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VAPOR DIFFUSION METHODS IN QUALITATIVE AND QUANTITATIVE ANALYSIS' HORATIO HUGHES College of Charleston, Charleston, South Carolina
AmLmIcnL procedures of high precision based on use of the "Conway Unit" have been described by several investigator^.^ Most of the applications reported have been in connection with biochemical materials. In the method described by Wercha for the determination of acetone in blood, milk, etc., Conway's procedure is simplified and shortened. In contrast with the original procedure, in which a reaction goes to completion and the end product is titrated, in this, which may be designated as a kinetic diffusion method, the time for the reaction to reach a definite stage is measured with a stop watch. In this laboratory the method has been found useful in both detection and quantitative determination of a variety of volatile ~ubstances.~Because of the simplicity of equipment and small space required, the method seemed promising for both the quantitative and qualitative analysis courses in the detection and determination of the anions, as well as for control methods. The Conway unit is a simple glass diiusion dish 67 mm. in diameter, having two concentric compartments, the outer wall having a ground edge to form a seal with the cover. Either. liquid reagents or spot tests on paper are used. The indicating reagent is placed in the inner compartment, and the sample with reagent to form the volatile product in the outer. The reaction in the inner compartment goes through a series of visible stages. Any well-marked change in color or pattern is selected for comparison, and by timing with a stop watch it is possible to estimate concentrations over a wide range with considerable accuracy. The purpose of the continued investigation was to ascertain the limits of accuracy and to develop details of procedure for those substances for which the method is suited. AMMONIA
Conway had devised a highly precise microdiiusion method for ammonia by the slower and more elaborate procedure. Although ammonia, because of its low density, seemed unpromising for the shorter method, its importance as a key substance in the analysis of a
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This research project was made possible by a grant from the Carnegie Foundation for the Advancement of Teaching. 2 CONWAY, E. J., "Micro-DifIusion Analysis and Volumetric Error," Crosby, Lockwood and Son, Ltd.,London, 1939. a WERCR,S. C., J . Lab. Clin. Med., 25, 414-20 (1940). Hnoms, H., Preliminmy report on "Use of the Conway Unit in Semi-Micro Analysis," South Carolina Academy of Science, April, 1939.
Vertical section on line AB Donwe). Unit rith C o r n Plat.
great variety of nitrogen compounds makes it desirable that we should have a reasonably accurate method for ammonia much simpler than those commonly in use. I t became apparent at once that ammonia does not form a pattern with Nessler's reagent. This is due largely to the fact that so light a gas must first mix with the air in the apparatus, and then diffuse more or less uniformly to the surface of the reagent. Therefore it was necessary to use a comparison method, for which a series of color standards had to be devised. The light absorption for different concentrations varies both in kind and intensity as well as in the proportion of reflected and transmitted light. Thirty-five series of liquid color standards were prepared before a satisfactory and reproducible one was found. One hundred and sixty-four determinations of ammonia were made in concentrations from 0.001 to 0.100 molar (17 to 1700 parts per million) with these standards. In addition, 34 determinations over the same range of concentrations were made with spot paper in the inner compartment of the dish.
JOURNAL OF CHEMICAL EDUCATION
The liquid standards for comparison were of two types: a suspension and a solution. The suitability of a particular standard depends upon the range of concentrations to be determined. Also, a standard should be selected for which the time readings are between ten and two hundred seconds. For the larger concentrations, an aqueous suspension of ferric oxide in ferric tartrate solution gave acceptable results. For concentrations of more than a few tenths per cent, better results were obtained by diluting the sample in ratio of 100 : 1. For low concentrations (which includes the range covered by this report), a standard solution was made by mixing equal volumes of 0.01 M femc thiocyanate and 1.0 M nickel chloride solutions. The ferric thiocyanate was made by combining exactly equivalent quantities of a ferric salt and ammonium thiocyanate to form the colored Fe(CN)s--- ion. To make the spot standards strips of C.S. and S. spot paper were cut, and the corners trimmed off. These fit loosely in the inner compartment of the Conway unit. The standard is made by placing one drop of 0.1 N ferric chloride solution on the middle of the strip. When the drop is absorbed this is exposed to ammonia fumes to develop a light-brown, nearly circular spot of ferric hydroxide, 10 or 11 mm. in diameter. Excess ammonia is thoroughly evaporated. The fuming may be done conveniently in a Conway unit. Care must be taken not to contaminate any of the materials to be used, since the reagent is sensitive to 17 of ammonia in the apparatus. The reagent used is a 4 : 1 dilution of the usual Nessler's solution. When
the liquid standard is used in the comparison unit, 2 ml. of reagent is put in the inner compartment of the test unit. When the spot standard is used, one drop of the reagent is placed a t the center of a 12 X 34mm. strip of Whatman No. 1 filter paper in the inner compartment of the test unit. This drop should spread to very nearly the same size as the standard spot on spot paper. The difference in absorbing quality of the papers allows for the differencein surface tension of the solutions used. It is, of course, unnecessary to use an additional unit in the spot procedure. Although more convenient, the spot procedure has so far proved less reliable than the other. The alkaline reagent used for releasing ammonia, that recommended by Conway: is a saturated solution of potassium metaborate. One milliliter is put in the outer compartment, and the unit tilted gently to distribute the solution. The unit is placed on a piece of white unglazed paper, and the cover plate, a piece of clear glass 75 X 75 mm., is put on, leaving a space a t one side just wide enough to admit the tip of a 1.00mm. pipet for delivery of the unknown. It is convenient to tip the unit slightly by resting one side on the edge of a spare cover plate. As soon as the solution stops flowing from the pipet, the cover plate is slid into position, the unit placed level, and the stop watch started. With practice this can be done in less than one second. Even if several seconds are required, the measurement will be as reliable, provided the operation is carried out every time in precisely the same way. The color standard, either a duplicate unit containing 0.9 ml. of fluid standard in the inner compartment, or the standard ferric hydroxide spot, is placed alongside. When the depth of color in the reaction vessel matches that of the standard, the stop watch is stopped. The measured time intervals depend upon a number of variables, temperature being one of major importance. The laboratory temperature should not vary by more than one or, a t most, two degrees Centigrade during the measurement of a series. Other variables, in addition to concentration of the sample and reagents, are volumes of contents of the two compartments, the thickness of the layer through which light is transmitted, and the type of light source. For measurements of small volumes, except that of the sample, dropping pipets are sufficiently accurate. These should be made from 7 or 8 mm. tubing, with tip ground even on a rather close grained stone. The other end is slightly flared to hold a 2-ml. rubber bulb. They are calibrated by counting the number of drops of the solution to make 1.0 ml. or 1.0 gram. No attempt should be made to deliver a portion of a drop. For example, in order to deliver 0.9 ml. from an 18drop pipet, 16 drops are close enough. The student should be cautioned that the same pipet will not deliver the same number of drops per milliliter of different solutions. At least a dozen dropping pipets should be a t hand. Several types of illumination have CONWAY, E. J., b e . dl.
