Determination of Calcium by Potentiometric Titration SORBERT URI Chemical Laboratories, Hebrew C'niversity, Jerusalem, Palestine
A new metkod is proposed for the determination of calcium by potentiometric titration. The method is based on the precipitation of calcium ions by potassium fluoride solution and the use of the oxidation-reduction potential of ferricferrous ion as an indicator for the determination of the equivalence point. The estimation is carried out in a 50% alcoholic solution saturated with sodium chloride. The method can be used to determine the sum of calcium and magnesium.
N
0 satisfactory potentiometric method for the determination
Riedel de Haen. The potentiometric cell may be indicated as' follons :
of calcium can be found in the literature. M o s t methods are based on the use of third-class electrodes such as Ag/AgGO,/ CaCs04/Ca++, Pb/PbC$O4/CaC2Os/Ca-+, and .4g/.4g?P04/ Car(POa)s/Ca+ (1). The method proposed here for the estimation of calcium was originally suggested by Treadwell ( 2 ) some 15 years ago for the determination of mixtures of magnesium and aluminum. It is based on the fact that ferric but not ferrous ion forms a stable complex with fluoride. If a solution of calcium chloride containing ferrous and a small quantity of ferric ion is titrated with potassium fluoride, the fluoride added is first precipitated as calcium fluoride. At the end point, the FeT++ is converted to FeF6---, causing an abrupt drop in the oxidation-reduction potential measured by means of a bright platinum electrode immersed in the solution. It is not practicable to Tyork in a purely aqueous solution, owing t o the considerable solubility of calcium fluoride and it is probably for this reason that the method, though allowing the rapid determination of calcium, has not been suggested so far. Accurate results were, however, obtained by working in a 507, ethyl alcohol solution saturated with sodium chloride. The method can also be applied to the estimation of the sum of calcium and magnesium ( a separate estimation of the two in a mixture was not Practicable under the working conditions), and thus by the additional determination of one of the components by usual analytical methods, the proposed potentiometric method may be used to ascertain the amount of both calcium and magnesium, an important fact ansidering the analytical importance of solutions of these ions. In the determination of the sum of calcium and magnesium the potential drop is less distinct than in the case of pure calcium chloride solutions, but t'he error never exceeds *1%. This disadvantage is partly offset by the ease and speed of the determination, since ordinarily in this separation many difficulties have to be overcome.
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1
1
Sample
solution
KCI (satd.)agar-agar bridge
I
(satd.) KC'"gzC1z reference electrode
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Hg
The calcium chloride solution was kept in a Pyrex beaker covered with a n ebonite block which could be fixed in a thermostat. The measurements were in general carried out a t a temperature of 0 " C. The ebonite block contained a number of holes for a stirrer, electrode, and electrolytic bridge and for the introduction of the solutions and addition of titrant solution from a 10-ml. microburet divided into 0.01 ml. The bright platinum electrode had the form of a square of 1 sq. cm. area and when not in use was kept immersed in distilled water. The e.m.f. of the cell was measured with a Hellige Roehrenpotentiometer. This apparatus is based on the followin'g principle: The potential to be measured is applied to the grid of a triode valve, and causes a change in the anode current. This change is measured by means of a dial-galvanometer. Thus it is possible to read directly (in millivolts) the value of the potential applied to the grid. As the accuracy is reduced when more than 200 mv. are applied to the grid, the apparatus possesses an arrangement whereby back e.m.f.'s of 200, 400, 600, 800, 1000 mv., respectively, can be applied. These back e.m.f.'s are obtained from a series of resistances connected to a special standard cell in
EXPERIMENTAL
All substances used were Schering-Kahlbaum products (puriss. or for analysis) except ferrous chloride which was obtained from
Table I. Potentiometric Titration of 50 Ill. of 0.05 M Calcium Chloride with M Potassium Chloride (Solvent 91% by volume ethyl alcohol saturated with N a C1) M KF, 1\11, E , mv. A E / 3 T7 0.00 0.50 2.50 3.50 4.50 4.80
... ... ...
+146 148
++I48 +146
cw(x1 CW(M2
70%), the potential drop is less pronounced. Data presented in Table I show the course of the potential change using 91% ethyl alcohol as solvent. By comparing the data in Table I with curve 1 (Figure 1) it can be seen that in the latter case the potential drop is less distinct, though even under these conditions I n solutions containing less the error does not exceed '1%. than 30% ethyl alcohol no results clear enough to allow the determination of calcium were obtained. PROCEDURE FOR DETERMINATION OF CALCIUM
Dilute a solution of calcium chloride with distilled water and ethyl alcohol so that its final concentration of calcium chloride is between 0.05 M and 0.2 M , and that of the alcohol 50% by volume. Then add 0.02 gram of ferrous chloride containing 0.4 mg. of ferric chloride and saturate with sodium chloride (add 6 grams of sodium chloride per 50 ml. of solution) and titrate with a molar potassium fluoride solution. Stir the solution well and keep it in an ice-water mixture during titration (this cooling is not compulsory when the room temperature is below 16" C.). Be careful to wait until constant potential is obtained after each addition of potassium fluoride, especially in the proximity of the equivalence point. Ascertain by graph or by calculation the amount of potassium fluoride added for which A E / A V becomes a maximum. One milliliter of M potassium fluoride corresponds to 0.5 ml. of M calcium chloride or 20.04 mg. of calcium in the tested solution. Table I1 shows the precision and accuracy of the analytical method under routine conditions. LITERATURE CITED (1) Furman, N. H., IND.ESG.CHEX.,ANAL.ED.,14, 367 (1942) (2) Treadwell, W. D., Helv. Chim. Acta, 13, 500 (1930).
Simultaneous Determination of Carbon, Fluorine, and Chlorine in Halocarbons Semimicromethod REBECCA O'D. TESTON'
AND
F. E. XIcKENNA2
S.A.M. Laboratories, Carbide and Carbon Chemicals Corporation, New York, N . Y . Carbon, fluorine, and chlorine (and bromine) can be determined simultaneously in halocarbons, which do not contain hydrogen, by a combustion method. After decomposition of the compound at 1000" C. in a stream of oxygen, the chlorine reacts with silver in an absorption tube maintained at 295" C. (if bromine is present, it reacts simultaneously). Fluorine reacts with quartz to form silicon tetrafluoride, which is absorbed on alumina at 175" C. Carbon dioxide is absorbed by Ascarite as in the conventional carbon and hydrogen determinations.
P
ARR bomb fusions of fluorocarbons with sodium peroxide have not yielded reproducible analytical results, and on a large number of occasions extremely high values for the halogen content (100 to 300yc of the theoretical) were obtained, even though halogen-free reagents were used in all cases. Fusion of Present address, Eastern Rubber Specialties CO., South Norwalk, Conn. Present address, Institute for Kuclear Studies, Tniversity of Chicago, Chicago 37, Ill.
these compounds with potassium for 30 minutes a t 400" C. in sealed Pyrex tubes has been recommended ( 2 ) . I t v-as found by IGmball and Tufts ( B ) that such conditions were not sufficiently vigorous for the quantitative decomposition of some compounds; they reported a fusion with potassium in specially designed nickel bombs. Simons and Block (9) suggested decomposition of fluorine-containing organic compounds by fusion with sodium in quartz a t red heat; more convenient sodium fusions have been reported in the hlanhattan Project literature ( 4 , 6 ) .
