N e w M e thod for the UItra microdetermination of Nitrogen SIR: A new method for ultramicrodetermination of nitrogen is based upon sealed tube dry combustion and the subsequent weighing of mercury which replaces the gaseous nitrogen. The method is simple, fast, accurate, and requires no special apparatus. METHOD
Reagents. Pure nitrogen-free oxygen prepared by electrolysis. Solid reagent grade sodium hydroxide. Copper wire gauze. Copper wire gauze for elementary analysis, having a diameter of about 0.15 mm. and 18 wires per cm. is heated briefly in glacial acetic acid and boiled several times with water, dried in a quartz tube and heated to 500" C. in a current of air for 5 minutes, heated to 800" C. in a current of nitrogen for 20 minutes, and cooled under nitrogen. It is kept in a clean glass stoppered tube (3). Procedure. The sample, a, is placed into a tube of quartz, Vycor, Supremax, or Pyrex 1720 glass with one end drawn out t o a fine capillary as shown in Figure 1, A . Pure oxygen is led through a t a rate of 3 to 5 cc. per minute, 50 mg. of copper wire gauze, b, and about 20 mg. of solid sodium hydroxide, c, are introduced. A split type furnace, d, a t 500" C. is drawn over the tube and allowed to stand there for 5 minutes. I t is then pushed back, and the tube is sealed off first a t point g and then immediately a t point h. At g a fine, slightly bent tip should be produced, which facilitates the breaking off described below. The sealed tube is shown in Figure 1, B. The tube is then placed in a furnace a t 700" C. for 1 hour-the whole tube should be in the hot zone of thc furnace -and allowed to cool slowly in the furnace. The sample is thus completely burned and all combustion products except the elemental nitrogen are absorbed by the reagents. Nitrogen oxides are reduced t o elemental nitrogen by the copper and the excess of oxygen is absorbed by the copper. The tube is then pushed into a thickwalled glass thimble filled with mercury as shown in Figure 1, C, so that the tip a t the lower end is broken off and mercury is drawn into the tube to point K . The capillary is now cut off a t a point about 20 mm. below the mercury meniscus and placed horizontally on the table as seen in Figure 1, D. A mark is made a t point m. The mercury is now flipped out from the capillary and the empty capillary is weighed. It is then filled to mark m with mercury as seen in Figure 1, E, with a microsyringe and reweighed. The volume of the nitrogen is then calculated from the 434
ANALYTICAL CHEMISTRY
weight of the mercury. One microliter gives a weight of 13.5 mg. If the capillary is very narrow it may be necessary to centrifuge the mercury to the bottom of it to fill it to the mark
capillaries of Supremav or Pyrex 1720 glass, but this is, of course, only a question of time. The fact that the meniscus of mercury lies in the opposite direction during the weighing as compared with the marking introduces no significant error. The height of the meniscus is very small as compared with the length of the capillary, and the error can be cancelled out with the blank-if uniform capillarics are used, and if the blank is large enough to be accurately measured. In our experiments the blank has been less than 0.1 PI. The method is considerably faster than most earlier described methods for the ultramicrodetermination of nitrogen. It is based upon reliable quantitative reactions and determines all nitrogen regardless of the chemical con-
m. DISCUSSION
Results of representative analyses are given in Table I. For routine use it is probably better to use prefabricated capillaries and seal them to the combustion tube instead of drawing out the latter. *4 more uniform capillary will thus be obtained which might further improve the accuracy of the method because a safer counterbalancing of the capillary depression during the marking procedure, Figure 1, D, will be obtained. I t has been difficult to obtain suitable
A -
f
a
x
e
D 1
I
m
t -
I
m
Figure 1. nitrogen A.
Apparatus for ultramicrodetermination of
Charging the combustion tube. a = sample, b = capper wire gauze, c = sodium hydroxide, d = furnace, 500' C., e = capillary, inner diameter about 0.5 mm., length about 60 mm., f = combustion tube, outer diameter 6 mm., wall thickness 0.6 to 1 mm., g and h = points where tube i s sealed off 6. Sealed tube ready for combustion C. Collection of nitrogen in capillary k = location of meniscus D. Marking of capillary m = location of mark E. Capillary filled with mercury m = location of mark
Table I.
Substance Acetanilide Acetanilide Acetanilide Trifluoroacetanilide 3,5Dinitrobenzoic
Weight Weight of of Sample, Mercury, pg. Mg. 86.3 59.0 97.3 67.9 83.8
Lussac around 1825 ( I ) and by Frerichs in 1877 (2). Both methods worked with dry combustion of large samples in closed systems.
Representative Analyses
103.4 72.1 122.0 62.0 135.6
Nitrogen Found rg.
70
8.77 6.07 10.13 5.20 11.32
10.16 10.29 10.41 7.65 13.50
Calcd ,
Deviation,
%
ACKNOWLEDGMENT
10.37 10.37 10.37 7.41 13.21
-0.21 -0.08 +0.04 f0.24 fO .29
This work was made possible by a grant from the Swedish Medical Research Council.