APRIL, 1951
been tried, the best by far heing fluorescent lamps either just over the work table or in the regular ceiling installation. PLOTTING RESULTS
Results are determined graphically by interpolation on a reference chart. The relation between dilution (reciprocal of molar concentration) and time for the reaction to reach any selected stage is nearly, but not strictly, linear. A straight-line graph is satisfactory if the range of concentrations is divided into two parts; the first from 0.100 to 0.005 molar, the second from 0.005 to 0.001 molar, and each of these he represented by a straight line. The slopes of these lines differ by a small angle. To make the reference chart, use two sheets of the commonly available letter-size graph paper divided into 24 vertical and 18 horizontal squares with sides subdivided by fifths. On one, let each division correspond to 10 units. Number the ordinates 0 to 240 units of dilution (liters per mol), and the abscissas 0 to 180 seconds. On the other sheet a smaller scale is used. Number the vertical divisions from 0 to 1200, each division corresponding to 50 units of dilution. The abscissas will have a range of 450 seconds, each division heing 25 seconds. Do not number these until the known points have been fixed. Make a solution of 0.100 M NH&1, and dilutions of this of 0.005 and 0.001 M concentration. Label these, respectively, M/10, M/200, and M/1000. Make four determinations of each, and plot the average values on the chart. The MI200 point will be the upper one on the first sheet, and the lower one on the second. For the high dilution chart, select for the M/200 value, a point near the left of the sheet, and number the ahscissas accordingly. These probably will start a t 100 and end a t 550 on the right. It is suggested that for a laboratory exercise, nitrogen samples containing not less than 0.007 per cent NH, (0.004 N) be assigned for analysis. This will eliminate preparation of the second (high dilution) chart, and time readings of more than three minutes. If determinations of dilutions between M/100 and M/200 (0.017 and 0.009 per cent NH,) are to he made, somewhat better accuracy will also the M/125 point on the reference be had by ~g graph, since the slope changes appreciably in this region. A portion of such a chart is shown in the figure. The broken lines d and d' indicate the reli-
ability of the timing with a deviation of f6 seconds at the M/125 point. The centers of the small circles show an actual series of known concentrations. It will be noted that the graph cuts the time axis a t about 5 to 7 seconds. This appears to he quite general and is interpreted as the interval that must elapse between the beginning of the reaction and the arrival of vapor a t the surface of the indicator. From this point the rate of change is that of a first-order reaction. Roughly quantitative detenninations have been made of hypochlorite, chloride, sulfite, carbonate, and cyanide ions, and methyl alcohol and formaldehyde. The procedure for hypochlorite is like that for ammonia. A strip of filter paper in the inner compartment is wet with one drop of a 0.05 M solution of potassium iodide buffered a t a pH of 5. In the outer compartment is put 1.0 ml. of 18 N (1 : 1) sulfuric acid. 1.0 ml. of hypochlorite solution is pipetted in, the cover slid into place, and the dish rotated. With a 100 : 1 dilution of the 5.25 per cent commercial sodium hypochlorite solution, corresponding to 0.007 N OC1-, a uniform brown spot appears in six seconds, and is entirely bleached except for a very pale stain in six minutes. For a 0.003 N hypochlorite solution the times are 10 seconds and 10 minutes. The test is made more delicate by adding 2 ml. of 0.2 per cent starch solution to each milliequivalent of potassium iodide used in making the indicator solution. For chloride, the procedure is similar. The indicator is the buffered starch-potassium iodide solution just described. One drop of this is put on a strip of spot paper in the inner compartment. In the outer compartment is put one ml. each of saturated potassium permanganate and 18 N sulfuric acid solution. One ml. of the chloride sample is pipetted into this. A 0.0001 M concentration (about 4 p. p. m.) of chloride produces a uniform lilac spot in three minutes. A 0.0002 to 0.0010 M solution requires about two minutes; and for a 0.005 M one, the time is less than one minute. Such rough-and-ready methods as these last are not offered for quantitative analysis, hut may serve for preliminary estimates. Research in progress leads to the expectation of several kinetic-diffusion techniques a t least as accurate as that developed for ammonia.
ERRATA In the table headed "Analogues of DDT," page 654 in the December, 1950 issue, the compound designated by RL = Rz = ~ C H I O C ~ H was A cited to melt a t 109". The correct melting point should be 89". In Table 1, page 27, in the January, 1951 issue, the precipitant for methyl ethyl ketone should be 2,4dinitrophenylhydralineand not 3,5-dinitrophenylhydrszine.
