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1912
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modification of the latter method which gives better and more concordant results, closely approaching the bismuthate method in accuracy. Experience has shown that the color method cannot be used with all kinds of steel, as various shades of color are obtained, which do not compare well. Johnson’s method gives fairly good results. The chief objection is that some solution upon decanting is always left in the test tube, the error increasing as the manganese in th-e steel varies from the manganese content of the steel used in standardizing .the sodium arsenite solution. Furthermore, various shipments of lead peroxide did not have the same oxidizing power. Using Walter’s persulphate method, and titrating the permanganic acid obtained with sodium arsenite, gives gratifying results, providing the permanganic acid is kept ice-cold. In first trying this method in warm weather, difficulty was experienced b y the solution rapidly re-oxidizing, so that unless the arsenite was run in rapidly, the results obtained were poor. I t was found that if the solution was warmed a few minutes after the titration was made, re-oxidation would be so complete that if again titrated as before, a practical check could be obtained. By keeping the solution ice-cold, there is no oxidation and good results are obtained, providing the operator is familiar with the end point, which is a suggestion of yellow, free from pink tints. A modification of this method was worked out, which produced better results. Briefly, the method is : Weigh out one gram of steel, dissolve with I O O cc. of nitric acid (sp. gr. 1.20) in a zoo cc. beaker b y placing on a hot plate and heating till no more brown fumes appear. Remove from the hot plate, cool and dilute to 500 cc.. mixing thoroughly. Take I O O cc. of the well-mixed solution, corresponding to 0 . 2 gram of steel, place in a 300 cc. Erlenmeyer flask, and heat on the water bath till warm. Add 15 cc. of silver nitrate (1.33 grams per liter) and about a gram and a half of ammonium persulphate, warming about a minute or two after the color commences to develop: 8001, add a slight excess of sodium chloride (6 cc. of solution containing I .4 grams per liter) so as t o precipitate all the silver; titrate with a standard solution of sodium arsenite till all pink shades are gone, and the white of the precipitated silver chloride alone remains. This end point is very sharp and more easily determined than that of the other methods. The silver nitrate being all precipitated, no re-oxidation of the reduced permanganic acid can occur. The sodium arsenite was standardized against a Basic Open-Hearth steel of the Bureau of Standards, sample No. 13-u,the results obtained b y the various analysts on this steel being more concordant than on the other standard samples.
203
The accuracy of this method was noted by determining the manganese in four Bureau of Standards samples of steel, the results obtained being within one per cent. of the general averages for these steers. TESTINGLABORATORY, AMERICAN BRIDGECOMPANY, AMBRIDGE.PA.
CARBON DIOXIDE: ITS VOLUMETRIC DETERMINATION. B y LEON T. BOWSER.
Some time ago there appeared the description of a procedure devised b y J. C. MimsI for the volumetric determination of carbon dioxide, and from i t the writer has succeeded in evolving an accurate and reliable method. There are no new reactions involved, merely an adaptation of well-known principles to a suitable form of apparatus. Stripped of details, the procedure is essentially that of releasing carbon dioxide b y means of hydrochloric acid, absorbing it in a strong alkaline solution and measuring the absorbed gas by titration with a standard acid. Absorption is accomplished in a tower especially designed to meet the conditions. No preliminary guard tubes are necessary, and instead of rigidly excluding water from contact with the potash solution i t is the practice to distil over a small amount, thus insuring the mechanical carrying over of residual carbon dioxide along with the water vapors. The solution previous t o titration contains a mixture of potassium hydroxide and carbonate, since bicarbonates do not exist in the presence of alkaline hydroxides. In titrating with an acid, using phenolphthalein as indicator, disappearance of the pink color marks the point a t which all hydroxide has been neutralized and the normal carbonate has been converted to bicarbonate or, in other words, the normal carbonate has been half neutralized. The titratiori being continued, after adding a drop of methyl orange, appearance of the usual acid reaction denotes complete neutralization of the bicarbonate. The volume of acid used in the latter titration, then, is just one-half of that required t o release all the carbon dioxide from the condition of a normal carbonate. I t follows t h a t I cc. of normal acid used in the titration between the two end points is equivalent to 0.044 gram CO,. The form of apparatus used is shown in Fig. I. F is a flask in which the carbonate is decomposed by an acid, which is introduced through a small separatory funnel, S. That used b y the writer was originally part of a Geissler alkalimeter, but doubtless could be supplied alone by dealers in apparatus. In Fig. 2 is shown an easily made substitute; the entire arrangement should be as small as possible, the capacity of the funnel body being about I O cc. The necessity for the constriction at the top will be explained presently. The condenser C, of Fig. I, is specially constructed so that the inner tube may be quite short and of as small a bore as possible. All tubing used in the apparatus, except the body of the tower, is of 2 mm. internal diameter, which allows 1 Bull.
