The apparatus described was a prototype and certain modifications are now being made to give still greater stability and sensitivity. These include replacement of the two-stage cylinder regulators by pressure regulators, 0-200 psi output (Model 8601, Brooks). A model 10-460 flow-through katharometer (Gow-Mac), having a lower internal volume, has been shown to give a n increase in sensitivity and an improved base line at the same filament current. It is hoped that these improvements will enable surface areas of 0.02 mz/g to be measured. At present the total time required to determine a surface area is three and a half hours of which one and a half hours
is taken up by degassing the sample. For the examination of large numbers of samples, it should be possible to operate a number of sample tubes in series, as suggested by Daeschner and Stross (2). This would allow simultaneous degassing, and following the establishment of a given relative pressure of adsorbate gas, a rapid adsorption/desorption could be performed on each sample in succession.
RECEIVED for review February 1, 1971. Accepted April 29, 1971. Crown Copyright. Reproduced by the permission of the Controller of Her Majesty's Stationery Office.
Hydrochloric Acid and Ammonia Gaseous Standard Solutions Elio Scarano, Michele Forina, and Giovanni Gay Institute of Analytical Chemistry, Faculty of Pharmacy, University of Genoa, Italy RECENTLY it has been shown ( I , 2) that concentrated lithium chloride or sulfuric acid aqueous solutions, containing small amounts of hydrochloric acid, possess a hydrochloric acid higher than that of hydrochloric acid partial pressure (PHCI) aqueous solutions at the same concentrations; a nitrogen stream, passing through these solutions in a wide range of flow rates, becomes saturated by hydrochloric acid at the corresponding equilibrium partial pressure, thus allowing the determination of the P,CI from the amount of hydrochloric acid contained in a given volume of gaseous mixture; the very low water content, in both the liquid and the gaseous phases, reduces or nullifies the condensation of water in tubing and the solubilization in this water of the hydrochloric acid of the gaseous phase (these phenomena take place when the water content is high, giving anomalous results in the P H Cdeterminations). ~ These results allow the possibility of preparation and use of gaseous standard solutions. A gaseous standard solution (GSS) can be defined as a mixture of known and constant composition of a gaseous reagent and an inert gas (really, also small amounts of aqueous vapor are present). The concentration of the gaseous reagent is expressed in this paper in molarity, i t . , in moles of reagent per liter of gaseous mixture at given temperature and pressure. A GSS is prepared at the time of use, allowing a stream of the inert gas to flow through a suitable liquid solution, the mother solution, where the reagent is dissolved. Then the GSS passes through the sample solution, leaving the reagent in it. Finally, the inert gas volume is measured. In this paper we discuss hydrochloric acid and ammonia gaseous standard solutions (HC1-GSS and "3-GSS). EXPERIMENTAL
Mother solutions were prepared by gravimetric and volumetric procedures in amounts of about 100 ml for each series of experiments. Titrations were carried out potentiometrically with the glass-saturated calomel electrode system, with 5-ml portions of standard Na2B40ior HC1 aqueous solutions. (1) E. Scarano, G. Gay, and M. Forina, ANAL.CHEM.,43, 206 ( 197 1). (2) E. Scarano, M. Mascini, and G. Gay, ibid., p 442. 1310
