Estimation of Carbonate Content of Base by Gran's Method A Semimicroscale Experiment Antti V e d a University of Turku, SF-20500 Turku, Finland Gran's method is a useful tool in linearizing potentiometric titrationcurves ( 1 4 ) . The early method ( 1 ) uses simply AVIAE relation acauired from observed deltas while the second (2) is more &idely useful applying mmolar function V[H+l. The method has been overlooked earlier by textbooks in analytical chemistry (3). However, some newer textbooks introduce this method (5, 61, which is important for many reasons. Recently the applications have been vastly increased due to automatization of titration processes (7).Gran's plot is also useful in the calibration of the glass electrode needed by many computer programs in calculation of equilibrium constants (e.g., 8). Although the method is mostly applied in determinations of strong acids and bases, it can be applied in the estimation of the pK. values of weak acids or bases as well (6).Since being introduced (1,2), it has been evident that the determination of the carbonate content always present in, e.g., the titrant base can be carried out with the Gran's plot. In the present paper an application of the early method by Gran (I)is reported. Here carbonate content of a base is determined by titration with hydrochloric acid using small scale samples and additions. A better accuracy can be achieved by applying the method of standard additions, i.e., the titration is repeated three or more times by adding known masses of analytical grade sodium carbonate. Experimental A thermostated titration vessel with a total volume of 5 mL purchased by Metrohm was used in the experimental work. The vessel was equipped with a cover through which the nitroeen gas, the electrode and the titrant could be transferred. '?he. temperature was held constant within f0.5 K by a Thermomix thermostat. All titrations werecarried out under nitrogen atmosphere. Water vapor was added to the gas, and the gas was led through a carbosorb tube. The electrode was a Metrohm combined glass silversilver chloride electrode and the potential was followed by a simple 4 112 digits potentiometer. An Agla micrometer syringe was used as a buret. By this it is possible to make
additions of the titrant of a magnitude of one microliter. Instead of this equipment a commercial mimpipet (e.g., Finnpipet) with a volume range of 5 4 0 fi can be used as well. The titration mixture was agitated with a magnetic stirrer. The titration procedure was as follows. A 5-mL portion of distilled water was transferred to the titration vessel and nitrogen gas was bubbled through the liquid for 10 min. ARer that the stream of gas was led above the liquid and a 200-kL sample of the base solution to be investieated was introduced and the potential recorded. The concekration of the titrant in this experiment had to be of a mamitude of 1mol L-'. Its more precise concentration doesnot need to be known when relative amount of carbonate is to be determined. The titrant was added in even intervals of volumes (e.g., 5 pL)to the solution and the potential was recorded. The titration was continued approximately 50% over the sum of hydroxide and carbonate neutralized. In addition, three other titrations were carried out with variable content of sodiumcarbonate (4.4.8.0, and 12.1%ofthe total amount of the base). Results As described in the very first paper of the G r a d method (I)the values of AVIAE (in units of pL V-') are extracted from the data and plotted against the consumption of the titrant. Two such plots are presented as examples in Figures 1(a) and (b). The asymptotes of the Gran's plot that describe the portions of strong base (OH-) and acid (H303 have been drawn on the basis of linear remession. The end points of the titrations, i.e., the points of ikersection of the asymptotes and the abscissa have been calculated with the aid oiregression equations froni the separate plots. These points and their differences due to the various amounts of the added carbonate have been reported in the table. With the aid of these values the carbonate content of the sample NaOH can be estimated. This is presented in Figure 2. If linear regression is again applied and the point ofintersection determined the value of 2.5 is obtained. This means
bV/nE
Figure 1. Gran plot of AWAEvs. V(a)without added carbonate, (b) with 12.1 mole-%of added carbonate. Volume 69 Number 7 July 1992
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The Evaluated End Points of the Titrations and Their Differences as a Function of Mole Percent of Added Carbonate NazC03
(Mole-%)
OK end pt. (pL)
Total base end pt.(pL)
Diff.( pL)
that approximately 2.5% of the total base is in the form of carbonate. Normally commercial, so-called, carbonate-free ampoules contain from 0.5-1.5% carbonate (9). The present samples have been diluted from this kind of concentrate. Being stored for a few weeks, the high carbonate value seems reasonable in our moderately concentrated (0.5M) solution. Besides the linearization of titration data this experiment demonstrates skills to work in semimicro scaleand the applicability of the method of standard additions, as well. Literature Cited 1.Gran, G . h Chem. S a d . 18W,4,559477. 2. Gran, G.Analyat 1952,77,661-671. 3. Rosaot" t? J. C.: Ropsottl, H. J. Chem. Edue. 1885.42,W W 7 8
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Carbonate contentlmol %
Figure 2. Differencesof the two end points in the table plotted against the mole-%of added carbonate. 4. Serjeant, E. PPoUntiomtrymdPotantiomtrir ntmtions;John Wiley & Sons: New York, 1984;pp 22&229: 465-469. 5. Christian, G. D . A w l y t k 1 Chmlstry, 4th ed: John Wiley & 9om:Hang Kong. 1988:
.n~. 31W21. 6. Hani8.D. C QuanfifafiwC k m k l A m l y s i s , 2nded; W H.Reeman & Co:NewYork, 1981:pp 243-244. 7.Unper. L.R.Int. LobomtoryNe~18.1980.2.6. 8.Dana, P.;S a b a ~ , AVaccq : A J.Chem. Soc. Dalton h. 1885,1195. 9.Maltell, A. E.;Motektls, R. J . The Determinotion ond Use of SfobilifyC m f f n f s ; VCH Publiahera:New York. 1988;p 50.