%
acid
46.32 46.65 -0.33 200.2 16.63 35.9 Urea 36.81 36.83 -0.02 184.2 15.39 Thiourea 41.8 Sodium-ammonium 31.6 26.3 2.19 6.93 6.70 +0.23 phosphate.4HzO The samples were weighed out on a Rodder Model E ultramicro quartz fiber balance.
stitution of the sample. It was found useful also for decimilligram and even milligram samples for use with the combustion technique of Kirsten (3). Sodium hydroxide must, of course, be
added to the other reagents in the tube. Combustion methods for the determination of nitrogen and of carbon, hydrogen, and nitrogen, respectively, have been published by Liebig and Guy
LITERATURE CITED
(1) Dennstedt, M., “Die Entwick$mg
der organischen Elementaranalyse, pp.
30 et seq., Hamburg, 1899. (2) Frerichs, F., Ber. deut. chem. Ges. 10, 26 (1877). (3) Xirsten, Wolfgang J., 2. anal. Chem. 181, l ( 1 9 6 1 ) .
KEIICHIRO HOZUMI WOLFGANG J. KIRSTEN Institute of Medical Chemistry University of Uppsala Uppsala, Sweden RECEIVED for review November 20, 1961 Accepted December 22, 1961.
Determination of Chromium and Copper on Trimetallic Lithographic Printing Plates SIR: -4 coulometric method (3) is used a t times in the printing industry for the determination of chromium thickness on trimetallic (chromium on copper on steel) lithographic printing plates. To confirm chromium thickness values on samples received from the platers, a n accurate volumetric method was desired. A method has been proposed for the determination of chromium on nickel (4), based on the use of a n iodine-HC1 stripping solution; however, this procedure is not suitable for trimetallic plates because iodine attacks the copper coating thereby affecting the chromium and copper results. ThP method described below requires no special equipment and can he employed for the determination of thicknesses of both the chromium and copper platings. The procedure i$ based on standard dichromate and iodometric titrations (2, 5 ) . PROCEDURE
Preliminary. T h e determinations are generally made only on the printing side of the plate. Select a sample area somewhat greater t h a n 1 square inch. If chromium alone is t o be determined, remol‘e and discard the chromium coating on the opposite side of the plate by placing warm dilute HCl (SAr) on it until the copper coating is completely exposed. If both chromium and copper are to be
determined, remove these coatings from the opposite side by swabbing with dilute HC1 containing a few drops of 30% hydrogen peroxide. Steel-wooling facilitates complete removal of the copper coating. Wash with water and dry. Cut or punch out a sample area of 1.00 square inch. The sample is now ready for analysis. Chromium. Immerse the sample in 20 ml. of dilute perchloric acid (70% HC10, a n equal volume of water) warming gently until the chromium dissolves and the copper coating is completely exposed. Transfer the solution plus aashings to a 250-mi. Vycor flask and heat until heavy .crhite fumes of HClOl finally reach t h e neck of the flask. At this point, Cr(II1) will have been oxidized quantitatively to Cr(VI). Remove the flask from the hot plate and plunge it immediately into ice mater. Add 50 ml. of water plus 2 grams of solid sodium bicarbonate and boil gently for 5 minutes to evolve chlorine ( I ) . Cool the solution to room temperature. Add a measurrd excess of 0.100,V ferrous ammonium sulfate containing 10 ml. of concentrated H2S04 per liter. Mohr’s salt is sufficiently pure for use as a primary standard in this mrthod (6). Kext, add 0.3 ml. of 0 05% sodium diphenylamine sulfonate and 10 ml. of sirupy phosphoric acid. Back titrate with standard 0.100N Ii2Cr207 solution to a purple end point. The weight of chromium is equal to the area X thickness X density; conbequently, when a 1.OO-square in( h qample
+
is taken for analysis, the thickness of the chromium coating in microinches is given by Cr
solution x 14.9
Copper. Place the above sample which has been analyzed for chromium in a dry 50-ml. beaker and cover with 5 ml. of concentrated nitric acid. ils soon as the copper dissolves, immediately remove the metal sample with a glass-encased magnet and mash quickly with absolute methanol. Do not use water in lieu of methanol. Transfer the liquid to a 250-ml. Erlenmeyer flask and boil for 5 minutes to evolve oxides of nitrogen. Cool and add concentrated ammonia dropwise until the blue color of the cupric ammonia complex just persists. Add 2 grams of solid ammonium bifluoride and dilute to 100 ml. Following this, add 2 grams of solid potassium iodide and titrate immediately with standard 0.1N sodium thiosulfate reagent. until the brown color of the solution changrs to a cream color. At this point, add 1 gram of ammonium thiocyanste followed by 2 ml. of 0.1% starch solution and continue the titration to the sharp disappearance of the blue color. The weight of the copper coating is equal to the area X thickness X density; hence, when a 1.00-square inch sample is taken for analysis, the thickness of copper in microinches is given by Cu thicknes6~mlcrolnLhes) = titer value X ml. thiosulfate X 6.8 VOL. 3 4 , NO. 3, M A R C H 1962
e
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