Hydrogen Chloride Partial Pressure of Dilute Hydrogen Chloride-Concentrated Lithium Chloride Aqueous Solutions Elio Scarano, Giovanni Gay, and Michele Forina Institute of Analytical Chemistry, Faculty of Pharmacy, University of Genoa, Italy
Solvent systems with low water activity (aHzo),such as aqueous concentrated solutions of salts or acids, containing small amounts of hydrogen chloride, show high hydrogen chloride partial pressure (Pncl). An apparatus and a procedure for PHClmeasurements in the range from 2 x to 2 m m Hg are described. A small quantity of hydrogen chloride is stripped from the solution in question by means of a known nitrogen volume, recovered, and determined potentiometrically. Because of the small quantity of hydrogen chloride required, measurements can be repeated several times. Most of the experimental data are relative to lithium chloride solutions, but some results in hydrochloric acid and in sulfuric acid solutions are also reported. Measurements are rapid (6-8 minutes) with PHLl 2 2 x lo-* m m Hg, and precise (relative standard deviation s’1%) with QHjO 5 0.5. A linear relation is observed between PHCl and hydrochloric acid concentration in lithium chloride and in sulfuric acid solutions. PHCl can provide information about the solvent system, information about acid-base equilibria in a solvent system, and a means for determination of very weak bases.
SOLVENT SYSTEMSwith low water activity (aH?o),such as aqueous concentrated solutions of salts o r acids, are arousing increasing interest. Strong acid solutions have been used for the measurement of base strength of very weak bases and for the interpretation of the kinetics of certain acid-catalyzed reactions through the acidity functions ( I ) . Salt solutions (LEI, NaBr, CaCle) have been used for polarographic investigations (2-4), in electrochemical (5) and nuclear (6) technologies and for the determination of weak bases (7,8). In this paper an apparatus and a procedure aLe described for the determination of the hydrogen chloride partial pressure (PHCI) (in the range from 2 X lo-‘ to 2 mm Hg) of solvent systems with low of120containing small amounts of hydrochlol give information about the solvent system; ric acid. P H Ccan information about acid-base equilibria, and a means for determination of very weak bases. Some P H C measurements ~ in hydrogen chloride-lithium chloride solutions have been previously made by Glietenberg and von Stackelberg (3) to investigate the effects of high lithium chloride concentrations on dissociation and volatility of hydrochloric acid. These authors passed a finely divided nitrogen stream for many hours through a lithium chloride solution (12 or 13M) containing some hydrochloric acid (4-8 X 10-3M). The stripped hydrogen chloride was determined as (1) M. A. Paul and F. A. Long. Chem. Rec., 57, l(1957). (2) T. S. Lee, in “Treatise on Analytical Chemistry,” I. M. Kolthoff
and P. J. Elving, Ed., Interscience, New York, N. Y., 1960, Part I, Vol. 1, p 246. (3) D. Glietenberg and M. von Stackelberg, Ber. Bunsenges. Phys. Chem., 72, 565 (1968). (4) N. K. Roberts and H. van der Woude, J. Cliem. Soc., Sec. A , 1968, 940. (5) K . Schwabe, Electrochim. Acta, 12, 61 (1967). (6) M. Lucas, Bull. Soc. Chim. Fr., 1966, 2767. (7) F. E. Critchfield and J. B. Johnson, ANAL.CHEM.,30, 1247 ( 1958). (8) Hisashi Kubota and D. A. Costanzo, ibid., 36, 2454 (1964). 206
0
a function of the nitrogen volume (up to 250 liters) from the decrease of the polarographic current of the hydrogen ion reduction. A final control was effected by collecting the stripped hydrogen chloride in 0.1M NaOH and then titrating. In our experiments a n accurately measured nitrogen volume ( VxZml) passed through a cell ( A ) containing the solution in question (Vs ml) and stripped a small quantity of hydrogen chloride. The nitrogen flow then passed through a cell ( B ) containing a n acidic KCl solution ( V K C ~ml) and the glasssaturated calomel electrodes system. From the starting and ending p H values of this solution, PHCl value was calculated. The B cell could be rapidly emptied, washed, and refilled with a new batch of the acidic KC1 solution for the successive measurement. Because of the relatively small quantities of the stripped HCl, the hydrochloric acid concentration in the tested solution (CRC1)remained practically unchanged and measurements could be repeated several times. Most of the experimental results are concerned with lithium chloride solutions. The experimental conditions relative to these solutions are reported in Table I. Some results in hydrochloric acid and in sulfuric acid concentrated solutions and the behavior of weak bases in dilute HC1-saturated LiCl solutions are also reported. EXPERIMENTAL
