V O L U M E 23, N O . 6, J U N E 1 9 5 1
927
CONCLUSIONS
The selection of a ferroin-type indicator suitable for the oxidimetric determination of iron in 0.1 to 4 or 6 F sulfuric or hydrochloric acid is now possible. The proper ferroin dye base can be selected to give a ferrous sulfate complex indicator Fvith any oxidation potential from 0.82 to 1.10 volts in increments of 0.02 to 0.04 volt ( 1 ) . I n every case the molecular extinct,ion coefficient is between 11,100 and 14,500 and therefore an intensely colored indicator is assured. All selections give rise to complex indicators of favorable instabilitj constants. A potentiometric titration of the reaction in question is the only necessary prerequisite to the selection of a suitable indicator. For every reaction condition, because of the marked change in oxidation potential of t.he e- = Fee+, and Cr+6 3e- = two half-cell reactions, Fe++’ Crt3, a potential is defined at which the change in e.ni.f. with added increment of oxidant is the maximum. The oxidation potential of the indicator selected should correspond precisely with this valur to provide maximum proficiency. The visual indication of indicator t,ransition is somewhat higher than the potentiometrically defined oxidation potential (0.05 to 0.06 volt). This
+
+
value is not of sufficient magnitude t o require a correction blank in titrational macroprocedures. Potentiometric study of the oxidation of ferrous iron in 0.1 F perchloric acid has shown that the two half-cells involved givp the following formal potentials: F e + + + e- = F e + + (am.f. = 0.735 volt) C r + + + + + +3e- = C e + + +(e.m.f. = 0.84 volt)
+
At these values the equivalence point break is practically negligible (0.01 to 0.02 volts), and the determination of iron under these conditions is impossihle. LITERATURE CITED
(1) Brandt and Smith, ASAL. CHEY.,21, 1313 (1949). (2) Salomon, Gabrio, and Smith, d r c h . Bzoehem., 11,433 (1946). (3) Smith and Brandt, ha^. CHEY.,21,948 (1949). (4) Smith and Fritz, Ibid., 20, 874 (1948). ( 5 ) Smith and Richter, “Phenanthroline and Substituted Phenanthro-
line Indicators,” Columbus, Ohio, G. Frederick Smith Chemical Co., 1944. (6) Stockdale, D., dnalyat, 75, 150 (19.50). RECEIVED July 14, 1950.
Ultramicronitrometer for Use in Determination of Nitrogen in Mineral Oil WOLFGANG KIRSTEN, Institute of Medical Chemistry, University of C‘ppsala, Sweden AND
BIRGIT WALLBERG-OLAUSSOK, A. Johnson & Co., Oil Refinery, .\yniishamn, Sweden I\‘ THE course of the analysis of mineral oils, high values of
I nitrogen were frequently obtained with the common Dumas apparatus due to incomplete combustion. These difficulties were readily overcome by using the modified Dumas method described by Kirsten (1,2’). However, the quantity of nitrogen in the analyzed oils was so small that it was impossible to get good readings with the common micronitrometer. As it was difficult to burn larger samples than about 60 to 70 mg. of motor oil because of the formation of tar and hard-burnt carbon in the capsule, a nrw nitrometer was constructed which allows the measurement of smaller quantities of nitrogen than the type commonly used. The nitrometer is shown in Figure 1. The graded part is a capillary with a length of ca. 140 mm. and a capacity of 0.2 ml.; s3 is a three-way sto cock with one connection to the leveling bulb, al,which is fillecfwith mercury. S t o p cock S I leads from the bottom of the nitrometer. The leveling bulb, a*, is filled a i t h potassium hydroxide. PROCEDURE
The capsule is filled with copper oxide grains up to the ground joint. The grains are covered with a layer of copper oxide powder. A 50-mg. sample of oil is weighed into a platinum or nickel boat and the boat is filled with copper oxide powder. The capsule is kept in a horizontal position and the boat is put inside the ground joint of the capsule by means of a pair of tweezers. The capsule is then turned upright and a mixture of copper oxide grains and powder is poured upon and around the boat until the ground joint is filled. Then the rest of the capsule is filled with copper oxide grains. Apparatus and nitrometer are now swe t with carbon dioxide. Residual nitrogen is taken out from tge nitrometer by raising a2 and turning s3 in such a manner that the nitrogen passes out into a funnel, u. After closing sa, a2 is lowered, al is raised, and s3 is turned so that mercury from al passes down the graded capillary and some drops fall down to the bottom of the nitrometer. Then s3 is closed and al is placed upon the table. The capsule with sample is put into the apparatus and the sweeping and combustion are carried out (b). The nitrogen of the sample is accumulated under the mercury. After sweeping, sg is opened cautiously, so that the mercury slowly passes up and partly into al. The nitrogen follows ,the mercury up into the graded part of the capillary. Then sg is closed and the quantity of nitrogen is read after 20 minutes. When reading, the length between the two menisci is taken for the calculation. Possible
errors from the manner of reading are compensated, as the blanks are read in the same manner.