The apparatus and the procedure have been already described ( I ) . The mother solution was kept in a cell (the A cell) connected by means of glass tubing to another cell (the B cell), containing the Na2B40ior the HC1 standard solution (these latter solutions substituted for the acidic 1 M KC1 solution described in reference I). A nitrogen stream passed continuously through the B cell. Prefixed nitrogen volumes (accurately measured with a soap bubble flowmeter downstream) could also be passed through the A cell, thus transferring known amounts of the gaseous reagent (hydrochloric acid or ammonia) in the B cell. These amounts of reagent were small compared to the reagent content of the mother solution. The composition of the mother solution and that of the GSS were practically unaffected, even after many titrations, provided that the following condition was fulfilled : 4
- n 100 < E
Q
where q = moles of reagent consumed for each titration; Q = moles of reagent in the mother solution; n = number of titrations; E = per cent error in titrations. The concentration C of the GSS was calculated by means of the following formulas (I):
(3)
P,, = C R T,,
(4)
where meq = milliequivalent of the standard substance (Na2B40ior HC1); V = volume (ml) of the GSS delivered from the A cell; V N 2= water saturated stripping nitrogen volume (ml) measured downstream, at the temperature T,; Tms= temperature ( O K ) of the mother solution; T, = room temperature ("K); P = atmospheric pressure; P H ~ O= water partial pressure, at T,; AP = bubbling overpressure in the A cell, due to the gas bubbling in the B cell; Psx0 = water partial pressure of the mother solution, at Tm8;Pgr = gaseous reagent partial pressure of the mother solution (all pressures in mm Hg); R = gas constant. When C > lO-4M (Pgr>, 2 mm Hg), C was calculated by means of Equations 5 and 6, obtained from Equations 2, 3, and 4:
ANALYTICAL CHEMISTRY, VOL. 43, NO. 10,AUGUST 1971
Table I. Results of Titrations with HCl-GSS
Composition of mother solution, mole 0.91 HzO 3.41 HC1 1.00 X 10-1 HzSO4 0.87 HIO 3.71 HC1 4.00 x HzSOi 0.91 H20 3.38 HCl 1.00 X HzSO4 0.74 Hz0 3.82 HCl 1.00 x
b
T,. = 298.15 "K Number of HC1-GSS titraequivalent tions volume, ml 4 27.27
Series of experiments 1
Na2B40,standard solution, N 1.OOo x 10-2
2 3
0.850 x 1.002 x 10-3
4 14
23.17 21.17
4a 5
1.002 x 10-3 1.002 x 10-4
4 13
21.19 41.49
6 7
0.850 X lo-' 1.002 x 10-4
6 4
8 96
0.850 X lo-' 0.850 X
4 4
1.834 x 10-3
Re1 std dev, 7i 0.01
1.834 x 10-3 2.367 X lo-'
0.12 0.39
4.40 0.223
2.364 X lo-' 1.201 x 10-5
0.45 1.50
35.22 401.7
0.224 0.0232
1.207 x 10-5 1.247 x 10-6
0.32 1.09
340.3 340.2
0.0232 0.0232
1.249 x 10-6 1.249 x 10-6
0.57 1.01
35.73
C (M) 1.954 x 10-3
Re1 std dev, 7 3 0.38
PHCI, mm Hg 34.12
C(M)
34.12 4.40
After 36 hours. After 24 hours.
Table 11. Results of Titrations with NH8-GSS
Composition of mother solution, mole LiCl 0.73 Hz0 3.76 NH3 6.30 X lo-' LiCl 0.96 Hz0 4.38 NH3 9.99 X lo-' LiCl 1.01 HzO 4.34 NH3 1.30 X lo-* a After 120 hours.
T,, = 293.05 OK Number of NHI-GSS titraequivalent tions volume, ml 4 25.59
Series of experiments 1
HC1 standard solution, N 1.000 x 10-2
2 3
0.850 x 1.000 x 10-3
4 4
21.83 33.97
35.58 2.69
1.946 x 1,472 x 10-4
0.38 0.07
4 5"
0.850 X 0.850 x
4 4
28.75 28.73
2.70 2.70
1.478 x 1.479 x 10-4
0.11 0.04
6
1.000 x 10-4
4
32.89
0.278
1.520 x 10-5
0.57
7
0.850 X loe4
4
27.96
0.278
1.520 x 10-5
1.14
PxHs,mm Hg
Table 111. Titration with HCl-GSS Titrated solution = 5 ml of 0.850 X 10-4M NazBaOi standard solution; meq = 4.25 X 10-4; T,, = 298.15 OK; T , = 295.15 OK; P = 762 mm Hg; P H ~=O 19.8 mm Hg; AP = 4.8 mm Hg ( I ) ; P&, = 4.5 mm Hg (3) HCI-GSS
added, ml 30
PH 5.760
A PH
A 2 pH
5.680 5.538
0.096 0.238
36
5,300
38
5.072
V x 2 = 34
-0.010
0.228
0.096 +2X m a= 35.81 ml;
C = 1.204 X 10-5M; P H C=~ 0.224 mm Hg;
10-5~.
+ AP - P&o)
(7)
RESULTS AND DISCUSSION 0.142
34
C = A (P
with a n error less than 0.3 %.
