ANALYTICAL CHEMISTRY
1678 circle using an accurately calibrated capillary tube. The adjacent spots should not coalesce. The spots were dried a t room temperature, and the chromatogram was developed using Partridge's solvent (butanol-acetic acid-water, 40: 10:50). The paper was air-dried and uniformly sprayed Trith 0.5% ninhydrin in 95% acetone. After drying in a current of air the paper was k t y t a t 65" C. for 30 minutes. The zones w r e cut out and extracted with 4 ml. of 75% ethyl alcohol containing 0.2 mg. of copper sulfate, care being taken t o avoid contamination of paper from the hands or by dust. While running the chromatogram the distance of the movement of solvent was closely controlled. For purposes of drawing calibration curve?, the distance of the solvent boundary from the center of the paper (9 cm.) was always kept constant. The optical density was measured using a KlettSummerson colorimeter with green filter (540 mp). d blank detprmination was also made with uncolored area, and correction was applied in each case.
determined accurately with ease and facility. Thompson et a / . ( 3 ) drew attention to the fact that considerable losses of amino acids occurred in two-dimensional chromatograms. Circular paper chromatography eliminates errors due t o this t o a very great extent, as shown by recovery experiments. Using this method the nitrogen metabolism of leaves and leguminous seeds and the role of transamination in protein synthesis are being investigated. LITERATURE CITED
(1) Giri, K. V., Krishnamurthy, K., and T'enkatasubramanian, T. A., CzLrrent Science, 21,44 (1952). ( 2 ) Giri, K. V., and Rao, N. A. N.. ,Vatwe, 169,923 (1952); J . Indiun Inst. Sci., 34, S o . 2, 95 (1952). (3) Thompson, J. F., and Steward, F. C., Plant Phusiol., 26, 421-40
ADVANTAGES OF THE METHOD
This quantitative procedure has proved to bc a very useful tool. I n view of the clear separation obtained by circular paper chromatography the individual amino acids in a mixture can be
(4)
(1951). Thompson, J. F., Zacharius, R. XI., and Steward, F. C., I b t d . , pp. 375-97 (1951).
RECEIVKD for reyiew April 4, 1952. Accepted .June 14, 195'2
Determination of Tracers in the Presence of Their Radioactive Daughters H. W. KIRBY Moccnd Laboratory, Miamisburg, Ohio
IXTURES of genetically related radioisotopes are frequently encountered when tracers are employed to follow a chrmical reaction. If the particles emitted by parent and daughter are of the same kind, and especially if they are of similar energies, the question arises as to what proportion of the measured radioactivity is due to each nuclide. very simple equation can be derived which is applicable to a great many tracers, since, in general, it is the percentage carried rather than the absolute amount of thp tracer which is sought. Consider the case of a radioactive tracer having only one radioactive daughter. Let = = -41 = kl = A\-l
X1
Equations 1 and 4 are now solved simultaneously for A ,
the activity of the parent a t time, tl. If this mixture undergoes a fractionation, so that unknown amounts of the parent and daughter are present in each fraction, and if each fraction is then counted a t times t 2 and t f T , it follows from Equation 5 that the activity of the tracer in the fraction a t time t? will be
+
the number of atoms of the parent a t an arbitrary time, t l the decay constant of the parent (X = 0 . 6 9 3 / T l / ~ ) the activity of the parent (A-1E.l) a constant representing counting yield ("geometry")
and let N,, A,, A*, and kl have similar connotations with respect to the daughter. The total counting rate in the mixture a t time tl will he
C1 At a later time, tl ( 11
klNlX1
+ kz'V2X2
= ki-41
+ k2A2
where C, and C, are the counting rates of the mixture a t f l and tP T , respectively. Since the total activity of the parent a t t2 will be Ale-A1(t2-Li), on dividing Equation 6 by Equation 5
+
(1)
+ T, the counting rate for the parent will be Furthermore, it frequently happens that t, - tl can be made very small compared with the half-life of the parent, in which case, as X l ( f 2 - t l ) approaches zero
and the counting rate for the daughter will he
The total counting rate for the mixture a t tl
+ T will be
Equation 8 defines the ratio of tracer in the fraction to total tracer, if aliquots of each are counted a t nearly the same time. I t is not necessary that the chemical procedure be of short duration in order that tx - tl may be kept small, since an aliquot of the original mixture can be retained for counting a t the later time. I n practice the time T , however, should be chosen equal to or greater than the shorter of the two half-lives involved. Equation 8 is applicable only to the case of a radioactive tracer having a daughter which decays to a stable isotope. Similar
V O L U M E 2 4 , NO. 10, O C T O B E R 1 9 5 2 methods may be applied to longer chains, however. ample, in a chain containing three radioisotopes = A 1
Ce
- C,(e-XzT
CS - C a ( e - h g T
1679 For ex-
+ e-hzr) + Cae-(Xn + X a ) T + e-XaT) + C,e-(Az + XdT
(')
is to be noted that the effect of this approximation is to eliminate the half-life of the parent from Equations 8 and 9. LlTERATURE CITED
(1) Rutherford, E., Chadwick, J., and Ellis, C. b.,"Radiations from
Radioactive Suhstances," London, Cambridge University Press, 1930.
+
I n this case, the original mixture is counted a t times t l , 1, T, and + 2T' and the fraction is counted at "7 " + and t z -k 2T* Herr, again, X l ( t 2 - t i ) must be aPProximateh' equal to Wro. It
RECEIVED for review March 28, 1952. .4ccepted August 22, 1952. U. S. .4tomic Energy Cornmission, Contract NO. AT-33-1-GEN-53.