65, 156. U. S. Bureau of Chemistry.
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very little space for carbon dioxide to collect and possibly escape absorption. I n the absorption tower T, there are two principal requirements to be fulfilled: securing complete absorption of carbon-dioxide, and making the construction such that the carbonated solution may be readily and completely washed out, which bars the use of
any form of potash bulb such as employed for gravimetric work. It is believed that the form of tower shown meets both requirements fully. Merely passing carbon dioxide through an unbroken column of an alkaline hydroxide fails t o give complete absorption ; it is necessary t h a t the bubbles be completely broken up by means of glass beads or the like. A suitable amount of sample, usually I gram, is now introduced into a I O O cc. Erlenmeyer flask, then about 50 cc. of water are added. The apparatus is connected up, I O cc. of absorbing solution ( 5 0 grams KOH in I O O cc. solution) pipetted into the tower, and sufficient hydrochloric acid introduced through S t o decompose all the carbonates in the flask. A word of caution as to the procedure in introducing this acid may not be amiss. First closing the stopcock, the acid is poured into the body of the funnel, then a piece of rubber tubing fitted over the upper end, constricted for this purpose. The admission of acid to the interior of the flask a t once releases carbon dioxide and produces sufficient pressure to force out the air, which finds it easier to escape through the funnel than through the absorbing tower, and this pressure must be overcome by blowing through the rubber tube. With very Fig. 2 effervescent carbonates the acid is introduced a few drops a t a time; with others the entire amount is added at once. After active effervescence has ceased the flask is gently heated, taking care not t o force bubbles through the tower too fast, and
Mar., 1912
water is allowed t o distil over until the tower is nearly filled. The carbonated solution is now transferred to a I O O cc. volumetric flask as follows: The bent end of the inlet tube a is placed in the neck of the flask and the solution forced out by blowing through the small outlet tube b set in the rubber stopper. This stopper is then removed, the tower filled with water and the latter blown into the flask. Two more washings suffice to rinse out the last traces of alkaline solution, when the contents of the flask are made u p to volume. While practical experience has not revealed any trouble from contamination b y the carbon dioxide of the breath, yet all possibility of danger may be obviated, if desired, by interposing a soda-lime guard tube properly connected. To an aliquot of 2 5 cc. in a 2 5 0 cc. beaker or other open vessel is added a few drops of phenolphthalein, followed by IO-rj cc. of 95 per cent. alcohol; then acid of approximately normal strength is run in until the color begins t o dim, after which decinormal acid is used to complete the discharge of the pink color. The burette reading is recorded, a drop of methyl orange added, the titration continued to the usual end point, and a second reading made. After subtracting the equivalent of carbon dioxide in the reagents ascertained by a separate blank determination, the difference between the two readings, multiplied by the factor 0.0044for strictly decinormal acid, gives the grams of carbon dioxide in the aliquot, from which the percentage in the original sample may be easily found. I t is probable that the titration will be found the principal difficulty in the use of the method. Ordinarily the phenolphthalein end point is supposed to give no trouble of any kind, but it is very apt to do so nevertheless. The writer has discovered t h a t the addition of a small amount of ethyl alcohol after introducing phenolphthalein entirely eliminates the difficulties from this source, but without such a precaution i t is improbable that a satisfactory end point can be secured. A fuller explanation of this difficulty and the simple remedy for i t will be given in a later paper. During this part of the titration the solution should be kept rotating vigorously, since any local excess of acid would carry the reaction on to the point of releasing carbon dioxide from the bicarbonates in the immediate vicinity. I n the addition of methyl orange not more than one drop should be added to 2 5 cc. of the carbonated solution. When this amount is used the change of color is quite easily noted, but the presence of larger quantities renders observation of the color change a matter of difficulty. The end point to observe is when the clear lemon-yellow of the alkaline solution becomes a shade darker from the admixture of pink of the beginning acid reaction. Dark days or poor light render the observation difficult. When the acid used for the titration is as strong as decinormal the color change is noticeable to the unaided eye, but for acids more dilute i t is advisable t o have a t hand a comparison solution of the same tint as the titrated
Mar., 1912
T H E JOURNAL OF I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y .
