240
ANALYTICAL EDlTlON
Vol. 3, No. 3
directly with dilute standard sulfuric acid from a microburet; and the end point was taken when the shade of color of the indicator exactly matched that of the blank. This proved to be a very faint pink a t a pH of about 5.7. By matching the blank it was unnecessary to subtract a blank correction from the amount of acid used. Table I-Comparative Accuracy of Micro-Kjeldahl Method SAMPLES NITROGEN ERROR WITH ERROR WITH ANAIN MACROMICROLYZED SAMPLE KJELDAHL KJELDAHL Mg. % % 8
Apparatus for Micro-Kjeldahl Method
reagents and procedure as with the samples, after the apparatus had been thoroughly steamed for a t least 15 minutes. carewas taken that the Same amountof liquid was contained in each flask to insure the same intensity of color. The &Ontents Of the receiving flask from a sample were titrated
c
21,114
6 4
1,000 0.583
4
0.100
0.62
0.29
1.85
1.20 0.24 3.00
It can be readily observed that this apparatus and procedure can be used with rapidity and accuracy on samples requiring as much as 10 cc. of concentrated sulfuric acid in the digestion mixture, as well as samples containing as little as 0.1 mg. of nitrogen. The comparative accuracy of this method with the macro-Kjeldahl method is shown in Table I. By running two sets of apparatus a t the same time it was found that 20 samples could easily be analyzed in a half day. Literature Cited (1) Kemmerer and Hallett, IND. E m . CHEM, l % 1295 (1927) (2) Preg!, “Quantitative Organic Microanalysis,” pp. 94-104, Blakiston, 1924.
(3) Scales and Harrison, J. IND
END.
CHEM.,12, 360 (1920).
Use of Boric Acid in Micro-Kjeldahl Determination of Nitrogen’ Norman M. Stover and Reuben B. Sandin DEPARTMENT OF CHEMISTRY, UNIVERSITY OF ALBERTA,EDMONTON, ALBERTA
The use of boric acid in absorbing ammonia in and thus a saving is made in H E use of a saturated nitrogen determinations by Pregl’s micro-Kjeldahl the use of standard reagents. solution of boric acid method has been found to give accurate results. Also, it is considered by some in place of a standard The distillate containing the ammonia does not need to beeasier toprepareastand.acid to absorb ammonia in a r d acid solution t h a n a to be boiled before titrating. pitrogen determinations by A mixed indicator containing methyl red and tetrastandard alkali solution, and $he K j e l d a h l m e t h o d has bromophenol blue has been found to give good results a carefully standardized acid been reported a number of solution-is less subject to &imes. I t was f i r s t proin boric acid solution. change in storage. posed by Winkler (8),has been mentioned by Scales and Harrison (G), and recomExperimental Procedure mended by Spears ( 7 ) . The authors ( 5 ) have also used boric acid to absorb ammonia in the determination of organic Attempts were made to find a suitable indicator for use in ,nitrogen in liquids by an alkaline fusion method. boric acid solution when absorbing small amounts of ammonia. The use of boric acid in nitrogen determinations is based on The following indicators were tried: bromophenol blue (0.1 the principle that ammonia is absorbed by the weak boric gram in 3 cc. of 0.05 N sodium hydroxide and diluted to 250 acid and is titrated in this solution with a strong acid, such as cc.), tetrabromophenol blue (0.1 gram in 20 cc. of warm alcosulfuric or hydrochloric. . Ammonium borate is probably hol and diluted to 100 cc.), sodium alizarin sulfonate (1 per formed, and in the presence of a strong acid the ammonia is cent in water), methyl red (0.1 per cent in 95 per cent alcohol), again released to form the salt of the strong acid. a mixed indicator containing methyl red and methylene blue The fact that boric acid gives results almost identical with (0.1250 gram of methyl red and 0.0825 gram of methylene $hose obtained when using a standard acid has led the authors blue in 100 cc. of 90 per cent alcohol), and a mixture of methyl to determine the possibility of using it in the micro-Kjeldahl red and tetrabromophenol blue (made by mixing equal method as outlined by Pregl (4). It has several advantages, volumes of solutions of the separate indicators). The mixed as stated by Scales and Harrison (G), and also a few other small indicator containing methyl red and methylene blue has been points in its favor. A minimum amount of the only standard recommended by Johnson and Green (3). The new indicator, Bolution (standard acid) required is used. There is no back tetrabromophenol blue,* was developed recently by Harden titration as the ammonia in the boric acid is titrated directly, 2 W. C . Harden of the laboratories of Hynson, Westcott, and Dunning
T
1 Received
January 30, 1931:
kindly supplied the sample of this indicator.
