A S A L Y TICAI, EDI T I O S
76
Vol. 2. s o . 1
Determination of Carbon Dioxide in Carbonates' Wilfred W. Scott and Paul W. Jewel U h I V E R s I T Y OF S O U T H E R N CALIFORNI.4, LOS .kSGELI?S, C A L I F .
T
HE investigation here reported was undertaken with
a view to developing one or more methods for the determination of carbon dioxide in carbonates which would be simple and easy of manipulation and a t the same time give results comparable with those obtained by the more complicated methods. An apparatus that could be set u p without the services of a glassblower, from materials usually available in chemical laboratories, that were neither fragile nor expensive, \\-as also desired. The method sought should be so rapid that a great many determinations could be run in one day and should giye results as free from error as powible. In other wordy a strictly routine method that could be really depended upon mas desired. A11 of the nietliocl~ cbcribed in the literature that appeared to have any possibility of being suitable m r e tested inthelaboratory. S o n e that fulfilled the requirements of the inyestigation n ere found. howeyer. A large number could be run quickly and were simple and easy of manipulation, but their accuracy left much to be desired, the error in many cases being as high as 5 per cent. T h e investigation showed that any new method to be entirely suitable would have to Figure 1-Apparatus for Alkalimeter be of the type generMethod allv r e f e r r e d t o a s the "loss of weight type." hIethods 'of the other types either require the use of complicated and expensive apparatus or are subject to certain inherent defects that make them unsuitable. The loss of weight methods call for weighing the sample in a suitable apparatus, liberating the carbon dioxide by some means, usually by allowing it to react with dilute acid, and weighing it again after the elolution of gas has ceased. The amount of carbon dioxide present can be calculated from the loss in weight. These methods are usually simple and easy of manipulation but their accuracy is not high, results showing an error of 1.0 per cent being regarded as very good. Unless a regular alkalimeter, the use of which is precluded b y its cost and fragility, is employed, the degree of accuracy to be attained by methods of this type is not high, even when they are run by capable and careful analysts. The problem thus resolved itself into the derelopinent of a method of the loss of weight type which would be accurate, having an error as low as 0.10 per cent, if possible. Experimental
The cause of the error in the loss of weight methods wa? determined by the following experiment : 1
Received October 8, 1929.
X 50-cc. Erlenmeyer flask was fitted l\-ith a glass tube 15.24 em. long, just coming through the rubber stopper and drawn out to a capillary tip at the outer end (Figure 2 ) . A carefully weighed sample of calcium carbonate was introduced in a glass thimble, prepared by cutting off the lower inch of a regulation 2-dram homeopathic vial and firedressing the edges, the entire assembly was weighed, and the carbonate was made to react with bhe acid by agitating the flask to overturn the thimble. The reaction was allowed to go to completion a t room temperat'ure and the apparat,us was again weighed after it had had ample time t'o readjust itself t,o the atmospheric pressure. The apparatus was again weighed, when t,he loss in weight 17-as found to correspond t o about 55 per cent of the carbon dioxide theoretically present in tlie sample. The contents of the flask were then hcatetl to gent'le boiling until the vapors issuing from tlic tip of the glass tube gave no test for carbon d i o d e with a drop of barium chloride solut'ion on the tip of a glass rod held above the tip of the t'ube. The flask was again weighed. The loss in Twight was found to correspond to an amount of carbon dioxide about 5 per cent in excess of that theroctically present in the sample. With the same apparatus used in the ~ a m eway results with about 1 per cent of error could be obtained if the contents of tlie flask ivere heated just to boiling over a small flame and the flask quickly cooled and T\-eighed. The results of this experiment indicated that the solution of the problem lay in the development of a method having a suitable means of drying the carbon dioxide as it left the apparatus and not requiring heat. M e r a great deal of ex p e r i m e n t i n g with various types of apparatus, t h e f o l l o w i n g method and apparatus were developed. This methodmas a t first used with heat, but, it was found t'hat with proper aspiration heat. JF-as not necessary and more X C I 1.2 P accurate results could Y be obt'ained without Figure 2-Apparatus for Simplest it. Quantitative Method The Method
APPARATUS(FIGCRE1)-Fit a 50-cc. Erlcnnie\-er flask with a 2-hole rubber stopper bearing a short calcium cliloride tube filled with granular calcium chloride of about 16 mesh. This calcium chloride tube is best prepared by cutting down the regulation tube so that its length is not over 7 . 5 cm. Fit the upper end of the tube with a 1-hole rubber stopper bearing a short glass tube just coming through the stopper and bent a t right angles above, so as to make a convenient attachment for the suction pump. This stopper and tube