samples in which the carbon dioxide had been determined both in this way and with the Knorr method, Sulphuric acid should not be used to release the carbon dioxide from a material. A long study of this point, involving numerous determinations, has shown that i t will not uniformly4 decompose carbonates in this form of apparatus. Hydrochloric acid, on the other hand, gives a very perfect decomposition. Numerous trials have shown that, contrary t o what might be expected, the acid is not appreciably carried over during the determination, although even if small amounts did accompany the steam no harm could be done. Another point studied was as t o the relative efficiencies of Ba(OH),, NaOH and KOH for use in the absorbing tower. It was speedily found that Ba(OH), cannot be used since it is not possible to prepare a Per cent. COz found. solution greatly exceeding N / 4 in strength, which Average. Per cent. is entirely too weak for the purpose. 7-Salt COn Successive O n det. On salt. No. used. taken. aliquots. Using NaOH or KOH there is no difficulty in 1 . . . . . . . . NasC03 41.19 41.10 preparing a solution of any desired strength, although 41.18 2. . . . . . . . 41.27 the best results are attained by the use of rather con3 ........ 41.10 41.18 41.18 4 ........ 41.27 centrated solutions. The I O cc. taken is more than 5 . . . ..... KzC03 31.27 31.15 sufficient to absorb one gram of carbon dioxide, which 31.15 6 ........ 31.15 7 ........ 31.15 would be equivalent to a two-gram sample of nearly 31.19 31.17 8 ........ 31.24 any of the purest carbonates. On the whole sodium 9........ CaC03 43.96 43.91 hydroxide is the less satisfactory in use, there being 43.95 1 0 . . ...... 44.00 1 1 ........ 44.00 too great a variation between the titration results 12... . . . . . 44 ,09 of successive aliquots. Potassium hydroxide has 13........ 43.91 44.00 given uniform satisfaction both in manipulation and 14........ 43.83 1 5 . . ...... 43.91 from the standpoint of accuracy 44.00 43.91 43.95 * 16 ........ Either hydrochloric or sulphuric acid may be used 17........ BaCOs 22.28 22.09 22.35 18........ for the titration, either giving good results. For ordi22.53 22.32 19.. ...... nary work the most satisfactory strength is decinormal, 22.53 29.75 20........ 21.. . . . . . . 22.35 which gives a clear, easily noted color change with 22.53 22.47 22........ both indicators. When unusually small amounts 23........ 22.35 of carbon dioxide are to be determined, acids of N / s o 24.. . . . . . . 22.35 22.35 22.35 22.38 25.. ...... or N / I O Ostrength are employed, the former being 26 ........ SrCOy 29.81 29.57 used in most cases. Their use is justified only in the 27.. . . . . . . 29.92 29.75 29.75 28.. . . . . . . most exact work, however, since the increased time 29.83 29.79 29.77 29.. . . . . . . and difficulty involved cuts down heavily on the speed of the work. While this method is on the whole as accurate as Of the salts used the latter three had been freshly opened, and presumably were of nearly the theoretical any of those now employed, the speed attainable by composition. In the case of sodium and potassium its use is much greater. With but a single apparatus carbonates the bottles had been opened several times it is possible to make 24 determinations in an eightpreviously and i t was thought some moisture might hour day, with abundant time between successive have entered. On this account the carbon dioxide distillations. There are no fragile nor unusual parts content was carefully determined by direct titration necessary in the apparatus, nothing to get out of order, with a standard acid, giving the percentages shown and’the entire construction is flexible enough to stand under “Per cent. CO, taken,” both of which are slightly a great deal of rough usage. It is easy t o construct below theoretical. All five salts were J. T. Baker’s in almost any laboratory, and in case of breakage analyzed products, purchased especially for the pur- replacement is the work of but a moment. In conclusion, the writer desires t o acknowledge pose. As may be noted the average results are, in all but one case, slightly lower than theoretical, his indebtedness to the following for many helpful which is as i t should be, since slight traces of im- suggestions: Profs. F. E. Edwards and C. E. Bradley, purities and moisture are nearly certain to be present. both formerly of the Oregon Agricultural College, As a whole this series is very satisfactory and shows and J . C. Mims, whose published results afforded a that the method is not lacking in accuracy. Equally starting point for the evolution of the method favorable results have been obtained in the case of DAYTON,OHIO.
solution when alkaline, and the slightest difference of color is then easily noted. Even with the aid of this device a considerable amount of care must be exercised should an acid as dilute as centinormal be in use, but with care and proper light even then the end point is distinguishable with accuracy. After the development of the method a large number of determinations was made, nearly every one of which was very satisfactory. I n the table is presented a summary showing the general run of results. In explanation i t may be noted that in some cases two, in others three titrations were made on aliquots from the same flask, given in the column headed “Successive aliquots,” while the average of these titration results is to be found under “On det.” The last column gives the average of all determinations on each salt.