h l y 15, 1931
INDUSTRIAL AND ENGINEERING CHEMISTRY
and Drake ( 2 ) . This name for the indicator has been suggested by Kolthoff as a trade name, the proper name for the compound being tetrabromophenol-tetrabromosulfonephthalein. Since Scales and Harrison ( 6 ) obtained good results with bromophenol blue when used in the ordinary Kjeldahl method, and the authors (5) likewise found this indicator satisfactory when used in their alkaline fusion method, it was thought the same indicator might be used in the present investigation. Preliminary trials indicated, however, that bromophenol blue possesses a sufficient salt error to render the indicator useless in boric acid solution when titrating small amounts of ammonia. This is readily shown by the effect of varying amounts of ammonium sulfate on the color of the indicator in boric acid solution. Also, when using N/70 hydrochloric acid for titrating the ammonia, the end point is not sharp. With the other indicators, no salt error was noticeable. This was determined for each indicator by adding 2 drops of the indicator (4drops in the case of the mixed indicator containing methyl red and tetrabromophenol blue) to each of three solutions containing 2 cc. of 4 per cent boric acid, varying amounts of ammonium sulfate solution (furnishing, respectively, 0.5 mg., 1 mg., and 2 mg. of nitrogen), and enough freshly boiled, cooled, distilled water to make 50 CC. Each indicator or mixture of indicators showed its characteristic color regardless of the concentration of ammonium sulfate present, thus showing the absence of any salt error. The colors shown were as follows: tetrabromophenol blue, pale blue; sodium alizarin sulfonate, yellow; methyl red, faint pink; mixture of methyl red and methylene blue, bluish green; mixture of methyl red and tetrabromophenol blue, pale gray-violet. The effect of varying amounts of boric acid on the indicators was also determined. This was done by adding 2 drops of the indicator (4 drops in the case of the mixed indicator containing methyl red and tetrabromophenol blue) to each of two solutions containing, respectively, 2 cc. of 4 per cent boric acid and 5 cc. of 4 per cent boric acid and enough freshly boiled, cooled, ,distilled water in each case to make 50 cc. of solution. Tetrabromophenol blue showed no difference in color in the two eolutions. With increasing concentration of boric acid, sodium alizarin sulfonate changed to a lighter yellow, methyl red to a deeper pink, mixed methyl red and methylene blue from bluish green to pure blue, while mixed methyl red and tetrabromophenol blue took on a reddish tint. Hence most of the indicators were affected by increasing concentrations of boric acid. In the case of tetrabromophenol blue, further work showed that the end point is difficult to determine when using N/70 hydrochloric acid to titrate the ammonia held by boric acid solution, as the color of the indicator in boric acid solution containing absorbed ammonia is only a somewhat deeper blue than a contrrol solution containing the same amount of boric acid and ammonium sulfate equivalent to 1 mg. of nitrogen. It should be mentioned that the color changes shown by tetrabromophenol blue are the same as those shown by bromophenol blue. The color change of sodium alizarin sulfonate. is from yellow in acid solution to violet in alkaline solution. Yet when small amounts of ammonia were distilled into a boric acid solution, this indicator underwent a color change only from yellow to a deeper golden yellow instead of to a violet color. Hence the two indicators, tetrabromophenol blue and sodium alizarin sulfonate, were found t o be unsuitable. The other three indicators proved more satisfactory. Their practical value was determined as follows: A solution of .ammonium sulfate was made so that 5 cc. contained approximately 1 mg. of nitrogen. A definite volume of this solution,
24 1