ISDCSTRIAL Ai\-D EiYGISEERIA-G CHEJIISTRY
January 15, 1930
may be left off during the neighing. If desired. tlie tube may be closed by means of a rubber policeman. Fit the other hole of the stopper with a glass tube drawn out to a capillary at the lower end and reaching nearly t o the bottom of the flask. Bend it at right angles above the stopper to make a conT-enient attachment for the purifying train, and, unless otherir ise specified, close it Tvith a rubber policeman during tlie determination. Sr.ssrax;ic
Table I SAMPLGTHEORETICAL FOLND Gram Gram Gram 0,5000 0,5000 0,5000 0.5000 0.8555 0.3302 0.5000 0,5000 0 5000 0.5000 0,6000 0.6000 0,5000 0.5000 0,3000 n ,3000
0.2200 0.2200 0 2200 0.2200 0,4482 0.1729 0.1ii-i 0.1774 0.15y2 0.1592 0 . lis9 0.li89 O.liS4 0.li54 0.1666 0.1366
0 2200 0.2202 0.2201 0,2202 0.4460 0.1726 0,liil 0.lii3 0.1593 0.1590 0.li86 0 .178.5 0.1751 0,1750 0.1566 0.1564
ERROR Grum Per cent 0 0000 0.00 n ,0002 0.10 0.0001 0.05 0,0002 0.10
n
0022 0 0003
0.0003 0 0001 0.0001 0.0002 0,0003 0.0004 0.0003 0,0004 0 0000 0 , 0002
0 . a 0,li 0,li 0.00 0.00 0.01 0.16 0.22 0.16 0.21 0.00 0.01
Tlie purification train consists of two fairly large U-tube?: the one next to tlie apparatus filled with calcium chloride and the other filled with soda lime. The air as it enters the apparatus is drawn through this purification train and is thus rendered free from carbon dioxide and dried. DETERUI~.~TIO~-~~5'1en the apparatus has been assembled as described, place about 15 cc. of dilute hydrochloric acid (1:l) in the flask. Introduce the carefully weighed sample (about 0.50 gram) in a glass thimble SO that the thimble stands upright in the acid. This t'hinible is best prepared by cutting off the loner inch of a regulation 2-dram homeo-
pathic vial and firedressing the edges. If desired, the apparatus may be weighed before the introduction of the thimble and sample and t'he two weights added to give the t'otal weight of the apparatus. After the apparatus has been carefully weighed overturn the thimble by tipping the flask. The inlet tube must he closed and the outlet' tube must be open during the reaction. K h e n the reaction has subsided attach the apparatus to the purifying train and pump and apply suction, so that the bubbles pass through the apparatus a t the rate of aliout two per second. Continue this aspiration for 15 niinutei a t this rate. Then detach the apparatus, replace the ruliber policeman, and weigh the entire apparatus. The loss iii weight is calculated as carbon dioxide. The time required for making the deterniination is ahoiit 20 minutes. The arerage raniple will he about 0.5000 gram, but wlic~n the carbon dioxide content is high 0.3000-gram samp1t.i should be used. CRmCISx-This method is very rapid and awurate! being capable of results having an error as low as 0.10 per cent or lower. It is, of course, open to .the usual objections to methods of this type, namely, that any othw substances which may be evolved as gases under the conditions obtaining during the determination vi11 be weighed and erroneously reported as carbon dioxide. Tlie method is not reconiriiended for basic carbonates or for lead carbonate. Just why these substance:;: should give difficulty is not l i n o ~ n ito tlie witers. Results
The results obtained x i t h the new method, using a number of different carbonates, are given in Table T.
Filtration Pipet for Spot Indicator Tests' Earle R. Caley PRINCETOS CSIVERSITY, PRISCETOS, S . J.
CERTAIS types of titrations wheie tlie spot indicator ItheSmethod is employed, it is desirable to withdraw froin solution being titrated liquid free from any suspended precipitate. This procedure is necessary when the color of the precipitate interferes with the spot reaction, or in cases where the precipitate reacts chemically with the outside reagent and leads to a n uncertain or false end point. A simple device for rapidly filtering small samples of turbid liquid for spot tests is shown in the figure. It is most easily constructed from a T-tube proportioned as shown. For the avoidance of capillary effects, which interfere with the operation of the instrument, glass tubing of not less than 6 mm. interiial diameter should be used. To the end of the short arni is attached a small rubber bulb, while the end of the long arni nearest the bulb is drawn out to a tip. The other end of the long arm is provided with a slight internal collar formed by holding the end momentarily in the flame. A small length of heavy-walled rubber tubing is then inserted into this end. The rubber tubing should fit the inside of the glass tube quite tightly and its upper end should be about 1 mm. from the end of the tube. As a filtering medium one or more closely fitting disks, cut from filter paper by means Received September 14 1929
spot test, the filtering end is dipped into the solution being titrated, while the rubber bulb is nianipulated b e t w e e n t h e t h u m b a n d middle finger with the forefinger closing the tip. To release the drop or two of filtered liquid thus ohtained, the instrument is inverted over the spot plate in an inclined position with the bulb uppermost. the spot plate. A device of this type can uiidoubtedly be used for other purposes than t h e o n e n i e n t i o i i e d
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