, 1175 (1946). (6) Lewis, J. .I.,Ministry of Supply, Great Britain, Chemical Research Laboratory, Sci. R e p t . CRL/AE 56 (1950). (7) Lewis. J. -i., and Overton, K. C., Ibid.. CRL/AE 41 (1949). (8) Lingane, J. J., Chem. Revs.,29, 1 (1940). (9) Rodden, C. J., and Warf, J. C., “Analytical Chemistry of the Manhattan Project,” pp. 77-122, Kew York, NcGraw-Hill EookCo.. 1950. (10) Ibid.,pp. 122-35. (11) Ibid., pp. 596-610. (12) Strubl. R., Colleclion Czechosloz. Chrm. Conzmuna., 10, 466 (1938). R E C E I T Efor D reiieiv Augu4t 20, 1953. Accepted July 21, 1954,
Tu rbidimet ric Microdete rmination of Zi rconium GUY WILLIAM LEONARD, JR., DOUGLAS E. SELLERS, and LEROY E. SWIM Department o f Chemistry, Kansas State College, Manhattan, Kan.
The recent interest in the chemistry of zirconium and its compounds has necessitated the development of improved methods of analysis. As the reaction between zirconyl ion and phthalic acid gives a very finely divided precipitate which tends to stay in suspension, the possibility of using this reaction for a simple turbidimetric determination of zirconium was in%estigated. The zirconium phthalate suspension was found to be very stable and to follow Beer’s law up to 123 p.p.m. of zirconium. No critical control of the order of mixing, length of heating, period of cooling, or hydrogen ion concentration was necessary. The interference of the fluoride ion in a solution of known zirconium concentration can be used to estimate concentration of that ion. A simple, direct, and rapid turbidimetric method is described for the microdetermination of zirconium. This technique is useful for the determination of zirconium in the presence of high concentrations of various ions.
T
HE widespread interest in the study of zirconium and its compounds has necessitated the development of improved methods of analysis, especially in the micro range. An investigation of the reactions between organic reagents and zirconium salts in an acid solution showed that the precipitate formed by phthalic acid with zirconium ions was very finely divided and tended to stay in suspension. Because phthalic acid had been suggested as a selective reagent for the gravimetric determination
of zirconium ( I ) , the possibility of a turbidimetric method was investigated. REAGEYTS A \ D EQUIP\IE\T
Stoch solutions \yere prepared containing 7 grams of potassium hydrogen phthalate in 1 liter of 0.255 hydrochloric acid and also 20 grams of zirconyl chloride octohydrate in 1 liter of 0.25-V hydrochloric acid. All further solutions were made by dilutions of these stock solutions. The zirconium solution was standardized gravimetrically by mandelic acid. The Bausch and Lomb monochromatic colorimeter with a 430 mp filter was used as a photoelectric turbidimeter. PROCEDURE
The acidic solution of zirconyl ions was diluted to such a volume that the final concentration of zirconium was from 10 to 125 p.p.m. in approximately 0.255 hydrochloric acid. Five milliliters of this solution was transferred to a test tube and 10 ml. of the phthalic acid stock solution was added. The solution was mixed by inverting the tube. The tube was lightly corked and suspended in a beaker of briskly boiling water. The mixture was heated for about 1 minute and then the cork was pushed firmly into the test tube. Heating was continued for a total time of 10 minutes; then the test tube n a s placed in a beaker of cool water. After cooling, the solution was shaken vigorously for a few seconds. The test tube was set aside for 3 to 4 minutes to allow air bubbles to escape. The solution was then placed in a square cell of the Bausch and Lomb monochromatic colorimeter and the per cent transmittancy measured a t 430 mp. The concentration of zirconium was read from a calibration plot showing either per cent transmittancy or absorbancy versus concentration of zirconium. This calibration curve was obtained by similarly treating selected amounts of zirconium from the zirconyl chloride stock solution.