measured from a buret, was introduced into the microKjeldahl distillation apparatus, an excess of saturated sodium hydroxide solution added, and the ammonia distilled off, using Pregl’s method of steam distillation as modified by Clark and Collip (1). In one series, the ammonia was determined by the usual micro method-i. e., absorbing the ammonia in an excess of N / 7 0 acid and titrating the excess acid with approximately N/70 carbonate-free sodium hydroxide solution (the relationship between the acid and alkali being known). In this series, methyl red wm used as indicator, the acid solution being boiled for about 1 minute and then cooled just before titrating with the N / 7 0 alkali. In other series, the ammonia was absorbed in a solution containing 2 cc. of 4 per cent boric acid solution, 2 cc. of distilled water, and 2 drops of indicator. The distillates (about 50 cc. in each case) were titrated a t once with N/70 hydrochloric acid until the color of the solution matched that of a control containing 2 cc. of 4 per cent boric acid solution, 5 cc. of ammonium sulfate solution (1 mg. of nitrogen), 2 drops of indicator, and boiled distilled water to 50 cc. The control is necessary because, as already shown, the colors of the indicators vary with different concentrations of boric acid. The amounts of nitrogen recovered from the original ammonium sulfate solution in each series were calculated ( 1 cc. of N/70 HC1 = 0.2 mg. of nitrogen). By comparing these amounts i t was possible to ascertain whether the boric acid method gave as good results as the ordinary method and also which indicator was most suitable. Some typical results are given in the first column of figures in Table I. Table I-Comparison
SAMPLEINDICATOR
of Results w i t h T w o Methods NITROGEN NITROGEN RECOVERED RECOVERED FROM AMMONIUM FROM SAME SULFATE SAMPLE SOLNS. OF URINE Mg. ME. ORDINARY METHOD
1
Methyl red
0.507 0 509 0.509 0.509
Av.
0.575 0.585 0.573 0.578
1,005 1.005 1.005 1.007
2
BORIC ACID METHOD
1
Methyl red
2
1
0.504 0.505 0.505
Av.
0.565 0.562 0.575 0.567
1,002 1.002 1.004 Methyl red and methylene blue
2
0.505 0.505 0.505
0.575 0.575 0.570 Av. 0 . 5 7 3
1,005 1,005 1.005
1
2
Methyl red and tetrabromophenol blue
0.502 0 505 0.506
Av.
0.575 0.578 0.684 0.579
1.005 1.005 1.006
Finally, the indicators were used in the determination of nitrogen in urine by both the ordinary micro-Kjeldahl method and the authors’ boric acid method. The procedure used was to dilute 5 cc. of urine to 100 cc., and then digest 1 cc. of the diluted urine with concentrated sulfuric acid, copper sulfate, and potassium sulfate as directed by Pregl (4). After digestion, the contents of the digestion flask were cooled and
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ANAL YTICAI, EDITION
242
transferred to the distillation apparatus and distilled as before, except that a small amount of sodium thiosulfate solution was added with the saturated sodium hydroxide solution to precipitate the copper, and 3 drops of Nujol to prevent frothing. Nujol was found to be better than amyl alcohol in that it did not boil off. Typical results are shown in the last column of figures in Table I. It will be observed that with the use of boric acid and the mixed indicator containing methyl red and tetrabromophenol blue, results were obtained which were very close to those obtained by the ordinary method. It was also found that this mixed indicator gave a sharp end point, the color changing from a clear green in alkaline solution to a gray color at almost the end point, and then to a pale gray-violet with slightly more acid. Scales and Harrison (6) found that 50 CC. Of 4 per Cent
boric acid solution would recover 95 mg. of nitrogen as ammonia with accuracy. Hence, 2 cc. of a 4 per cent solution would be quite sufficient when using the micro-Kjeldahl method for the determination of nitrogen in urine. Also, instead of diluting 2 cc. of a 4 per cent solution with 2 cc. of distilled water, 4 cc. of a 2 per cent solution could be used directly. Literature Cited (1) Clark and Collip, J . B i d . Chem., 67, 621 (1926). (2) Harden and Drake, J. Am. Chcm. Soc., 61, 562, 2278 (1929). (3) Johnson and Green, IND. ENG. CXEM.,Anal. Ed., 2, 2 (1930). (4) Pregl, “Quantitative Organic Microanalysis,” Blakiston, 1924. (5) Sandin and Stover, Can. J . Res., 2, 264 (1930). (6) Scales and Harrison, IND. END. CHEM.,12, 350 (1920). (7) Spears, J . Assocn. 04icial Agr. Chem., 5, 105 (1921). (8) Winkler, 2. aizgew. Chem., 26, 231 (1913).
