Routine Determination of Equivalent Acidity or Basicity of Fertilizers E. W.CONSTABLE, North Carolina Department of Agriculture, Raleigh, N . C..
A
KUMBER of states a n d agencies have interested themselves i n the analysis of fertilizers for acid-forming qualities, but none, so far as t h e information at hand shows, to t h e extent required t o carry the program of t h e Department of Agriculture i n S o r t h Carolina. This state is t h e first to require the determination, individually, of acid-base balance on all officially collected samples of mixed fertilizers. This requirement necessitated extended routine F o r k and it was in order t o expedite this routine t h a t the assembly nonbeing used was devised. In principle the Pierre method (2) as prescribed by t h e Association of Official Agricultural Chemists (1) is followed, but in lieu of the specified apparatus there has been substituted an assembly designed t o reduce work a n d personnel t o a minimum, to curtail tedium, and t o incorporate flexibility. The Pierre method consists of treating weighed samples of fertilizers with sodium carbonate solution; evaporating and burning at moderate muffle temperatures to destroy organics and remove nitrogen; treating with water, hydrochloric acid, and heat t o drive off carbon dioxide; and filtering t o remove debris to enable colorimetric titration with alkali. From this titer, in combination with results from previously established routine analyses for nitrogen and phosphate, t h e final results are calculated. The assembly, characterized by mass processing, automatic measuring and control, and electrometric titrating, has effected the speed and economies sought.
SECTIOX OF LABORATORY, SHOWING FILTERING ASSEMBLY 214
Assembly Containers for weighed samples are Pyrex beakers, Griffin. low-form with lips, of 150-ml. capacity. These, arranged in four rows of seven each, 28 per set, are placed in shallow Chrome1 wire baskets with handles, wherein they remain throughout all handling and processing up to the point of filtering, if colorimetric titration is to be used, or all the way through to titration if thib is to be done electrometrically. The advantages of handling the beakers in numbers are obvious. Automatic or self-leveling pipets of 10-ml. and 30-ml. volumes, respectively, having a maximum deviation of 0.04 ml. for the former and 0.05 ml. for the latter, are used for adding sodium carbonate and hydrochloric acid. The crossbars of the petcock plugs have one slightly elongated end to indicate direction of flow. The pipets, as \vel1 as the buret described below, have an added tip guard inside the overflonbulbs to prevent accidental spraying of solutions through the vents. The pipets, attached rigidly to 75-em. (30-inch) lengths of glass tubing which s v h g from rubber siphon hoses drawing reagents from overhead 19-liter (5 gallon) bottles, hang in batteries of four each t o match the four roxs of beakers in the baskets; the pipet tips reach just into and bear against the beaker lips for discharging and contact draining. hlanipulation is progressively across the baskets and from end to end, the baskets being slid along the table as needed to maintain suitable relation. Ovwflolv from leveling the pipets is carried to receivers by small rubber hoses of sufficient length to afford freedom of manipulation, but with anchored discharge ends. When not in use, the pipets are held out of the way by hooks on reagent shelves. Filling is much quicker than for hand-transfer pipets. Discharge and draining, which are held to standard, proceed unattended. The arrangement enables very rapid and accurate transfer of large quantities of reagents. For evaporation, the baskets are transferred to a hot plate. Filter-paper cones, inserted into the beakers u.ith tips down and points not clipped, are used to prevent spattering. This step is continued until the papers are heavily brovned. For burning, the heated baskets are slid into a large-sized electric muffle furnace, the floor of which is at the same level as the hot plate. The furnace is automatically maintained at constant temperature by a pyrometer control assembly n-hich requires no more attention than conventional drying ovens. .4 time snitch may be added, which will permit the termination of burning unattended and therefore the burning of an additional charge beyond the end of a norking period. A siphon is used for the addition of water to approximate quantitv previous to the hydrochloric acid treatment. For expelling carbon dioxide the baskets of beakers, rvith covers in place, are returned as units to the hot plate. FILTRATION. When the material is to be filtered, in order to permit the employment, of colorimetric titration-a necessity because of debris from processing which produces turbidity of solutions-an assembly is used consisting of fourteen KO.1 Buchner funnels of 100-ml. capacity; 5.5-em. filter papers; and large-mouthed (3.5-cm.) Pyrex Erlenmeyer flasks, 250-ml.. fitted with No. 8 crepe rubber stoppers. Funnels and rightangle glass suction tubes are inserted through the stoppers. The filter rack is a 2.5 X 3.75 em. (1 x 1.5 inch) wood bar Kith 14 holes, tapered outiyard, evenly spaced along its center, through which