RAPID PREPARATION of ANHYDROUS Nn,C03 for USE in ACIDIMETRY Dissociation and Dehydration
of NaHC03 under
G. FREDERICK SMITH
AND
Reduced Pressure
V. R. HARDY
University of Illinois, Urbana, Illinois
T
HE standardization of acids is most commonly carried out using anhydrous sodium carbonate. The reagent is cheap, easily purified, satisfactorily stable in air and practically all possible variables associated with its preparation and applications in acidimetry have been exhaustively studied.' The dissociation of sodium bicarbonate to the normal anhydrous carbonate under reduced pressure at temperatures up to and including 305OC. has not been studied. The present paper shows the very considerable advantage resulting from such a study toward the improvement of conditions governing the preparation and use of anhydrous sodium carbonate as a fixed standard in acidimetry.
The conditions affecting this dissociation to be studied were:
PREPARATION OF REAGENTS USED
INITIAL REACTION TEMPERATURE
(1) The temperature at which the reaction is initiated (2) The variation of the speed of the reaction with increase in temperature (3) The variation in the speed of reaction with decrease in pressure (4) The determination of the maximum allowable temperature without further dissociation of the sodium carbonate formed (5) Factors influencing the practical method of preparing anhydrous sodium carbonate for use as a standard of reference in acidimetry.
Samples of sodium bicarbonate were placed in small porcelain boats and inserted in the clean, dry reaction tube, and the apparatus was evacuated until the manometer showed a "zero" reading. The heating coil was centered over the sample and boat with the thermometer adjusted and the heat slowly applied. The rise in temperature was noted and at the point at which the pressure within the system began to increase the temperature was read. The determination was repeated a number of times in this manner and the averAPPARATUS EMPLOYED The vacuum dissociations and dehydrations camed age temperature of initial dissociation with rising temout in this study employed a straight glass tube with perature was found to be 78% The determinations manometer attached, evacuated by use of an efficient were repeated with falling temperature and the point rotary oil vacuum pump. The required temperature at which the change in pressure per unit time interval was obtained using electrical energy and temperatures became zero was noted. The average value for these estimated using an enclosed mercury thermometer. determinations was found to be 7 6 T . The mean of both values is therefore 77'C. which is probably more Details are omitted for the sake of brevity. accurate than either of the individual average determinations since the temperature lag is eliminated. EXPERIMENTAL PROCEDURE That the change in pressure involved represented a The reaction to be studied was the following: dissociation accompanied by the liberation of carbon 2NaHCOs +NazCOs + H1O + COz. dioxide was proved by detection of the same in the gas (1) liberated within the reaction chamber at that tem'For an extensive bibliography reference should he made t o KOLTHORP AND FURMAN, "Volumetric analysis," John Wiley & perature.
Sodium Bicarbonate. This material may be prepared in pure form using the procedure described by Kolthoff and Fnrman.' Standard Solutions of Hydrochloric Acid. Constantboiling acid was prepared following the directions of Foulk and Holling~worth.~The apparatus and procedure in this preparation exactly duplicated their description of the method.
Sons, Inc., New York City, 1929, vol. 2, pp. 87-93. Additional literature references dealing with this dissociation of direct rend., 147, interest in this connection are: S ~ W R YCompt, , 1246 (1908); RAKUZIN AND BRODSKI,Z. angeu. Chem., 40, S36 (1927); COCKSEDOE, Brit. Patent 202,678 (Apr., 1922); and roc. chim., 43, 744 (1928). h c o u s ~ m Bull. , I. Am. Chem. Soc., 45, 1220 FOULKAND HOLLINGSWORTH, (1923).