An Easy Method of Marking Chemical Glassware‘ K. H. Morkert and W. D. Hatfield SANITARY DISTRICT, DECATUR,ILL.
TCHING glassware in the ordinary routine laboratory with hydrofluoric acid is not an easy task, consequently labeling of flasks, bottles, and rough calibrations is usually done with gummed labels, sometimes covered with a wax coating, or with colored wax pencils. These labels are not permanent, particularly with articles which must be washed regularly. For some time the stoppers and the bottles, as well as the capacities of the bottles when stoppered, used for incubation of the dilutions for determination of the biochemical oxygen demand have been numbered in this laboratory. The use of these calibrated bottles greatly decreases the routine for setting up and later titrating the B. 0. D.’s. The scratching of numbers and Ietters on the stoppers and bottles with a glass marking pencil is a hard and tedious job and cannot be done neatly. A merchant stuck pennants on his plate glass windows with water glass, and when he removed the pennants he found his plate glass etched so permanently that he could not wash the etching off with acid, alkali, or abrasives. His call for help aroused the interest of the authors in this problem. It occurred to the authors that a permanent etching might easily be obtained by writing the letters and numbers on glassware using water glass and a steel pen. The results have been very satisfactory and, so far as is known, this method of marking chemical glassware is not commonly used. The technic is summarized as follows:
E
Reagent
Water glass as purchased from a drug store is 40 per cent sodium silicate. This solution is a little too thick for easy application with a steel pen. A 30 per cent solution obtained by diluting 75 cc. of the 40 per cent solution to 100 cc. with distilled water gives a solution of the proper consistency. One hundred cubic centimeters will mark a large number of bottles. Care should be taken to rinse out glassware used in measuring and diluting the reagent. The reagent may be kept in an ordinary bottle with a rubber stopper. 1 Received April 8, 1931. Presented before the Division of Water.‘ Sewage, and Sanitation Chemistry at the Slst Meeting of the American Chemical Society, Indianapolis, Ind., March 30 to April 3, 1931.
AIcommon steel pen is used to apply the reagent to the glassware. Marking of Pyrex and Resistance Glass
The article to be marked should be cleaned and thoroughly dried. Dip the pen in the sodium silicate solution, drain the excess reagent from the pen point by touching the pen to the mouth of the reagent bottle, and mark the desired letters or numbers on the glass. A few experimental markings will show the amount of reagent and the best pressure on the pen for the desired results. Allow the markings to dry for a few minutes and then go over them with a second application. This will leave a colorless marking raised about 1 mm. above the glass surface. Where there are a number of articles to be marked, the pen must be washed frequently to prevent the reagent from crystallizingon the pen point. After the applications have been made and the markings dried for a few minutes, the markings are heated in the hottest point of the Bunsen flame or in a blast-lamp flame until the markings, which frost at first, turn red. This heating will take about 1 minute, The intensity of heating will depend on the piece of apparatus and the kind of glass, but with more intense heating, a better etching is obtained. On cooling, the etching is a heavy white frosting, some of which will wear off, but there will remain a good permanent white etching which will not be removed by daily washing in acid, alkali, or soap. The heavy frosting appears to become still more permanent if allowed to stand a few days before being washed. Marking of Flint Glass or Cheap Glass Bottles
Cheap glass bottles cannot be heated as described above without considerable breakage. If the cheaper glass bottles are heated to about 55” C. (by placing on a steam radiator or in a 55” C. oven), or a temperature that can be just comfortably held in the hand, the markings can then be burned in quite satisfactorily without breakage. Of course one cannot heat as hot as with Pyrex apparatus. If a light etching is satisfactory, the burning need be only sufficient to cause the sodium silicate marking to frost solidly. In this case, the frosting will easily wash off but will leave a distinct but fainter etching than where more heat is applied.
