Analysis of Sodium Nitrite by Sulfamic Acid ' A Gasometric Analysis
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ROBERT C. BRASTED University of Hawaii, Honolulu, T. H.
T HAS long been the opinion of the author that.the student in the &st semester of quantitative analysis supervision was given the set of directions, which are should have an opportunity to deal with a t least one noted below, with highly satisfactory results. The time precision-type gasometric analysis. The previous ex- allotted for the construction of the apparatus and the perience of the student with this type of measurement determination was found to be legs than that allotted has probably been only in the laboratory of general to the more com'mon analyses. freshman chemistry. Samples of sodium nitrite for student analysis were The instantaneous reaction between nitrites and sul- prepared by the author, using weighed quantities of famic acid has been known for some time. Baumgarten sodium nitrite dried a t 110°C. and sodium chloride of and Marggratfl utilized this diazotization type of reac- analytical purity as a diluent. The samples are stable tion for both gravimetric and gasometric estimations of under ordinary oven-drying conditions, and an accunitrites. Their emphasis was placed mgre particularly racy of 0.1 to 0.2 per cent may be expected. on analysis of nitrites in the presence of nitrates. GraviDIRECTIONS metric and gasometric data indicate an accuracy well within the limits generally prescribed. The reaction Introduction: For some years it has been known that sulfamic acid (HSOsNH2) would react instantaneously MNOn + HSOsNH2 MHSO, H2O 4- N 9 and quantitatively with nitrite nitrogen according to suggests the gravimetric analysis by precipitation of the the following equation : sulfate with barium ion and the gasometric analysis by HSOINHl + NaNOl Ne NaHSOa Hs0 measurement of the nitrogen evolved. The entire apparatus is constructed by the student Baumgarten and Marggraff' have utilized this reaction from material in his own desk or which is readily ob- in two ways for the analysis of nitrites. A gravimetric tainable from the stock room. No little pride is expe- method involves the precipitation of the sulfate by barrienced by the student when an apparatus of his own ium chloride and the gasometric analysis involves the construction proves to be a precisioninstrument. The measurement of the nitrogen formed. Since no reaction practice in preparing solutions of limiting reagents, cal- results from the treatment of a nitrate in dilute soluculations involving gaseous systems, manipulation of tions with sulfamic acid, a useful differentiation between apparatus heretofore not familiar, and the use of the nitrites and nitrates is suggested from both a qualitahighly versatile chemical, sulfamic acid, were all points tive and a quantitative standpoint. Certain advanof worth-while benefit from a pedagogical point of view. tagesof the gasometric analysis over either a volumetric An averace - " section of students not under the author's or a gravimetric method are notable. No standard solutions are necessary, no indicator is needed, no weighed BAUMGARTEN, P., AND I. MARGGRAPP, Ber. 63,1019 (1930).
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crucibles are needed, and the apparatus for the gasometric method is easily prepared. Procedure: Using the figure as a guide, construct the apparatus from equipment found in the desk or obtained from the storeroom. The reaction vessel A is a 250-ml. suction flask. Any larger size is not recommended. The rod B is a heavy glass stirring rod bent at the end to 90' to facilitate lowering the vial C (empty litmus bottle) to the bottom of the reaction flask. A solid stopper may be substituted a t H, using the rod B independently as a lowering mechanism. A rubber band or a thin section of rubber tubing around the top of the vial will prevent slippage of the copper wire handle that is to be secured to the vial. Delivery tube D may be slightly constricted to fit the size 00 rubber stopper for the Mohr buret F. A tight fit of the 00 stopper a t E may be assured by looping a rubber band around the delivery tube and securing i t to a copper wire coil and hook around the buret. The leveling bulb G may be a Gooch funnel, ordinary funnel of a t least 60-ml. capacity, or a separatory funnel. A light stopcock grease applied to the 00 stopper will be found advisable to prevent gas leaks. Do not grease the stofiper at H since this may result in slippage of the stopper during the determination with an over-all change in volume of the apparatus. A check should be made for gas leakage before starting a determination. Lower the leveling bulb against a tight system, creating a low pressure within the apparatus. Constant readings of the meniscus over a ten-minute period will assure a gas-tight system. Prepare a 50-ml. portion of sulfamic acid solution containing approximately 0.1 mol. A 10-ml. portion of thm solution should provide at least twice the sulfamic acid necessary for a complete reaction with the unknown nitrite assuming 100 per cent sodium nitrite. The unknown should be dried for a t least one hour a t 110°C. Again basing the calculations on 100 per cent sodium nitrite, prepare a 250-ml. aliquot so that a 25-ml. portion will liberate no more than 45 ml. of nitrogen. Pipet a 25-ml. aliquot of the unknown solution into the reaction flask. A 10-ml. portion of the sulfamic acid is then transferred to the vial and lowered to the bottom of the flask by rod B. An adjustment of the length of rod extending below the stopper may be necessary. The leveling bulb is raised nearly to the zero mark of the buret, the stopper H set M y , and the leveling bulb so adjusted that atmospheric pressure exists within the system. The initial volnme and the temperature are noted. The vial is upset by tilting the apparatus, and the solutions are allowed to mix. Slight motion of the ring stand will result in swirling of the solution in the flask. A complete reaction should take no more than a minute, as evidenced by no further evolution of bnbbles of nitrogen. Since the reaction is exothermic, the slight elevation of temperature may be rectified by cooling the flask with a beaker of water which has been allowed to assume room temperature. The bulb is lowered slowly until the pressure is again
equalized and the temperatye and volnme noted. Make several readings over a ten-minute period to ascertain the establishment of equilibrium. Avoid rapid raising and lowering of the leveling bulb. (Why?) Since drafts of warm or cold air will cause errors, the apparatus should be so placed that this condition will be minimized. The use of the average temperature and volnme readings will minimize this error. The refinement of a jacketed Mohr buret may be added with very little trouble, but results of sufficient accuracy are possible without this if excessive temperature variations within the laboratory can be avoided. Run four or five aliquot portions, rinsing out the flask before each run. From data recorded and correcting for the vapor pressure of water, calculate the number of mols of nitrogen evolved, the weight of nitrogen, and the percentage of sodium nitrite in the unknown sample.
