L. I. Brown
Connecticut College New London, Connecticut 06320
An Improved Equivalent Weight Apparatus
The reaction of weighed samples of metal with hydrochloric acid and the measurement of the volume of hydrogen evolved is a common experiment in general chemistry. As the experiment is usually performed, however, the concentrated acid is rapidly diluted by the water surrounding the metal sample and slow react,ion rates result which can only be corrected by using a very large excess of acid. The apparatus shown in the figure is simply constructed from two thistle tubes, a rubber stopper, and a short piece of glass rod. The tapered end of the glass rod is lightly ground into the top of the thistle tube so as to make a seal. The entire operation is carried out in a 5-1 plastic pail or other container and requires a total time of 5 min from thc insertion of the weighed sample. The procedure is as follows: Fill a flask or bottle complct,ely with water and invert it in a pail of water. The weighed sample of metal is placed in the lower thistle tube hulb and water is added unt,il all the air has been displaced and the apparatus is full of water up t.o a point just above the lower part of the thistle tube hulb, which is sealed by a slightly tapered glass rod which has been lightly ground with abrasive powder to act as a valve. Concentrated hydrochloric acid is added to the upper bulb, the assembly inserted under the inverted flask, and the acid permitted to run in to displace the water in the lower bulb and come in contact with the metal. The first gas formation forms a gas pocket at the top of the lower bulb, which prevents dilution of the acid by the water in the pail. More acid may be added as needed to complete the reaction, taking care that the level of liquid docs not fall below the valve in the upper bulb. When the reaction is complete, water is added to the upper bulb t.o displace
the remaining gas from the lower bulb into the flask. Inside and outside levels in the flask are made equal and the mouth of the flask is blocked by a sheet of paper, or better by a small piece of sheet rubber, until the flask is removed and set upright on the desk top. A measured quantity of water is added from a graduated cylinder to fill the flask to the brim, or if desired the flask may be weighed and the weight of water to fill the flask to the brim may be determined. This method has been used successfully by students in this laboratory for several years. Undergraduate assistants have had no difficulty in fabricating the apparatus, which requires only a minimum of glass working skills. The apparatus may be made free standing by the use of rectangles 3 X 4 in. cut from '/,in. tempered hardboard perforated a t the corners of 1-in. squares. The bottom (3-in.) edge is slotted to straddle the horizontal portion of the glass tubing, and the rectangle is attached to the vertical tube under the upper bulb by a copper wire running through the two perforations directly above the slot. The device is useful as a means of collecting pure hydrogen by water displacement without having to sweep air out of a generator. I t is also useful in generating pure NO by reaction of comer with dilute nitric acid, &d in the generation of oxygen from hydroSchematic diagram of oppordur forddtermining equivalentweights gen peroxide the use of or metals by reaction with HCI. manganese dioxide.
Volume 46, Number 9, September 1969