James T. Corless
Narragansett Marine Loboratory University of Rhode Island Kingston
I
I
Determination of Calcium in Sea Water Analytical experiment using the radionuclide Ca45
The major solutes in sea water, CI-, Na+, Mgz+, S O P , Caz+, and K+, constitute over 99% by weight of the dissolved matter (Table 1); an accurate knowledge of their concentrations is of fundamental importance to oceanographers. The concentration of dissolved calcium is of particular interest since it is a prime participant in calcium carbonate formation by shell-forming marine organisms and by inorganic precipitation. Calcium concentration bears directly on the question of the solubility of calcium carbonate in sea water and the factors that control precipitation and solution. However, the analysis of a solution containing magnesium, calcium, and strontium has traditionally posed a problem, particularly where a high degree of accuracy and precision is required. Table 1. North Pacific Ocean Surface 19% . -- (12). Cations Na Mg2+ Caz+ +
Kt
Sr2+
Eq/k 0.4590 0.1046 0.0200 0.0097 0.0003
%o
10.556 1.272 0.400 0.380 0.013
Anions C1SOF HCOsBr-
Water,
Eq/kg 0.5353 0.0551 0.0023 0.0008
Chlorinity %O
18.980 2.649 0.140 0.065
When Dittmar (6) performed his analyses on the sea water samples collected from the world's oceans during the cruise of H.M.S. Challenger (1873-76), he emdosed a aravimetric vrocedure for calcium which invoGed a &gle oxalate precipitation followed by ignition to the oxide. I n Barnes' 1955 review on the analysis of sea water (I) he notes that "precipitation of calcium as the oxalate is still the principal method employed." Various modificatious of the oxalate procedure have included dissolution of the precipitate in acid followed by a permanganate titratiou (X),dissolution and reprecipitation (II), and in silu generation of oxalate ion from methyloaalate (7). Difficulties with the oxalate procedure have, however, persisted and been discussed for several decades (1). The application of the oxalate method to rigorous studies of the calcium content of sea water has been limited by several sources of error, among which the coprecipitation of magnesium and strontium is vrobablv the most significant. Contribution No. 83 from the Graduate School of Oceanography, University of Rhode Island. The assistance of L. Donald Ray and the support of the Ofice of Naval Research under Contract Nonr (396)08 are gratefully acknowledged. The suggestions of Drs. John Winchester and David Scbink have been appreciated.
Chow and Thompson (3) have described a procedure based on flame photometry for the determination of calcium in sea water. They report a precision of 1% using a method of standard additions to compensate for the interference by other ions. Carpenter (2) has used an analytical method for calcium in natural waters which employs an ion exchange separation of calcium from magnesium and strontium followed by an EDTA weight titration with a spectrophotometrically-determined end point. The precision attainable by this procedure is reported to be of the order of 0.1% or better and represents a significant advance over earlier methods. Pate and Robinson (IS) have analyzed 35 sea water samples by a direct EDTA titration using Cal-Red as an indicator. This procedure calls for the precipitation of Mg(OH)2a t a pH of 12 prior to the addition of the last milliliter of titrant. Strontium is titrated along with calcium and must be corrected for. The authors report "excellent results in respect to precision and accuracy." I n this paper an experiment is described which employs radiotracer, ion exchange, complexometric titration, and spectrophotometric techniques for the determination of calcium in sea water. Although the ion exchange procedure is rather different, the analytical methods used in this experiment are basically a modification after Carpenter (%). The Experimenl
The calcium in an aliquot of sea water is separated from the magnesium and strontium using a column of Dowex 50TV-X8 ion exchange resin. Ca4&is used as a tracer for monitoring the elution of the calcium from the column. The calcium fraction is then titrated with EDTA using a weight buret and murexide as the indicator. The endpoint is determined spectrophotometrically. Using 50-ml burets, two columns of the Dowex 50WX8 resin (200400 mesh in the H + form) are prepared. Approximately 15 ml (wet volume) of resin, which corresponds to a load factor considerably less than l7& is convenient. The columns are cleaned with several column volumes of 6 M HC1 and conditioned with 2 M T.r.1
nu. Standardization of the EDTA Solution. An EDTA solution (approximately 0.007 M made from the disodium salt) is standardized against copper. A standard solution of copper is prepared by dissolving approximately 360 mg of 99.99% pure capper in dilute HNOa. The solution is quantitatively transferred to a preweighed 60-ml polyethylene bottle and diluted to volume. The battle and contents are thenreweighed. Three 1-ml sliquats are weighed out to the nearest 0.1 mg. An aliquot is transferred
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Number 8, August 1965
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ta the titration cell' and concentrated NH40H added until the solution turns dark blue from the formation of the copper ammonia complex. One ml of a murexide indicator solution (15 rng murexide in 19 ml ethanol plus 6 ml H20) is added; this turps the solution a dark amber. Weigh the polyethylene buret,l titrate ta the onset of the visual end point, which is best taken as the point where the solution turns from light yellow to purplish red, and reweigh the buret. Transfer the cell to the spectrophotometer (Beekman DU or equivalent) and, titrating with a microburet, determine the end point spectrophotometricaUy. This is accomplished by adding the EDTA solution in small increments and plotting the absorbance at 450 mil versus volume. The end point is characterized by a, sudden change in slope (see figure). The reaction which takes place in the vicinity of the end point may be represented by
Cu-Pur
+ EDTA = Cu-EDTA + Pur
where Pur is the purpurate ion. The volume added from the microburet to reach the end point will he smsll (less than 3% of the total volume) and is added directly to the weight loss of the weight buret.
