Thermometric Precipitation Titration of Calcium in the Presence of Magnesium Kinetic Masking and Application to Limestone Analysis JOSEPH JORDAN and
E. J. BILLINGHAM, Jr.
Deparfmenf o f Chemistry, The Pennsylvania State University, University Park, Pa.
b When a soluble oxalate was titrated rapidly into a dilute solution of calcium in a borate buffer of p H 8, a well defined thermometric titration curve was obtained, corresponding to instantaneous exothermic precipitation of calcium oxalate. In contradistinction, the analogous titratio! curve of magnesium with oxalate was quasiisothermal in shape because of a slow precipitation mechanism involving a complex intermediate. A method for the determination of calcium in the presence of magnesium is based on these differences in kinetic behavior. The procedure involves an automatic thermometric titration with standard oxalate and has been adapted to the nonseparative determination of calcium in limestone and dolomite. It combines the advantages of a macrotechnique in terms of sample size, with the convenience of a microtitration procedure in the determinative step.
T
HE problem of determining calcium in the presence of magnesium is a tiresome ghost which still haunts limestone analysis in this day and age. The conventional gravimetric oxalate procedure for the separation of calcium from magnesium is highly precise and accurate (=t0.05%), but involves repeated precipitations and is laborious (9). Other methods use x-ray diffraction, thermal analysis, sucrose extraction, heats of solution, and iodometric titration (10,20). Generally, these procedures yield an accuracy and precision within 1%. The difficulty of determining calcium in the presence of magnesium is inherent in the similarity of the chemical properties of the two ions. The free energies of most determinative processes applicable to calcium do not differ significantly from those of the analogous reactions with magnesium. Generally, there are only two known chemical nonseparative procedures for the determination of calcium in the presence of magnesium-via., iodometric titration (20) and chelometric
120
ANALYTICAL CHEMISTRY
titration with (ethylenedinitrilo) tetraacetate (6, 16, 19, 21). Therefore, the possibility of using an enthalpy-dependent thermometric titration was explored, because such an approach was likely to be less sensitive to free energy limitations than methods based solely on equilibrium considerations. Thirty years ago Mayr and Fisch (18), and more recently Chatterji (4), using discontinuous point by point manual thermometric procedures, reported that calcium could be determined by monitoring in a primitive adiabatic titration cell, the change in temperature occurring upon titration with standard ammonium oxalate. I n the study described here the thermotitrimetric characteristics of the calcium oxalate precipitation process were investigated using modern automatic thermistor instrumentation. Results indicate that under specified experimental conditions calcium and magnesium, in dilute buffered solutions, yield strikingly different thermometric titration curves upon addition of ammonium oxalate. Based on these observations, a method has been developed for the quantitative determination of calcium in the presence of magnesium. The applicability to limestone analysis has been tested with simulated limestone solutions and National Bureau of Standards limestone and dolomite samples. The method appears to be ideally suited for rapid and convenient calcium determination. Magnesium (up to a twofold mole ratio excess), aluminum, and iron do not interfere. Extensive use is anticipated in the limestone industry. EXPERIMENTAL
Materials. Reagent grade chemicals were used throughout. All solutions were prepared in triply distilled conductivity water, protected from contamination by carbon dioxide. Reference concentrations were determined by classical methods. Apparatus. The instrumentation was similar to t h a t developed previously in this laboratory (6, 11). A 1-mv. Bristol direct current recording
potentiometer (Model 560, 0.4-second pen speed) was used to follow the temperature change during the course of the titrations. The thermistor circuit had a sensitivity threshold of 0.0002" C. Full scale deflection of the recorder (28 cm.) corresponded to 0.07" C. a t maximum sensitivity setting. Throughout the study the automatic buret (11) wm operated a t a delivery rate of 0.02037 f. 0.00004 ml. per second, using a 20ml. standard hypodermic syringe to dispense the titrant. A coiled delivery tube (made of borosilicate glass, 50 cm. in length and 0.3 cm. in diameter) was fused to the syringe. This "delivery spiral" was jacketed, and water from a constant temperature bath circulated around it, affording accurate temperature control of the titrant. A capillary tip, immersed in the titrate solution [titrate = substance titrated (17')], emerged from the end of the coil as titrant dispensing terminal. Titration Procedure. I n each experiment 50 ml. of solution