Industrial nitrogen from a cylinder was used (99.95 vol. nitrogen, not purified for the small carbon dioxide content). It was stabilized for its water content by bubbling through an aqueous saturated lithium chloride solution. The nitrogen stripping volume was measured with a soap bubble flowmeter ( V K=~ 5-100 ml), or with the device shown in Figure 1 (VN2 = 100-2000 ml). The nitrogen flow rate during the stripping was 0.1-1 ml/sec. It was not necessary to maintain it constant. O n the contrary, it could almost be stopped, approximating the levels. for an accurate estimate of VX?* C. Erba high purity KC1 and C. Erba reagent grade LiCl were used. The acidic potassium chloride stock solutions (1M KC1 and lO-4M HC1) were stored in polyethylene bottles. Before using the acidic potassium chloride solution, a nitrogen stream passed through it for about 30 minutes t o lower the carbon dioxide content. The resulting p H was 4.47-4.59. Saturated lithium chloride stock solutions were prepared by evaporating unsaturated solutions which had been previously filtered through sintered glass to eliminate insoluble materials. These stock solutions were stored in glass bottles with solid LiCl. Withdrawings of aliquots of these solutions were made at 25 O C after 2-4 hours agitation followed by a 2-4 hours settling of solid salt. For the 25 OC-saturated LiCl solution, the following values from the literature were used: density 1.296 g/cm3, molarity 13.93M (9), U H ~ O 0.11 (10). Derived values were: molality 19.747m, LiCl content 45.57x in weight. Our measurements gave: density 1.294 g/cm3, molality 19.80m. Density, water activity and molarity (9) D. Rosenthal and J. S. Dwyer, ANAL,CHEM., 35, 161 (1963). (10) L. Meites, “Handbook of Analytical Chemistry,” McGrawHill, New York, N. Y., 1963, pp 3-29.
ANALYTICAL CHEMISTRY, VOL. 43, NO. 2, FEBRUARY 1971
(MI CHCI(MI
CLicl
(cHCI/cLiCI) v s
x 100
(ml)
Table I. Experimental Conditions 11-13.93 5.5-11 0.0001-0.1 0.1-0.3 0.0007-1 1-5.5 75-100 75-110 5-50 5-100 2.5-10 3-5 0.05-0.5 0.1-0.3 7-700 30-300 lo-10,000 10,000-35 ,000 6-8 6-10
4.7-7 0.02-0.03 0.3-0.7 50-100 500-2,000 2.5-5 0.05-0.3 7-100 1,000-2,500 30-60
(d) (ml) ApHa (pH unit) Stripped HC1 (nanomole) Total HCl (fimole) Timeb(min) Starting pH = 4.47-4.59. * Time required for the entire cycle of a PHCIdetermination (Le., emptying, washing, and refilling of the B cell; stripping of HCI; pH measurements before and after the stripping). vN2 VKCl
KC1 s o l n . I
t
Figure 2.
Diagram of apparatus
(a) A cell; (b) B cell; (c) flowmeter; (d) stopcock S ; ( e ) and cf) stopcocks; (g) water pump
values, as a function of molality for the lithium chloride solutions in the whole concentration range investigated, were derived from the data of the literature (9-//), by means of a fourth-order polynomial which fitted the data within 0.1 %. Beckman General Purpose glass electrodes (Sinch), fiber type saturated calomel electrodes (SCE) (2j/, inch), 4.01 and 6.86 p H buffer solutions were used. p H measurements were
made with a Beckman 1019 Research pH-meter (readability 0.0005 p H unit). Calculations were made with an IME Sales Corporation System computer. Figure 2 shows the diagram of the apparatus. Figure 3 shows the cells. A buret with the closure on the tip (12) (1- or 5-ml capacity) was used to introduce a standard hydrochloric acid aqueous solution in the lithium chloride solution. The buret was fitted t o the A cell with a ground glass joint and its tip dipped into the solution. The 5-ml buret was equipped with a solution reservoir (50-ml capacity) for its refilling during the experiment. Solutions to be investigated were prepared directly in the dry A cell by weighing. A top loading Mettler P 1200 Balance (1200 g capacity, 0.01 g sensitivity) was used. In the case of a saturated lithium chloride solution, about 130 g and 1.2 g of solid LiCl were weighed. In the case of unsaturated lithium chloride solutions, water and saturated lithium chloride solution were weighed in varying amounts. In the A cell, the solution was magnetically stirred t o obtain an intimate contact between gaseous and liquid phases. The B cell was connected by means of tygon tubing t o (a) the flowmeter, (b) a water pump for emptying of the cell, (c) an all glass 10-ml buret with reservoir (1-liter capacity) containing the acidic potassium chloride solution. The buret was protected from the atmospheric carbon dioxide by means of a soda-lime trap.