Sea water samples are prepared by weighing two 10-ml aliquots to the nearest mg and, to each sample,
adding 2 ml of cone HC1 and 2 pc of Cads.% These samples are quantitatively transferred to the columns. The samples are allowed to soak in, followed by a 15-ml wash of 2 M HC1. The columns are then m e d and the elution performed with 2 M HC1. A drop of the eluant should be collected on a stainlesssteel plauchet, evaporated to dryness, and counted every 10 ml and an elution curve constructed. Each 10 ml is discarded until the activity is reached. The calcium fraction is then collected, evaporated to dryness, and taken up in 5 ml H20. The student should note that the drops collected for counting during the elution of the calcium comprise a significant volume of the calcium fraction and must, therefore, be reclaimed. The CaCl2which has been evaporated onto a planchet is dissolved in approximately one ml of H,O and combined with the sample. The calcium titration is carried out in a manner very similar to that described for the EDTA standardization by copper. The sample is transferred to the titmtion cell, the pH adjusted to approximately 11.6 with piperidiue and one ml of the indicator solution added. Weigh the polyethylene buret, titrate to the onset of the visual end point, which is best taken as the point where the solution turns from light red to red purple, and reweigh the buret. Transfer the cell to the spectrophotometer and, titrating with the microburet, determine the end point spectrophotometrically. This is accomplished by adding the EDTA solution in small increments and plotting the absorbance a t 510 mp versus volume. The end point is characterized by a sudden change in slope (see Figure). The reaction which takes place in the vicinity of the end point may be represented by Ca-Pur
Table 2.
Data from a Calcium Determination
1. Standard copper solution 352.3 mg Cu in 44.631 g solution (0.1 M HNOa) 1.242 X lo-' moles Cu/g soln 2. Titration of three aliquots of the Cu solution: Aliquot No. 3 4 2 Wt. of aliquot, g 0.9497 0.9439 0.9624 Wt. of EDTA, g , 15.730 15.642 15.945 Wt. EDTA/wt. ahq. 16.56 16.57 16.57 Ado~tedvalue of 1.000 e Cu solut~an= 16.57 e EDTA solution 3. Titration of two aliquots of a see. water sample*: Aliquot No. 12 13 Wt. of aliquot, g 10.713 11.623 Wt. of EDTA, g , 15.294 16.607 Wt. EDTA/wt. ahquot 1.4276 1.4288 Adopted value of 1.000 g sea water = 1.428 g EDTA solution 4. Calculation of calcium concentration: 1.428 1.242 X lo-&X - = 1.0704 X 10"moles Ca/g 16.57 sea water 1.0704 X 10F X 40.08 X 10' = 429.0 =t0.6 mg Ca/kg sen. watter
OSurface, 30°N, 64'35'W, salinity = 36.624%0.
small (less than 3% of the total volume) and is added directly to the weight loss of the weight buret. The two determinations should agree to within 0.3% or better. "Average" sea water has a calcium concentration of 400 mg/kg. A set of results obtained by this procedure in the author's laboratory is presented in Table 2. As a demonstration of the separation of calcium from magnesium and strontium by the cation column, two pc each of Srs9 and Mus4, a tracer whose cation exchange properties are very similar to magnesium, may be added to the samples in addition to the Ca45. MnZ+ has a distribution coefficient almost identical to that of Mg2+under those conditions (Table 3). In the absence of a ready supply of sea water, the
+ EDTA = Ca-EDTA + Pnr
where Pur is the purpurate ion. The volume added from the microburet to reach the end point will be
' We use a cell 4.5 X 3.3 X 9.3 om made from a/& Lucite. The polyethylene weight huret is made by inserting the tip of a polyethylene pipet into the cap of a 2-0. polyethylene bottle. a Oak Ridge National Laboratory Cat. No. Ca-45-P-3, high specific activity. 422
/ Journol of Chemical Educofion
ml
EDTA
A plot of absorbonce of sample rolvtia versus volume of tifront in the vicinity of the end point.
