236
ANALYTICAL CHEMISTRY
should be covered with a watch glass to prevent loss of solution during the resulting effervescence. As soon as the effervescence subsides, the contents of the beaker are boiled for 20 to 30 minutes to expel all carbon dioxide from the solution. The excess acid is then titrated, using 0.5 S standard sodium hydroxide with phenolphthalein as indicator in the case of sodium and potassium salts. In the case of barium and calcium salts, it is necessary to use hydrochloric acid and not sulfuric acid, since the barium and calcium sulfates are insoluble and hinder reaction between the carbonate and the acid. In using hydrochloric acid, 50 ml. of standard 0.5 N acid and a few drops of methyl red indicator are added to the calcium and barium ignition residue. When the samples are completely dissolved, the excess acid is titrated with standard alkali till the solution is just alkaline, then standard hydrochloric acid is added by buret till just acid. This is to prevent the presence of too much hydrochloric acid which can be lost on boiling. With the contents of the beaker just acid, the mixture j s boiled for 20 to 30 minutes, and the excess acid is titrated with standard alkali to the methyl red end point. The solution must be acidic throughout the period of boiling. If it becomes alkaline as the boiling progresses, the methyl red becomes yellow; then more standard hydrochloric acid should be added. DISCUSSION
The ignition residue in the case of sodium and potassium salts can be dissolved in water and titrated with standard acid, but the residue is slow to dissolve. It is generally quicker to dissolve the ignition residue in excess acid, boil off the carbon dioxide, and titrate the excess acid. The barium and calcium carbonates are water-insoluble, so that the excess acid method is necessary in ,those cases. Sulfuric acid will work with calcium salts, even though the
.
calcium sulfate is insoluble. A period of reaction longer than 0.5 hour is needed, however. Barium salts cannot be determined with sulfuric acid; a coating of barium sulfate forms over the unreacted carbonate and prevents further reduction. Hydrochloric acid, however, gives no trouble with these salts. Methyl red indicator must be used in the case of barium and calcium salts because of the acidity of the barium and calcium chlorides. Free hydroxides or carbonates have t o be determined on an unignited sample t o correct the final result. Samples containing more than 10% sodium or potassium hydroxide should not be run by this method, since the free caustic attacks the platinum crucibles. Sulfates and chlorides do not interfere in the analysis. The simplicity of the method is the main factor contributing to the high reproducibility. Using a sample containing 0.01 equivalent of salt will make possible a reproducibility within +0.570. The simplicity enables a technician to run many determinations a t one time; the number of determinations is usually limited by the number of platinum crucibles available. One determination run alone seldom requires more than 2 hours; however, four determinations run simultaneously can be done in almost the same length of time as one. Barium and calcium salts usually require a longer period for ignition than do the sodium and potassium salts. LITERATURE CITED
(1) Hurd and Fiedler, IND. ENG.CHEM.,A x ~ LED., . 9, 116 (1937). (2) Palit, S., Ibid., 18,246-51 (1946). (3) “Scott’s Standard Methods of Chemical Analysis,” 5th ed., p. 2251, New York, D. Van Nostrand Co., 1939. RECEIVED July 10,1947.
Potentiometric Determination of lead LADISLAUS FARKAS
AND
NORBERT URI, Department of Physical Chemistry, The Hebrew University, Jerusalem
Determination of lead by potentiometric titration with alkali fluoridein the presence of alkali chloride (or bromide) is proposed. The lead ions are precipitated as lead chlorofluoride (or as lead bromofluoride) and the equivalence point is determined by a drop in the ferric-ferrous oxidation-reduction potential.