was titrated with 0.2M ammonium oxalate titrant. The initial temperature of the titrant and titrate solutions was controlled to +0.2" C. All titrate s o h tions were buffered a t p H 8. Stoichiometric end points and reaction heats were determined graphically from automatically recorded thermometric titration curves, using calibration and extrapolation procedures previously described (11). Dissolution Procedure for Limestone and Dolomite. A 1- or 2gram sample was dried for 1 hour at 110' C. and dissolved in 15 ml. of 6 M hydrochloric acid. The solution was evaporated to dryness, and the residue was heated to 110' C. for 30 minutes. Twenty-five milliliters of 3 M hydrochloric acid was added and any insoluble residue mas filtered off and washed with five 20-ml. portions of 1% hydrochloric acid. The washings were then added to the filtrate, which was subsequently reduced to 10 ml. on a hot plate and cooled, and the p H adjusted to a value between 4 and 6 by the addition of 1M sodium hydroxide. A precipitate of hydrous oxides, mainly of iron and aluminum, may form at this point. The solution (20 to 50 ml. in volume) was subsequently made up to 500.0 ml. by diluting with an
Table I. Determination of Calciumn by Thermometric Precipitation Titration with Standard Ammonium Oxalateb
A
B
Ca + +, Precisiond MmoWLiter ( ~ccuracy Taken Foun& Deviation)(% Error) 3.1 1.97 f20 4- 60 4- 0 . 3 5.30 f 0 . 3 5.29 - 0.6 9.15 9.09 f 0 . 5 - 0.4 20.87 f 0 3 20.95 - 0.1 40.48 f 0 . 3 40.54 4- 0 . 4 94.40 f 0.2 94.01 Solutions of pure CaCh in borate buffer of pH 8. Oxalate titrant standardized with potassium permanganate. c Mean of 3 to 6 replicate determinations based on end points extrapolated as shown in Figure 1,II. d Standard deviation of single measurement.
I D
0
Figure 1.
Typical thermometric titration curves at pH 8 1.
Titrate. 0 . 0 0 9 0 4 5 M M g + + Titrant. 0 . 2 3 9 5 M (NH&CzO, II. Titrote. 0 . 0 0 9 1 4 5 M C a + + Titrant. 0 . 2 3 9 5 M (NHa12CzOd AB. Time-temperature blank prior to titrotion 8. Start of titration C‘. Extrapolated end point CD. Excess reagent line 88’. Temperature change during titrotion
aqueous borate buffer of p H 8 (0.005M in total borate). The precipitate was allowed to settle and aliquots of supernatant liquid were pipetted off and used as titrate. The volume of the precipitate was negligible compared to the total volume of solution; accordingly, no corresponding correction was made in computing results. RESULTS
Characteristics of Thermometric Curves of Ca++ and Mg++. Upon titration of 0.01M magnesium ion with 0.2M ammonium oxalate, a continuous quasi-isothermal thermometric titration curve x a s obtained] as illustrated in Figure 1, curve I. A precipitate was not formed in the Dewar: T h e solution stayed clear for a n hour after the addition of the titrant. I n contradistinction, upon titration of 0.01M calcium ion with the same titrant a rectilinearly ascending-type thermometric titration curve was obtained (as shown in Figure 1, curve II), and precipitation occurred instantaneously. Within experimental error] the “effective heat of reaction” was 0.0 kcal. per mole in the magnesium 0.1 kcal. per titration, and -6.1 mole in the calcium titration, in a temperature range between 24Oand 26” C. Determination of Calcium in the Absence of Other Multivalent Cations. It was anticipated t h a t titration curves of the type shomm in
*
Figure 1, curve 11, could be utilized for the determination of calcium a t least in solutions free of other multivalent cations. This was ascertained by titrating (with standard ammonium oxalate) a series of calcium solutions of accurately known concentrations. The results are summarized in Table I. As can be seen in columns 3 and 4, the method yielded a precision and accuracy of 0.5% in a range of concentrations between 5 X and 10-1M. However, when [Ca++] < 0.005M the determination was unreliable, involving deviations and errors of 20% to 50%.
Table II.
Interference Studies. Comparison of curves I and I1 in Figure 1 suggested t h a t calcium may be titrated in the presence of magnesium. To ascertain whether this was indeed feasible, the experiments summarized in Table I were repeated in the presence of varying amounts of magnesium, everything else being equal. The corresponding thermometric titration curves were similar to those obtained in the absence of magnesium, as long as the molar ratio between magnesium and calcium did not exceed a critical threshold value of 2. Table I1 summarizes the pertinent analytical results. The data indicate that calcium can be determined in the presence of a twofold molar excess of magnesium with an accuracy and precision within 1%. Under the experimental conditions it was not possible to locate end points on curves obtained upon titration of solutions containing a larger excess of magnesium, because of delayed (and probably gradual and incomplete) precipitation of calcium oxalate: An induction period of 2 t o 15 seconds elapsed
Determination of Calcium in Presence of Magnesium
Calcium Mo