(11) D. Rosenthal and J. S. Dwyer, Can.J. Chem., 41, 80 (1963).
(12) E. Scarano and M. Forina, J . Chem. Educ., 47,482 (1970).
Figure 1. Flowmeter (a) nitrogen inlet; (b) hole for refilling reservoir; (c) and stopcocks; (d) reservoir with water; ( e ) water manometer; (g) calibrated flask Nitrogen enters in d and water drops in g, with a low underpressure in d. When g is filled, f is
cf)
closed and nitrogen is allowed to flow in d until underpressureis nullified
ANALYTICAL CHEMISTRY, VOL. 43, NO. 2, FEBRUARY 1971
207
ture T OK inside the flowmeter; P = atmospheric pressure; AP = bubbling overpressure in the A cell due to the gas bubbling in the B cell; PHso= water partial pressure at T OK; P H l o A = water partial pressure of lithium chloride solution in the A cell at 25 "C(all pressures in mm Hg). Equation 1 was obtained as follows. From the perfect gas law applied to HCI at 25 O C :
h
PHCl = nHCl
R 298 V ~
where V is the volume of the gas leaving the A cell during the stripping. From the gaseous partial pressure law:
+
+ P H Cin~ the A cell at 298
P A = P . v ~ PH,O*
+
P = PXZF
Figure 3.
Cells
(a) A cell; (b) B cell; (c) stopcock S; ( d ) SCE compartment (filled with the acidic potassium chloride solution, with a sintered glass disk at the bottom); (e) glass electrode; (f) nitrogen outlet (to flowmeter or to waste); (g) inlet for the acidic potassium chloride solution; (If) sockets with O-rings, making a gas-tight junction; (i) solution outlet (to water pump)
A nitrogen flow (2-4 mlkec) dispersed by a sintered glass disk, was passed through the solution in the reservoir for 30 minutes before the experiment. Then nitrogen continued t o pass over the solution. Experiments were carried out with the A and B cells in a thermostatic bath at 25 i 0.1 OC. The glass-SCE electrodes system was standardized in the B cell before and after each experiment. p H standard solutions were introduced through the solution inlet g (Figure 3). The variation of standardization was mostly negligible. With the stopcock S (Figure 2) connecting A and B cells (cia 4-3), with no nitrogen flow, a volume (VHrl) of the standard hydrochloric acid aqueous solution was introduced in the A cell. Immediately afterward, the A cell was isolated by turning the stopcock S, and a 0.4-0.6 mlkec nitrogen flow was passed through the B cell cia 1-3. This nitrogen flow rate was successively maintained for the whole time of the experiment, except during the hydrogen chloride stripping. The B cell was washed with batches of the acidic potassium chloride solution delivered from the reservoir, then filled with the volume VJtcl. The solution reached thermal equilibrium and p H stabilized in 2-4 minutes within the pH-meter readability. Then by means of the stopcock S,the volume VN?was allowed to pass through the two cells cia 1-2-4-3 (gas flow rate 0.1-1 mlkec). p H lowered and stabilized in 1-2 minutes. Sometimes p H variation continued for a longer time. This tailing in p H variation was always observed when the solution in the A cell possessed high water activity such as in lithium chloride solutions with lithium chloride concentration (CL,CI) less than 10 m. In these cases in order to reduce waste of time after stripping, the solution in the B cell could be made to rise up t o the stopcock S, thus washing the inside of the glass tube connecting the two cells. After a PHCl determination, the B cell was emptied, washed (1-2 times) with a few milliliters of the acidic potassium chloride solution. It was then filled with the volume VKCIfor a new determination. Determination of Plrcl from pH Measurements. PHCIat 25 "C was calculated by means of the following equation:
where nlrrl = stripped HCl(nanomo1e); Vs, = water saturated stripping nitrogen volume (ml) measured at the tempera208
PHI0
in the flowmeter at T OK
O K
(3) (4)
where P A = total pressure in the A cell; PN2A and PN,F = partial pressure of nitrogen in the A cell and in the flowmeter. Besides : PA = P
+ AP
(5)
AP is a constant for a given apparatus, procedure, and VKcl.
