Analysis for Nitrite J. Kenneth Bartlett
Southern Oregon College Ashland, Oregon 97520
by Evolution of Nitrogen A general chemistry laboratory experiment
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trend toward experiments with individual unknown samples is well established in the freshman chemistry laboratory. The numerous advantages of this approach will not be reviewed here. On the other hand, instructors who have worked with this goal in mind are aware that certain disadvantages are also encountered. In particular, a few topics do not readily lend themselves to experiments that produce the desired results without introducing major difficulties. As a result, unknowns are sometimes issued for which very poor results must he considered acceptable or which may very easily be analyzed by simple methods unrelated to the real purpose of the experiment. A gas measurement, one of the procedures which has proved to be especially hard to adapt to quantitative determination, is employed in the experiment described in this paper. Various experiments have been used through the years which require the student to submit an answer which is in some way based upon a gas measurement. Problems accompany most of these traditional experiments. Every student knows the correct value before he determines the per cent oxygen in the air; as a result, he may alter his data to fit the desired answer. The accuracy which can be expected in determining the molecular weight of a volatile liquid by vapor density measurement is, at best, so poor that the student should be expected to become discouraged. In addition, the readily available organic liquids with appropriate properties for use iu the experiment can be sorted out by the students through simple sniff tests. The weight of an unknown dry gas can be determined accurately, but this is often carried out exper mentally in sucha way that the student does little other than weigh, and with a single-pan balance this is not very taxing t o the student's imagination The equivalent weight of a metal by liberation of hydrogen is limited by the availability of samples which are not easily identifiable by other methods. The present experiment involves analysis for the sodium nitrite content of a sodium nitrite-sodium chloride mixture by decomposition to nitrogen while heating in the presence of ammonium chloride solution. Samples are inexpensive and easily prepared, the variety of sample compositions is unlimited, and analysis by an alternate method is not likely to be within the capabilities of the typical college freshman. The required calculations, including certain simplifying assumptions, tax the ability of the average student. Considering the possible sources of error in the experiment and the calculations, the attained results are surprisingly good.
The Experiment
The apparatus is assembled as illustrated. The reaction vessel is a 50-ml Erlenmeyer flask; the collection vessel is a 38 X 300-mm test tuhe. The volume of the flask plus fitted delivery tube is determined by weighing them empty and again when filled with water. The student places a 1-g sample of the nitrite A unknown in the flask and adcis 35 ml of s solution containing about 1 g of ammonium chloride and one drop of 6 JM ammonia. Mixingdissolves the sample. With the delivery tube in position, gentle heating of the flask is started so that gas is liberated st the rate of two or three bubbles per ~i~~~~ I. ~ ~ for co~~e.tian ~ second. ~ ~ The ~ solution t of nitrogen. must not be heated enough to cause foaming up into the delivery tube. The reaction is complete when a steady noise of condensingsteam is heardandno additional gas bubbles are observed. Warm or cold tap water is added to the pneumatic trough as needed to bring to room temperature and the apparatus is allowed t o stand until it can be presumed that the gas is at room temperature. The height of water column and the volume of gas in the collection vessel are measured. The student calculates the per cent sodium nitrite in the original sample using the data obtained. He is told to keep in mind the following: (1) the gas collected is a mixture of displaced air, nitrogen, and water vapor; (2) the pressure on the gas mixture differs from atmospheric pressure to the extent that a column of water is in the collection tuhe when the measurement is made; (3) upon decomposition in the presence of ammonium ion, one nitrite ion produces one nitrogen molecule.
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Comment
Since the experiment has proved satisfactory as first developed, studies of limiting conditions, side reactions, and possible modifications have not been performed. Samples ranging from 24.6% to 56.2% sodium nitrite content have been analyzed by approximately 200 students over a three-year period. The sodium chloride-sodium nitrite mixtures have remained stable during this time. Approximately 35% of the Volume 44, Number 8, August 1967
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studeuts reported answers within 5 relative per cent of the correct value and approximately 60% within 10 relative per cent. Calculation errors, which are the most common and serious errors, account for many, if
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not most,unsatisfactoryresultsreported. The author would appreciate hearing about results, problems, or modiications from any instructor who asks his students to perform this experiment.