A
METHOD for the potentiometric determination of calcium by titration with fluoride has been described (3). In the present paper the principles of a similar method for the rapid and accurate determination of lead are given. When alkali fluoride is added to a saturated solution of lead chloride or bromide, lead chlorofluoride or lead bromofluoride is precipitated. Both lead chlorofluoride and lead bromofluoxide are practically insoluble in the presence of an excess of chloride ions or bromide ions; in fact, the precipitation of fluorides as lead chlorofluoride is the most reliable gravimetric method for their estimation. If an excess of sodium or potassium chloride is added to a solution containing lead ions, the lead is first partially precipitated as lead chloride; if, then, standard alkali fluoride solution is added with vigorous stirring, the precipitate and the lead chloride still in solution are quantitatively converted into lead chlorofluoride. The equivalence point in the titration, corresponding to the molar ratio Pb:F = 1:1, is marked by the drop in the oxidation-reduction potential of a ferric-ferrous platinum indicator electrode, since ferric ions form a stable complex with fluoride. Whereas in the determination of calcium (3) or of magnesium and aluminum (1) by a similar method, the titration has to be carried out in the presence of alcohol, the estimation of lead may be carried out in an aqueous solution. In 507, alcoholic solution the potential drop is greater than in aqueous solution, but reaching a constant potential after successive additions of titrant takes too long for most practical purposes. While in the precipitation of calcium fluoride, solubility cannot be decreased by a common ion in the proximity of the equivalence point, the’absence of this restriction is a deci-
sive advantage in the case of lead chlorofluoride, which contains a third ion that is not titrated. Lead ions can also be estimated by adding.an excess of an alkali bromide to the aqueous sample solution and titrating with fluoride. In general, however, the lead chlorofluoride method is to be preferred to the lead bromofluoride method, since because of the lower solubility of lead bromide, it takes longer for a constant potential to be established. EXPERIMENTAL
The potentiometric cell and the methods for standardizing the fluoride solutions are described in a previous paper (3). The lead content of lead nitrate solutions was determined gravimetrically as lead chromate and lead sulfate. In the gravimetric determinations the authors followed the procedure described by Treadwell ( 2 ) . The potentials were measured by means of a pH meter of the Cambridge Instrument Co., by which direct measurement of e.m.f. up to a potential difference of 1.4 volts can be carried out with an accuracy of + 1 mv. Preliminary experiments showed that identical results were obtained where sodium or potassium chloride was added to the sample solution, and whether sodium or potassium fluoride was used as titrant. To 100 ml. of the sample solution 0.04 gram of ferrous chloride containing a small quantity (0.8 mg.) of ferric chloride was added as indicator. The pH of the solution influences the oxidation-reduction potential considerably. The reason for this is that the complex FeF6--- is decomposed by strong acids, and therefore the determination cannot be carried out below pH 3. Generally in these experiments the pH of the sample
V O L U M E 20, N O . 3, M A R C H 1 9 4 8 I
237
1
1
!
Table I. Potentiometric Titration of 100 Ml. of 0.1 Lead Nitrate against 0.833 M Sodium Fluoride 1 Gram of KC1
SaF, nil.
2 Grams of KCl
Added (Molar Ratio C1:Pb = 1 . 3 : l ) A E / A V E , my.
0
10 5
11 0 11 3 11.7
0.2
330 0.3
300
2
4
6 18
328
4
325
150
110
290
200
260
12.06 12.10
15.5
12.3 12.5
130 72
13.0
42
14.0 1%
.
B
l
lI1. Y of Titrant 10
11 m
12
11
I?
Figure 1. Potentiometric Titration Cur5ees
+ +
+
225
300 213
225
275 204
217
175
186
120
169 145
160 98
114
51
72
96 45
m
100 m l . of 0.1 M P b (IuOa)s 1.5 grams of R-aCl ) titrated 100 m l . of 0.1 M Ph (&Oa)2 (50% ethyl alcohol NaF (A) solution) 1.5 grams of NaCl 4. 100 m l . of 0.1 M Ph (N0a)t 3 grams of KBr 2. 100 ml. of 0.1 M Ph (Nod2i1.5 grams of NaCl titrated against 1 M K F (B) Calculated 4E/laV m a x i m a a t : (1) 12.02 m l . ; (2) 10.0 m l . ; (3) 12.0 m l . ; (4) 12.05 m l . 1. 3.
239
350
235
250
200
298 2 58
275 245
250 12.02
320
250
253 200
11 98
8
326
200
175 11.94
327
324 8
316
130
11.90
15 Grain- of KCl Added ( l l o l a r Ratio CI:Pb = 2 O : l ) LE/ A V E , mv. 262 2.8 234
Added (Molar Ratio CI:Pb = 2 . 6 : l ) A E / A V E , mv.