It can be accurately measured with a water manometer connected with the A cell (in our case AP was 4.8 mm Hg). From the perfect gas law applied to N1:
(6) From Equations 3, 4, 5 , and 6: 298
P
- Pq"0
Finally, neglecting Ptrrl in Equation 7, from Equations 2 and 7, Equation 1 results. nHcI was obtained (from V K and ~ ~the p H values of the acidic potassium chloride solution before and after the stripping) by means of the least-square fifth-order calibration polynomial, giving p H as a function of the added HCl in nanomole/ml in the pH range from 3.9 to 4.6. The polynomial was obtained from 176 points of 16 experiments carried out with 200-rnl portions of two acidic potassium chloride stock solutions in experimental conditions close to those described for PlrcIdeterminations. Hydrochloric acid was added as lo-' or 2 X 10-2M HC1 (up to 1 ml) by means of a buret with the closure on the tip, after removal of the carbon dioxide from the acidic potassium chloride solution. The polynomial fitted the experimental data with a relative standard deviation (RSD) less than 0.75 %, or with a standard deviation (SD) less than 0.3 nanomoleiml (whichever greater). The choice of the 3.9-4.6 p H interval was a compromise between maximum sensitivity (achievable at higher p H values, in proximity of 7) and interferences due to carbon dioxide and to the unknown base contained in the potassium chloride. (The p H of a n 1 M KCI solution appeared alkaline with the removal of carbon dioxide, and slowly shifted toward higher values). The carbon dioxide content of the acidic potassium chloride solution was maintained at low values. High values caused pH instability due to carbon dioxide loss. Lower values were difficult to maintain and time-consuming to obtain. The stability of the decarbonated acidic potassium chloride solution was proved by the fact that its pH was able to remain constant within 0.002 p H unit for at least 30 minutes in the p H range from 3.9 to 4.6, in the B cell, with the 0.4-0.6 mlisec nitrogen flow. Depending on the stock of KC1 salt and on the acidic potassium chloride stock solution, as well as the age of the solution, the p H of the decarbonated acidic potassium chloride s o h tion could vary in the range from 4.47 to 4.59.
ANALYTICAL CHEMISTRY, VOL. 43, NO. 2, FEBRUARY 1971
Equilibrium between Gas and Liquid Phases. Experimental PHCl values can only have significance if the flowing gas phase comes into equilibrium with the liquid one. Glietenberg and von Stackelberg (3) quintupled the nitrogen flow after 1-2 days from the beginning of the experiment and saw that the CIIcIdecrease was only a N*-volume function. From this occurrence they inferred that the nitrogen flow was saturated by hydrogen chloride at the corresponding equilibrium partial pressure. Besides the limits indicated above, for the same purpose we have carried a number of PHc1 determinations with the saturated lithium chloride solution in more stringent conditions: stripping nitrogen flow rate from 0.05 t o 5 mllsec; magnetic agitation in the A cell from zero t o the maximum value; time interval between two successive determinations from zero to 24 hours; V s , up to 200 ml. A few similar experiments were carried out also with unsaturated lithium chloride solutions. PHClvalues obtained under variable conditions agreed, thus proving the achievement of equilibrium.