VARIATION REACTION VELOCITY WITH TEMPERATURE
The dissociation of sodium bicarbonate. while it begins at 77%, increases rapidly in speed with increase in temperature. Starting with equal weights of sample and noting the change in pressure per unit of time at
507
various temperatures, comparisons in the speed of dissociation are found in the comparative increase in the d p l d t ratios obtained. At 100°C. dissociation according to reaction (1) above was complete in 50 minutes. At 305OC. this dissociation was complete in less than two minutes. The rate of change in pressure per minute was too great a t a temperature of 305OC. to be accurately recorded. The time required for the preparation of samples of anhydrous sodium carbonate using sodium bicarbonate is not determined by the speed of the dissociation of the latter into the former but rather by the speed of dehydration of NaaCOn.HzOpresent. VARIATION I N RATE WITH PRESSURE AT A GIVEN TEMPERATURE
The maximum temperature which may be used for the dissociation of bicarbonate to normal carbonate is governed by the temperature a t which the latter is further dissociated to form sodium oxide according to reactions (2) and (3) following:
T I I B - P R B S S U R & - T E M F & R & T m ~R&LATII)NSHPS
T
'C. 123 123 216 216 250 270 270 270 270 305 305 305 305 305 305
Normality of referenee standard hydroehiorie aeid = 0.2001 Normdily of Acid Rcmorkr P Time COI evolution mm. min. Pound Deviation 46 min. 0-11 60 0.2017 +0.0016 +0.0019 SO min. atm. 60 0.2020 8 min. +0.0016 G26 20 0.2017 +0.0016 8 min. 0 12 0.2017 +0.0017 5 min. afm. 10 0.2018 +0.0006 6 mi". 0 15 0.2007 No spattering +0.0008 atm. 30 0.2009 Spattering +0.0005 0 83 0.2006 135 0.2000 -0.0001 +22 15 0.2002 +0.0001 2 min. 0 No spattering +0.0004 atm. 30 0.2005 Spattering *0.0000 0 30 0.2001 +0.0001 0 60 0.2002 -0.0002 0 90 0.1999 0.1996 -0.0005 195 0
Preheated 5 min. at atm. oressure. them evacuated t o totaltime stated No spattering +0.0005 305 0 15 0.2006 No spattering +0.0003 30 0.2004 305 0 +0.0003 Nospattering 305 0 30 0.2004 No spattering +0.0001 305 0 60 0.2002
PRACTICAL FEATURES OF THE PROCEDURE
As shown in the table, if the dissociation of sodium bicarbonate is carried out under atmospheric pressure a t 270' to 300°C. as recommended for the procedure up to the present, the reaction requires two hours' heating. That this time requirement is not necessary is also shown by the tabulated data. The time-consuming reaction is that of the dehyIn comparison a t a given temperature above 7PC. dration reaction (2), which may be expedited by the reactions (1) to (3) above are progressively less rapid. application of a vacuum. The dissociation of sodium Reaction (1) is complete in a very short period of bicarbonate a t 305OC. is so rapid that the individual time a t atmospheric pressure and a t 300' to 305'C. crystal aggregates are exploded accompanied by some Reaction (2) requires two hours for its completion a t loss of sample but leaving each particle with an abthe same temperature. Reaction (3) does not progress normally large surface which results in subsequent to a measurable extent during two hours a t the same rapid dissociation to form the anhydrous salt. The temperature. complete preparation of the sample for use in analysis To determine these relationships, samples of sodium when dissociated a t 305°C. in wacuo requires hut 15 bicarbonate were nlaced in a weiehed ~orcelainhoat and minutes as compared with 120 minutes when carried " dissociated in the reaction tube under various conditions out a t atmospheric pressure. of temperature, pressure, and time. The boat plus It is not particularly necessary to avoid spattering sample were then removed, cooled in a desiccator over of the sample during dissociation since only that por"anhydrone" to insure anhydrous conditions, and tion remaining in the hoat is used. If it is desirable to rapidly and accurately weighed. The samples were avoid spattering this can be done by heating a t 305°C. then dissolved in distilled water and after the addition for five minutes and then applying a vacuum for an of methyl red as indicator were titrated, using standard additional 55 minutes under which conditions the same hydrochloric acid made from constant-boiling-strength result is accomplished as by the former procedure. Hcid following the directions of Foulk and HollingsAnhydrous sodium carbonate is noticeably hygroworth.% The solutions of sodium carbonate were scopic and should he stored under strictly anhydrous titrated almost to completion, boiled a few minutes conditions and weighed in closed capsules. to eliminate COz, and cooled to complete the titration. SUMMARY The indicator titration error under these conditions for 1. Directions are given for the preparation of the strength of standard acid used has been shown to he negligible. The standard factor for the hydro- anhydrous NazC03from NaHC03for use as a standard chloric acid was then calculated using the weight of of reference in acidimetry by the method of dissociation sodium carbonate employed. Failure to complete under reduced pressure. 2. The initial temperature of dissociation was found reactions (1) and (2) results in the calculation of too large a factor for the standard acid required. If reaction (3) to be W C . Dissociation is complete in less than two has taken place, a low factor is obtained. Completion minutes a t 305'C. 3. The dehydration of the NaaCOs prepared in of reaction (1) is shown by the determination of the presence or absence of COz in the reaction tube. The eracuo a t 305°C. is complete in 15 minutes and no further results of a series of determinations are given in the weighable dissociation took place up to two hours' heating with small samples used in acid standardization. accompanying table. L