332
10 0
,M
%%%
sdution a t the beginning of the titration was set a t between 4.0 and 4.2. As titrants were used (A) a 0.833 M sodium fluoride solution xith a pH of 7.25 and (B) a 1 -If potassium solution with a p H of 9.0. In Figure 1, potentiometric titrations are compared, with curves 1 and 2 representing titrants A and B, respectively. With' the more alkaline potassium fluoride solution the potential drop is steeper, but for practical purposes titrant A is more convenient, since the equilibrium potential is attained more rapidly. Curve 3 (Figure 1) shows the change of potential in (by volume) ethyl alcohol as solvent. I n this case, too, the advantage of a greater potential drop is counterbalanced by a sloLTer attainment of the equilibrium potential. Curve 4 shows the change of potential in the event that lead ions are precipitated as lead bromofluoride. Table I illustrateq the influence of the chloride concentration on the change of the potential (with 4 E I A V values) a t various molar ratios of chlorine to lead. At high chloride concentrations the potential course is also influenced by the formation of a labile chloro-complex with ferric ions. The authors found optimum n orking conditions with the addition of amounts of potassium or sodium chloride corresponding to a molar ratio of -2.5:Cl: 1 Pb. At higher chloride concentrations or a t a very small excess of chloride over the molar ratio C1:Pb = 1: 1 the potential change is less pronounced. The accuracy of the lead determination is uithin the limits of *0.5Vo (for concentrations of 0.05 to 0.1 JI lead). With 0.01 &%I lead nitrate solutions (titrated v,-ith 0.1 JI sodium fluoride) no greater accuracy than * l . 5 5 could be obtained. At lead concentrations lower than 0.01 JI the method i i not practicable for accuiate analytical n-ork. PROCEDURE
T o a dilute lead nitrate solution (0.05 to 0.1 M ) , add 1 gram of potassium chloride (or 0.75 gram of sodium chloride) for each gram of lead in solution, and 0.04 gram of ferrous chloride (con-
Table 11. Precision and Accuracy of Determination of Lead Test
so.
SaF Found,
Deviation, Id i
111.
d2
X 102
Sample 1, 100 hI1. of 0.1 'it. Pb(S03)z (Theoretical value, 12.00 ml. of 0.833.11 XaF) 1
+0.01 -0.01 -0.03 + o . 01 -0.01 +0.03
12.02
2
12.00
3 4 5 6
11.98 12.02 12.00 12,04
Av. = 12.01
Idla.. = 0.017
+o
100 100 900 100 100 900
d\/Td"/j =
2 0.0 -0.2 +0.2 0.0 t0.3
0.021
Sample 2 , 100 ml. of 0.0750 31 Pb(N0a)z (Theoretical value, 8.00 111. of 0.833 .If S a F ) -0 4 0 -0 +O -0 0
Av. = 9 . 0 1
02
01 01 03 03 00
; d ' a v . = 0.0017
4 00
100 100 900 900 0.00
-0 1 +o 2 +0.2 +0.4 -0.2
+o. 1
dVd?:j 0 , 0 2 2
5 Concentrations determined by sral-inietric analysis taken as theoretical standard.
taining 0.8 mg. of ferric chloride) for each 100 ml. of solution; then, while stirring vigorously, titrate with standard alkali fluoride. Lead chloride solutions and precipitates of lead chloride (or lead bromide) dispersed in water can be titrated directly against fluoride without addition of alkali chloride (or bromide) to the sample solution. The change of potential is measured by a suitable arrangement, and one should wait until constant potential is reached after each addition of titrant. Titrations are carried out a t room temperature and with vigorous stirring. The amount of fluoride added for which AE/ 4 V becomes a maximum is evaluated by graph or calculation. One ml. of a 1 M fluoride splution corresponds to 0.2072 gram of lead in the sample solutions. Table I1 shows the precision and the accuracy of the method under specified conditions. LITERATURE CITED
(1) T r e a d w e l l , W.D., 'feh.Chim. Acta, 13,500 (1930). ( 2 ) T r e a d w e l l , W. D., T a b e l l e n u n d Vorschriften s u r q u a n t i t a t i v e n A h a l y s e , "L e i p s i g , F r a n e D e u t i c k e , 1938. (3) U r i , X . , .%X.~L. CHEM.,19, 1 9 2 (1947).
RECEITED January 16,1947