0.080 32
When C < 10-4M (Pgr 5 2 mm Hg), P,, was neglected and C was calculated by means of Equation 7:
V
C*
= =
35.31 ml; 1.187 x
Results are reported in Tables I and 11. Examples of titrations are shown in Tables I11 and IV. Tables I and I1 show the range of concentration of the GSS's; the precision [expressed as relative standard deviation (re1 std dev)] and the accuracy of the titrations; and the stability of the mother solutions. The rather wide range of precision was attributed to variable skill in performing titrations. The best results were obtained after sufficient experience in using the soap bubble flowmeter was achieved. Accuracy was determined by matching the values of C obtained with two different liquid standard solutions. Accuracy was very high in each case. The stability of the mother solutions was excellent, providing (3) N. A. Lange, "Handbook of Chemistry," McGraw-Hill, New York, N. Y., 1967, p 1435. (4) D. Rosenthal and J. S . Dwyer, Cm7. J . Chem., 41, 80 (1963).
ANALYTICAL CHEMISTRY, VOL. 43,
NO. 10, AUGUST 1971
1311
Table IV. Titration with “3-GSS Titrated solution = 5 ml of 1.OOO X 10-3MHClstandard solution; meq = 5 x 10-3; T,, = 293.05 “K; Tr = 292.15 “K; P = 765 mm Hg; PHlo = 16.5 mm Hg; AP = 4.8 mm Hg (I); = 5.3 mm Hg ( 4 ) NHrGSS added, ml
33
PH 4.485
34
5.195
35
7.458
A PH
A z pH
0.710 2.263 0.424
1.553 -1.839
36
7.882 1.553 - 34.46 ml; V = 33.97 ml; C vNz = 34 I1.553 1.839 1.472 x lO-4M; P N H=~ 2.69 mm Hg; C* = 1.451 X 10-4M
+
=
that the A cell was accurately closed when not in use, and avoiding large changes of temperature, which can cause water evaporation and water condensation on the wall of the cell. A difference was seen between the two kinds of titrations, regarding the stability of the pH near the equivalence point. In titrations with HCl-GSS, after each hydrochloric acid addition, the pH shifted toward acidic values, stopped, then came back slowly to higher values (in titrations the following procedure was followed: the pH value at the inversion point was read and immediately after another portion of HC1-GSS was added). With NHo-GSS, no pH shift was observed. The shift of pH toward higher values was attributed to the presence and the elimination of the carbon dioxide in the Na2B407solutions. The absence of the pH shift in titrations with NHa-GSS was attributed to a very low content of carbon dioxide in the NHa-GSS’s. To confirm that, an experiment was carried out. An amount of a “8-GSS, corresponding to 0.01 mole of ammonia, was bubbled through a 10-ml portion of a clear barium chloride solution. The observed turbidity was somewhat less than that obtained when a 10-ml portion of a l M N H 3 solution was added to the same quantity of the barium chloride solution.
1312
e
Some experiments have been carried out also with mother solutions having high water content. Results were encouraging, even if not so good as those obtained with mother solutions with low water concentration. CONCLUSIONS
High precision and accuracy in titrations, stability of mother solutions, easy preparation and use give reliability to the HCl-GSS’s and to the NHa-GSS’s. These GSS’s offer two major advantages over HC1 and NH3 liquid standard solutions. Their use doesn’t imply any addition of solvent to the sample solution and very high dilutions are achievable (useful in ultramicroanalysis). Preparation of mother solutions in a wide range of concentration is a simple and inexpensive matter. A mother solution can be used for many titrations, provided that standardization of the GSS is repeated after a suitable number of titrations. Standardization of the GSS is a very simple and accurate operation. For practical use, calculations can be simplified very much, if titrations are carried out at nearly constant atmospheric pressure and room temperature. Indeed, the following equation
C*VK,= meq
(8)
can be used for standardization of the GSS, obtaining a value C* different from C, but utilizable in a subsequent titration, where the unknown is meq. Besides their use for titrations, the GSS’s allow the possibility of addition of very small, accurately known amounts of hydrogen ion, chloride ion, hydroxide ion, and ammonia to systems under study ( e . g . , 1 ml of 10-6M HC1-GSS corresponds to 1 nanomole of HCl). We are now investigating the reliability of other GSS’s and of lower GSS concentrations; and we are developing a new apparatus and procedure to make the production and use of GSS’s simpler and easier. RECEIVED for review February 1, 1971. Accepted March 22, 1971. Work supported by the “Consiglio Nazionale delle Ricerche,” Rome.
ANALYTICAL CHEMISTRY, VOL. 43, NO. 10, AUGUST 1971