Table 11. Precision in PHCl Determinations Lithium chloride saturated solution 135.52 grams: solid I.iC1 1.3 g, VRCI = 5 ml
1M
1 7 ~ ~ 1 ,
DH
HCI, ml 0.30
Vh%
ml 20
Starting 4,442 4.440 4.444 4.444
0.60
10
1.00
5
nano-
PHCI,
Ending 4.054 4.051 4.054 4.054
mole 285.9 288.3 286.9 286.9
4.448 4.443 4.443 4.446
4.047 4.042 4.044 4,045
296.5 299.3 297.6 297.8
4.444 4.442 4.442 4.442
4.090 4.087 4.093 4.089
248.7 250.9 244 8 248.8
mm Hg 0.2708 0.2731 0.2717 0.2717 M = 0.2718 SD = O.OOO9 R S D = 0.33% 0,5636 0.5688 0.5656 0.5661 M = 0.5660 S D = 0.002 RSD = 0.35% 0.9487 0.9569 0.9336 0.9490 M = 0.9470 SD = 0.010 RSD = 1.0%
RESULTS AND DISCUSSION Feasibility of PHClDeterminations. Precision and time
determination appear strictly related t o required for a PHCI two factors: the presence of solid lithium chloride in the A cell and the water activity of the solution. The first situation occurred in experiments with the saturated lithium chloride solution. Some solid lithium chloride was added in this case to form a saturated solution with the water of the standard hydrochloric acid solution. This resulted in a lengthening of the experimental time due to the slowness of lithium chloride dissolution. Consequently, after HC1 was added, PE1c1increased for 1% hours, then stabilized. Instead equilibrium was soon reached with unsaturated lithium chloride solutions. The water activity in lithium chloride solutions (as well as in the other solutions studied) diminishes with concentration. At low U H ~ Ovalues ( 5 0.4), PHCIdeterminations are rapid, precise, easy to carry out. At medium or high uH20values, a
number of inconveniences arise : tailing in p H variation, lowering of precision, loss of time, drift in values relative to the same conditions. All these inconveniences should be attributed to the presence of a second liquid phase produced by splashes, fogs, and adsorption on the wall of the A cell and of the glass tube connecting the A and B cells. Because of its immobility, this second liquid phase should come into equilibrium with the gas phase very slowly. Precision in PHCI measurements is reported in Table I1 for
209
~~
Y
Relation between PHCIand CHCIat Different C L i C l Values
Table 111.
M
-- 2
CHCi x io3 (M) 1.00 2.00 2.99 3.98 4.97 5.96 6.95 7.94
0
K
=
24.4(SD
=
0
1.00 2.00 2.99 3.98 4.97
-1
K
=
-2
IO
20
15
C'tC1
Figure 5. PHCI/(CHCI - Co)as a function of
' CLiCl
the LiCl saturated solution. The same R S D figures held for CL,CI2 10 m ( U H ; O ,< 0.4). R S D in PHCI determination increased at lower C L ~ C and I reached 5 - 1 0 z about at C L ~ C =I 5-6 m. PHCI as a Function of CEcl. The relation between PHCI and CHCIin the lithium chloride saturated solution is shown in Figure 4. In the CHCl to 1OP2M,the relation range from was linear
P A C=~K(CHCI- Co)
(8)
47.1 (SD
(13) E. Sqarano, M. Mascini and G. Gay, ANAL.CHEM., in press. 210
0
(mm Hg) 0 0.0209 0,0458 0.0701 0.0936 0.1185 0.1413 0.1645 0.1923
=
0
0,0207 0.0451 0.0683 0.0904 0.1135 0.1341 0.1548 0.1794
7 X 10-6M);
0.4); Co
13 X 10-'M);
= 2.09 X 10-4M (SD PHCI RSD = 0.90 % 19.75 (satd) 0
=
0
0.0367 0.0840 0.1324 0.1779 0.2239
2.86 5.69 9.44 K = 102.6(SD = 0.4); Co
0.2718 0.5660 0.9471 = 1.96 X 10e4M(SD = 10 X lo-"); PHCI RSD = 0.37 a The small CL,CI diminution is due to the dilution caused by the water added with the HC1 standard solution. b Values corrected for C L ~ C variation, I referred to the starting
(
;E )
CIXI value. ( P H c I = ) ~( ~ P ~H~c I ) ~1~+~-~. m , where K and
dK/dm were approximate values obtained from the curve in Figure 5 and related data. This approximation introduced absolute errors in the order of mm Hg. Table IV. Relation between PHCI/(CHCI - C,) and CLCI LiCl saturated solution 91.08 grams, with no addition of solid LiCl PHCd (CHCI -
