A HIGHLY ACCURATE, RAPID-ACTION, MICRO OR MACRO BURET JAY
E. TAYLOR'
Miami University, Oxford, Ohio
A B U R E T W ~ may ~ C ~ heusedfor veryrapid titrations and which, at the same time, operateswnthexceptionally high accuracy was desired by the author for certain kinetic studies. The buret described has answered both of these qualifications. I n addition it may be built with micro or macro dimensions, or i t may be used for the dispensing of liquids unstable in the atmosphere. Once put into use it needs little attention over long periods of time. It is useful only for routine titrations, however. The buret embodies the principle of replacement of the solution by a volume of dry mercury measured in a graduated buret barrel. Since a number of mercury burets have been described in the literature, the author is not a t all certain of the extent of originality of the design presented in this paper.
Figure 1 . The Complete Burst A. 3-way stopcock leading to a vaouurn source and the atmosphere; B, calibrated buret barrel; C, reservoir tank containing mercury and o-dichlorobenzene tamed by the standard solution: D. inlet for the standard solution:. E.. titration outlet:. F.. mound elass . ioint:, G., manometer. ~~~~~~~. -
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The drawing of the buret (Figure 1) shows the details of construction. Stopcock A leads either to a wcuum Pump or to the atmosphere and is connected to the Tygon tubing. The graduated barrel is attached to tube C by a standard-taper joint a t 8'; this allows different barrels to be substituted without otherwise disturbing the apparatus. The buret as shown is for Present address: Department of Chemistry, The 0 h i o University, Columbus 10, Ohio.
use with standard sodium thiosulfate. Since this reagent attacks mercury, presumably with formation of mercury sulfides, the solution is protected from the mercury by a layer of o-dichlorobenzene. This liquid was chosen because of its relative availability, its high density, its insolubility, and its inertness. The layer of odichlorobenzene should be of sufficient thickness to protect the mercury from contact with the wetted sides of tube C. Stopcock D is provided for intake of the standard solution from a reservoir and stopcock E for the outlet. Since these stopcocks are subjected t o pressures of one atmosphere for short intervals, it is important to use pressure stopcocks and a grease of high consistency. Silicon high-vacuum grease is satisfactory since the tendency of this grease to creep does not affect the efficiency of the buret. Only very pure mercury should be used. If the mercury contains substances which are easily oxidized, the calibrated barrel will become coated with metal oxides and require cleaning at regular intervals. With pure mercury the buret need not be disassembled for long periods of time. It is advisable not to allow the mercury to stand in the calibrated part of the buret when it is not in use. The only attention required by this apparatus is to regrease the stopcocks as needed. This is easily done without draining the standard solution. Tube C should be about three times the volume of the calibrated barrel B, and the size of the latter is dependent only on the requirements of the user. Burets of 40-ml. and 5-ml. capacities have been used in this laboratory with results of equal accuracy; much smaller burets also should be accurate. The advantage of the design is particularly evident when the capacity is small. The buret is extremely rapid. Since there is no possibility of error due to uneven wetting of the glass as in an ordinary buret, it is allowable to have the opening of the tip sufficiently large to allow drainage in 10 or 15 seconds or even less. A coatine of antiwettine agent on the outside of the tip will prevent inaccuracies due to wettirlgby the standard solution. The buret should contain more than mercnrv to fill the calibrated barrel and a t least, an
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solution should fill all the remaining space, and there should be no air bubbles anywhere in the buret. ~ h procedure , for using the buret is quite simple. T~ fill it, stopcock E is closed, D is opened, and A is turned to the vacuum, causing the mercury to be drawn into the barrel B. Fine adjustment to the zero mark
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AUGUST, 1953
can be made by squeezing the tubing connecting D with the reservoir. Standard weight tubing which collapses when evacuated connects A and B and G to prevent over-shooting of the mercury. Next, stopcock D is closed, A is reversed, and the standard solution is dispensed through stopcock E as rapidly as desired. Due to the compressibility of the liquids in tube C and the possibility of undiscovered air bubbles in the system, each observation of the mercury column should, for greatest accuracy, he made a t equalized pressures in tube C. This is easily done by using the manometer shown a t G. When a reading is to he taken, the system is evacuated until the height of the mercury column in G equals approximately the height of the column in B. Even without using the manometer the error is relatively small, and it may be practically eliminated by making rough corrections. The usual corrections due to temperature are necessary. It is important that the time lapse between filling to zero and emptying and reading the buret should be short, particularly if the room temperature is changing. Otherwise a temperature correction must be made for the organic liquid as well as for the aqueous solution. For standard solutions which are not attacked by mercury the protective layer of o-dichlorobenzene may be omitted. Certain solutions such as standard iodine cannot be used in the buret as described since the organic solvent will extract the reagent. For use with iodine a substitate for tube C has been designed, as shown in Figure
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2: A glass syringe tube and accurately fitting plunger H replace the o-dichlorobenzene, The plunger, lubricated with but a trace of light-weight and unreactive grease, should effectively prevent contact of the mercury and iodine or other reagent.
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2. Rese~voir Tube forUsewith I o d i n e S o l u t i o ~
H,glaaa eyringe plunger inside ~ ~fitting barrel. ~
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A buret barrel with a 30-ml. calibrated bulb on top of a 10-ml. graduated tube was used for acid iodometric titrations of potassium periodate and iodate mixtures. By choosing the proper concentration of sodium thiosulfate the first 30 ml. could.be added very rapidly and the last few ml. more cautiously. If it is desired to use the buret with solutions unstable in air, only the reservoir need be protected against the atmosphere.
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