Determination of Calcium and Strontium in a Mixture GUNNlR 0. ASSARSSON
AND
.41NO BALDER
Chemical Laboratory, Geological Survey of Sweden, Stockholm, Sweden
HE estimation of calcium and strontium, occurring together Tin a sample, is doubtful or troublesome, when using such methods as are described in the handbooks or in the periodicalsc.g., the nitrate method ( 4 ) or methods founded on the selective solubility of one of the salts of the romponents in organic solvents (1-3). The polarographic method of Zlotowski and Kolthoff ( 5 ) is not suitable because of the small difference between the polarization voltage of calcium and strontium (0.15 volt in 4 N tetraethyl ammonium iodide solution; half-n-ave potential = 2.1 volts us. the saturated calomel electrode). For investigations on the system calcium chlorides-strontium chloride-mter the authors needed a rapid accurate method, giving values correct to within 0.5% of the total amount of the alkaline earth chlorides. Some methods of indirect determination were tried. The best one a-as found to be the physicalrhemical determination of the electrical conductivity of aqueous solutions. Cslrium and strontium chlorides give solutions, the conductivities of which are on a straight line when t h r niixed diluted solutions have the same concentration. PROCEDURE
Dissolve the weighed sample in water; if it is necessary to use acid, remove the excess acid by evaporation. Transform the salts to carbonates by precipitation with ammonium carbonate in hot solution, cool, and collect the precipitate in a filter crucible, wash with cold water containing a little ammonium carbonate, and lyith alcohol. Drv a t 550" C. and weigh. It is not sufficient to drv a t 120" C. only, because the carbonates contain about 1.5 to 2% moisture which can be removed only a t higher temperatulr. lyhen the temperature is higher than about 650" C., the carbonates attack the porcelain if the precipitate is collectrd in a crucible of this material. P u t the precipitate or an aliquot part of it in a platinum dish, dissolve it in several milliliters of purest redistilled hydrochloric acid, and evaporate to dryness on the steam bath. Dissolve in distilled water, of good qualitj- but not necessarily conductivity water, and evaporate; repeat the procedure about three times and dry in an air bath a t 120" C. in order to remove every trace of excess acid. Dissolve the purified chlorides of calcium and strontium in distilled water, and dilute to the concentration required. Measure the conductivity of the solution and read off the proportions of calcium and strontium carbonate on the nomogra h calculated from solutions of knoxn mixtures of calcium ancfstrontium a t different relative concentrations. The sizes of the conductivity vessels determine the most suitable concentration for the measurements. DISCUSSION
The authors use an apparatus which consists of a calibrated lyheatstone bridge, an accurate resistance box, and pipetformed conductivity vessels with a volume of about 25 ml. The platinum sheets are 1 sq. cm. and the distance betwren them is 3 cm. Using a telephone as nullpoint instrument and a water thermostat, constant within 0.02 O C., the authors have a determination error of about 0.2 mm. in the middle of the bridge, corresponding t o maximum 0.2% of the weighed carbonates. See Table I. The amount of sample used corresponds to 0.4 to 1.0 gram of weighed carbonates. The solutions of the rhlorides
are diluted in such a n-ay that 1 gram of the carbonates corresponds to 500 grams of the chloride solution. This solution is used for the conductivity measurements. The authors work at 25' C. a t which temperature the conductivities of pure strontium chloride and pure calcium chloride are 3.063 X and 4.423 X 10-3 reciprocal ohm, respectively, a t the concentration mentioned above. iit this dilution the impurities in the distilled water can be neglected. If small conductivity vessels are used (the platinum sheets about 0.5 sq. cm., the distance between them 2 cm., and the volume about 5 ml.), it is possible to lower the amount of the sample to about 0.05 to 0.2 gram of weighed mrbonatee R-ithout reducing the accuracy of the determination essentially.
Table I. Conductivity Measurements of Known IIixtures of the Chlorides of Calcium and Strontium at 25.00" f 0.02" C. Known Mixture Calcium Strontituil carbonate, carbonate, gram gram
0.4423
Found from Nomograph Calcium Strontium carbonate, carbonate, gram gram
Error, % o Carbonates
....
.....
For the preparation of solutions of known concentration, thr pure salts of strontium and calcium chloride with impuritics determined spectrochemically ( SrC12.6Hs0 containing 0.01% CaCI2and BaC12; CaCI2.2H2Ocontaining 0.01% SrClz and 0.01 % BaC12) were used. Other impurities (magnesium and alkali salts) were removed by precipitation of the carbonates. Although the determination described above is intended to be used only for the determination of calcium and strontium occurring together, it is also possible to use it when they occur in the presence of harium, which is the only impurity which caninterferewiththeresultq. In this case barium must be removed by separation [precipitation with ammonium chromate(l)] ordeterminedspectrochemic~ally directly. The discussion of this problem is not within thr scope of this note. LITERATURE CITED
(1) Fresenius, R., 2. anal. Chem., 29, 413-30 (1890); 32, 1'39 94 (1893).
(2) Kobe, K. A., and Motsch, R. L., ANAL.CHEM.,23, 1498-9 (1951)
and literature cited there. (3) Rose, H., and Strohmeyer. F., Pogg. Ann. 110, 296-300 (1860). (4) Willard, H. H., and Goodspeed, E. W., IND.ENG.CHEM.,h u i . ED.,8,414-18 (1938). (5) Zlotowski, I., and Kolthoff, I. XI., J. P h y s . Chern., 49, 398-402 (1945). RECEIVED for review April 9, 1952. Accepted July 11, 1952.