protrude the ends of slightly larger rubber suction hose3 leading to vacuum cocks, which in turn are mounted on reagentshelf uprights in the usual manner. The rack, attached to pivoted extensions from the reagent-shelf uprights, lifts up to permit placement of flasks for receiving filtrates. Stops are provided to maintain it a t the working position. The flasks (used for both filtering and titrating), in sets of 28two r o w of 14 each-are contained in suitable wooden trays. Upon completion of the filtering and washing of 14 samples, the flask stoppers are loosened, the entire filtering assembly 15 lifted as a unit, and the tray is shifted to bring the remaining 14 flasks of a set into position for receiving filtrates. Adequate washing is determined by pressing bits of blue litmus upon the
APRIL 1.5, 1940
ANALYTICAL EDITION
315 T h e greatest advantage of electrometric titration is the elimination of filtering and mashing, with their inherent possi bilitg of introducing error. There is also a gain in speed and accuracy, since titration can be followed progressively and end points r u n closer. The maximum capacity of the laboratory, approximately 100 samples per work day, is set b y h o t plate and furnace capacity, and could be extended to about 150 by the addition of a time switch. This laboratory, on occasion of exceptional rush, has maintained a n average of above 200 samples per d a y for 22 working days by using additional hot plates and furnaces. However, the chief advantage of t h e assembly is the speed and facility with which a limited personnel can carry on t h e day’s work. The accuracy and reliability of the systeni, as attested by checking, by comparison with results from other laboratories, and b y thousands of analyses, are satisfactory.
METHOD OF HANDLING PIPETS ASD BEAKERS
filters. Filter papers are used repeatedly to the point of slowing filtration. This filtration assembly has the advantage of enabling the emptying of entire beaker contents into funnels in a single operation, two beakers at a time if desired, and liken-ise for rinsing beakers (siphon used here). It eliminates side-arm filter flasks, or the transfer of filtrates, and facilitates the handling of funnels. Filtering, however, except where compliance with the ”letter of the law” is wished, is eliminated bj- the employment of electrometric titration. For titration a 25-ml. self-leveling or automatic TITRATION. Schellbach buret of 0.05-ml. maximum deviation is used. This is convenient and rapid in filling and leveling, and, since it is refilled for each titration, gives final readings wit>hout subtractions. As an indicator, the bromocresol green-methyl orange mixture recently developed by Pierre, Tully, and Ashburn ( 3 ) , in double strength, fed by dropping siphon, is used in connection with the daylight lamp as prescribed. For electrometric titration, an assembly consisting of pH meter, calomel and glass electrodes, thermometer, and motor stirrer is used. The pH meter is the direct-reading type in which balancing and key tapping are eliminated-that is, the potential that is developed at the electrodes is pointed out directly in pH units. The assembly is made of stock laboratory items, requiring no special ivork in preparation, and a 25 per cent saving in time and work can be attributed to its use. The electrode assembly-electrodes, thermometer, and motor Atirrer-is mounted rigidly on a vertical rod. Beakers containing the unfiltered reaction mixture are held in position by a swinging 5helf attached to this rod. The swinging of the shelf operates the on and off switch of the stirrer motor. Beakers are lifted from baskets to the proper position on the electrode assembly, the supporting shelf is swung into position, the t a p button of the pH meter is locked in the down position, and sodium hydroxide solution is fed into the rapidly stirred reaction mixture until the indicator needle has shifted to indicate the pH accepted as the end point. The meter is standardized daily and repeatedly checked n-ith a standard buffer solution having a pH near t,hat taken as an end point. Temperature corrections for the general laboratory temperature range are found negligible. The washing of electrodes, except for standardizing, has been found needless. The buret, a st,andard stock type for pinchcock delivery, i i mounted conveniently off-side and is controlled ii-ith a rigidly held, extension-mounted pinchcock, Delivery of reagent to beakers is by an offset tip.
ELECTROYETRIC TITRATION ASSEMBLY
Summary An assembly for reducing t o a minimum the routine work and personnel necessary for t h e determination of the acidbase balance of fertilizers on a n extended scale is described. Characterized b y mass processing, automatic measuring and control, find electrometric titration, the design has effected speed and economy.
Literature Cited (1) -4ssoc. Official Agr. Chem., Official and Tentative Methods of Analysis, 4th ed., pp. 34-5 (1935). (2) Pierre, W. H., IND. ESG. CHEar., Anal. Ed., 5, 228 (1933). (3) Pierre, W. H., Tully, Kelson, and Ashburn, H. V., I b i d . , 10,72-6
(1938). PRESEXTED before t h e Division of Fertilizer Chemistry at of the American Chemicai Society, Boston, Mass.
the
98th
3Ieeting