As the quantities of nitrogen are very small, special attention must be paid to possible errors. The dry ice in the thermos flask should be of good quality and a blank without copper oxide should be run after every filling of the flask in order to check it. With the present apparatus the value of the blank lies a t about 0.002 to 0.003 ml. of nitrogen, when the dry ice is of good quality. The copper oxide in the capsule should be used only once. Higher and somewhat irregular values were obtained when the copper oxide had been regenerated for further analyses, probably because of changes in the surface structure caused by the reduction and reoxidation. With good reagents the total value of the blank lies between 0.005 and 0.007 ml. of nitrogen. The filling of the combustion tube has to be reoxidized after about ten analyses by 53 leading a stream of oxygen through the hot apparatus for about 2 hours. After the tube is kept a t working temperature (1000 O C.) for 1hour in the carbon dioxide stream it can immediately be used again for the analysis of compounds containing nitrogen not bound to oxygen. After one analysis of mineral oil so much nickel oxide has been reduced that the tube can be used also for analysis of samples that give off nitrogen oxides during comhustion. When the quantity of mercury on the bottom of the Figure 1. Nitrometer nitrometer has increased after
ANALYTICAL CHEMISTRY
928 -.
Table I. Analyses of Mineral Oil Weight o! Bample.
S?,
Borrnal Pressure N Found. Mg. and Temp., MI. 5% A 42.7 0,0022 0.006 45.9 0,0023 0.006 B 40.3 * 0.0020 0.006 41.7 0.0020 0.006 40.2 0.0023 0.007 44.5 0.0027 0,008 D 44.0 0.0027 0.008 42.4 0.0026 0.008 47.3 0.0027 0,007 E 45.5 0 0036 0,010 40.7 0,0036 0.011 I; 0 0033 0.011 39.3 41.2 0.0032 0.010 c; 47.6 0 0040 0.011 41.2 0.0040 0.012 tI 42.3 0.0065 0.019 41.8 0.0074 0.022 I 42.6. 0.0081 0 024 41.6 0.0083 0.025 R 40.5 0.0089 0.028 43.5 0,0098 0,028 L 38.5 0.1624 0.527 38.0 0,1647 0 512 .Ill a n a h e * were c a r r i f , ~ rli:i by oiie of t h e authors (B. W,),
Oil
c
-___
~~~-
~
several analyses, az i p raised and the excessof the mercury is :illowed to flow out by opening 84. When the m o u n t of barium hydroxide specified by Pregl(3) is added to the solution of the potassium hydroxide available to the authors, an excem of barium hydroxide goes into solution, and when tlhe liquid ie used in the nitrometers this excess is pre-
cipitated barium carbonate. This can be avoided by using less barium hydroxide, but in this case the liquid seem to retain a tendency to foam. Satisfactory nonfoaming potassium hydroxide was obtained by dissolving the quantity of barium hydroxide specified by Pregl ( 3 )in the solution and precipitating the excew by adding some dry ice. After the precipitate has settled, the liquid is decanted and filtered through a sintered-glass funnel, or still better centrifuged. When the gas bubbles in the nitrometer tend to stick to the mercury, a trace of carbon disulfide is added to the potassium hydroxide in the leveling bulb and the latter i shaken. After a few minutes the carbon disulfide hap dissolved and is n-ashed down into the nitrometer. A thin layer of a black precipitate is formed on the mercury, which effectively preventa the sticking of the bubbles. The stopcocks or stoppers of the nitrometers should be lubricated only with v a d i n e and never with iiilicone grease, as the latter s e e m to cause foaming. ACKNOWLEDGMmT
The authors are indebted to Einar Stenhagen for reviewing the paper and to Lars Finn for drawing the figure. The apparatus may be obtained from Norstedt & Sijner, Stockholm. LITERATURE CITED
(1) Kifsten, W., ANAL.CHEY.,19,925 (1947). (2)Ibld,, 22, 358 (1950). (3) Pregl-Grant, “Quantitative Organic Micro-iysir,” London, J. &- A. Churchill, 1945. .kpril
1950,
Modified Vacuum Fusion Apparatus for Determination of Oxygen, Hydrogen, and Nitrogen in Certain Metals A. F. TORRISI AND JEAN L. KERNAHIN’ Transforrrier & :lllied Products Laboratory. General Electric Co., Pittsjield, Mass.
HE vacuum fusion method for the determination of hydrorgen, oxygen, and nitrogen in metals has been modified to increase its rate of output. This should enable this valuable method of analysis to receive more widespread use. The changes suggested are in the construction of the apparatus rather than in the theoretical principlw of the method. With these revisions the vacuum fusion apparatus becomes a more easily assembled piece of laboratory equipment, less evpemive to construct and to maintain than the basic apparatus deqcribed by Slexandcr, liurray. and rlhhley (1). Three major strurtural changes were made: A meltin chamber similar to that introduced by Guldner and B e a h f2) wy Bubstituted Ground-glass joints were locakd a t strategic points throughout the apparatus (note Figure 1) to facilitate assembling, repairing, and cleaning of the equipment. Two measuring systems were attached to the single melting furnace and the pum ing system. Each is a complete unit, consisting of a mercury &flusion pump, oxidizing furnace, trap, and calibrated volume. This enables one operator to analyze twice as many samples per day as with the ordinary single-unit system. This important c o s t and timesaving feature makes the vacuuni Yufiion method a more practical tool for many laboratories
To facilitate the most advantageous use of laboratory space and equipment, the apparatus has been installed on a small ta1)Ie (3.5 X 2 X 2.5 feet) on casters which enable the equipment to be rolled to the less easily moved 5-kv-amp. oscillator. I n many laboratories, several other kinds of vacuum equipment mwt he euclP
Prkwnt addreas Chcmiqrrs Deparriwnt, Cornall Uni\-ersity. Itlmca 3-.Y.
Figure 1.
Schematic Diagram
p. 72,