Table 3. The Distribution Coefficients and Tracer Characteristics of Mg2+, Ca2+, and SrZ+
Ion
KA16).
Tracer
Decav Modes
a Dowex 50W-X8 resin, 2 N HCI lKd(MnZ+) = 6.0.
experiment may be performed using a solution of the chlorides of magnesium, calcium, and strontium in the mole ratio 540: 110: 1. Students should be familiar with the material in the chapters on chemistry in references (9) and (l7), where the composition of sea water is reviewed, and also with references ( 1 3 , 10, 13, and 16). Recommended supplementary readings might include selected chapters in references (15) and (18). Some Study Questions
1. The specifications slip of a shipment of Ca-45-P-3 from Oak Ridge provides information on the specific activity, calcium concentration, and activity concentration of the CaCI, solution. Using the data given with your shipment and the original volume of the solution, calculate the amount of calcium added to the samples in the Cs" 'spikes. Considering the precision of the analytioal procedure, must this amount be taken inta account? 2. If strontium were equal in concentration to cslcium in sea. water, how would the procedure outlined in this paper have to be modified? 3(a). Given the equation
+ COsZ-(aq) CaCOs calcite) AFQ = - 11.38 kcal/male
Ca2+(aq)
=
(8,
calculate the solubility product, K,,, of calcite in pure water. (b). Using typical oceanic concentrations of 1 X 10-2 M and 3 X lo-' M for Cali and COa2- respectively, calculate the apparent ion product of calcite in sea water. Discuss possible Cause8 of the difference in values between (a) and (h).
(c). Using the concentrations of the major solutes in sea water (Table 1 ) and the basic form of the DebyeHuckel equation for activity coefficients (6),calculate the activities of Ca" and COaa- in sea water and, using these calculated activities, the ion product. Discuss the spplicability of the Debye-Huckel equation to sea. water. For further study on the problems related to carbonate precipitation in the sea, the student may refer to the papepers by Cloud (4) and Revelle and Fairbridge (14). Literature Cited
(1) BARNES,H., Analyst, 80, 573 (1955). (2) CARPENTER, J. H., Limnol. Oceanop., 2, 271 (1957). T. G., A d . C h a . , 27, 910 (3) CHOW,J. J. AND THOMPSON, (1955). (4) Cmun, JR., P. E., Geochim. Comochim. Ada, 26,867 (1962). R. A,, "Physical Chemistry," (5) DANIEI~, F. AND ALBERTY, 2nd ed., John Wiley & Sons, New York, 1961, p. 392. (6) DITTMAR,W., Report on researches inta the composition of sea water, collected by H.M.S. Challenger, "Challenger Reports" (Physics and Chem.), Val. 1, H. M. Stationery Office, London, 1884. (7) GORDON, L. AND WROCZYNSKI, A. F., Anal. C h . , 24, 896 (1952). (8) GEIPPENBEEQ, S., J. Cons. Int. Ezplor. Mer., 12,284 (1937). (9) HARVEY,H. W.. "Chemistry and Fertility of Sea Waters," Cambridge University Press, New York, 1960. (10) THOMPSON, T. G., J. CEEM.EDUC.,35, 108 (1958). E. G., Ind. Eng. Chern. Anal. Ed., (11) KIRK,P. L. AND MOBERG, 5, 95 (1933). (12) LYMAN, J. AND ABEL,R. B., J. CEEM.EDUC.,35.113 (1958). R. J., J. Marine Res., 17, 390 (13) PATE,J. B. AND ROBINSON, (1958). (14) REVELLE,R. AND FNRBRIDGE,R., Geol. Soe. of Amer. Mmoir 67, Val. 1, 239 (1957). (15) SAMUELSON, O., "Ion Exchange Separation in Analytical Chemistry," John Wiley & Sons, New York, 1963. (16) STRELOW, F. W. E., Anal. Chem., 32, 1185 (1960). M. W. AND FLEMING,R. H., (17) SVERDRUP, H. V., JOHNSON, "The Oceans," Prentice-Hall Inc., New York, 1942. (18) WELCEER,F. J., "The Analytical Uses of Ethylenediaminetetraseetic Acid," D. Van Nostrand Co. Inc., Princeton, 1957.
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