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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.