0.5M HC1
The meaning of the intercept C Ois the presence of basic impurity in lithium chloride (13). Its value depended o n the lithium chloride stock solution. A typical value was 2 X 10- 4M. Points above the straight line at CRCIvalues CL,cl/lOO, a definite deviation from Equation 8 was observed. With the increasing of CHCI,the value of PHCI/(CHCI - Co) increased. This must be attributed to the effect of hydrochloric acid on the water activity, on the chloride ion activity, and o n the hydrogen ion activity coefficient in the solution. K as a Function of CLIC1.K values [K = PHCI/(CHCI C,)] with CHCI< CLicl/lOO were obtained from experiments carried out for checking Equation 8, as well as with experi-
(PHC1)oorr'
(mm Hg)
0.2); Co = 1.37 X 10-4M(SD = PHCI RSD = 0.84 % 17.30 0 17.27 0.0363 17.25 0.0825 17.22 0.1289 17.20 0.1717 17.17 0.2142
0 5
(PHC1)meas
(m)
15.60 15.58 15.56 15,54 15.52 15.50 15.48 15.46 15.43
0 1
CLIClQ
(ml) ... 5
15 20 25 30
CLiCl
(CHCI
- CO)
(m)
(M )
19.75 17,98 15.24 14.16 13.22 12.40
,..
0.0329 0.0875 0.1102 0.1305 0.1489
CO)
PHCl
(mm
(mmHg)
Hg/W
1.9'16 1.767 1.302 0.9345 0.6753
... 58.2 20.2 11.8 7.16 4.54
ments allowing investigation of a wide range of C L , ~as ] , that reported in Table IV. Data from the two sources agreed. Figure 5 shows log K OS. CL,CI. Roughly, log K was a linear function of log C L i C l (molality) with a slope of about 7.4 in the C L ~interval C~ between 5 and 19.75 m. The least-square precision-weighed fifth-order polynomial log K
=
+ 3.299134 m - 0.4412603 m 2 + 0,0326595 m 3 - 0.001208848 m 4 +
-11.2557
0,000017489 m 5 (9) fitted the experimental data within 3 % of K at CLICI > 10 m. At lower C L , ~maximum ), deviation between experimental and calculated K values increased u p to about 20z at CLICI= 5-6 m.
ANALYTICAL CHEMISTRY, VOL. 43, NO. 2, FEBRUARY 1971
With C H C l I C L i C I 5 5 % ( M / M ) ,Equation 9 still held satisfactorily by substituting C L ~with C I chloride ion concentration (molality). Other Results. Known amounts (200-300 pmole) of weak bases and excess amounts of hydrochloric acid (500-700 pmole) were added to N100-ml portions of saturated lithium chloride solution which contained some solid LiCI. From the experimental PHclvalues, the degree of protonation (per cent of the base which reacted with protons to give the conjugate acid) was calculated. I n the case of 4-bromo aniline, glycine, 4-nitro aniline (pK, 3.86, 2.35, and 1.0, respectively) the degree of protonation was 100%; in the case of urea (pK, 0.1) about 30%. P H C l measurements were carried out also in concentrated (10-20 rn) sulfuric acid solutions containing small amounts of hydrochloric acid, and in concentrated (6-7M) hydrochloric acid solutions. I n sulfuric acid solutions, Relation 8 held with Conegative (acid volatile impurities) and R S D E 1 %. CONCLUSIONS
determination, validity of Satisfactory precision in values in a wide range Relation 8, and self-consistence of PHCl of experimental conditions, give reliability to the described apparatus and procedure. Paclcan be defined as a measure of the ability of a solution to give undissociated molecules of hydrogen chloride. It is a function of water activity and of the activities of hydrogen ion ( a ~ +and ) chloride ion (ac1-). HCl,,,
e HCI.,I
for gas-solution equilibrium
(10)
where a H C I is the activity of the undissociated HCl in the solution.
HCI,,l
+ n H20 e H+ + CI- in the solution,
(11)
with Hfand C1- hydrate ions, and
Then
PHcl can provide information concerning a solvent system. I n lithium chloride solutions with CL,CIconstant and with C H C L
