Electr ica I I y Operated Buret FREDERICK C. N A C H O D The Atlantic Refining Company, Philadelphia, Pr.
N series of experiments which were carried out several years Iliquid ago, the problem of avoiding contamination of the titration by stopcock grease was encountered. The solutions A
mainly consisted of bromine dissolved in light and “heavy” methanol (CHIOH and CHSOD), and were used in the determination of keto-enol equilibria (1). The apparatus employed, shown in the diagram, consista of a conical seal, S which must be ground with eat care; the male part of the seal is formed into a plunger, P wEch contains an iron core, C, auitably made of about 6 iron nails. This core is about 1.5 cm. long and 0.5 cm. in diameter. A top view of the upper crosa section of the plunger is shown a t the right. The four protrusions maintain the plunger in its vertical position during motion. Two solenoids, A and B , made from bell wire, and consisting of about 20 windings each, are connected over a &volt batte by means of a Morse key, K . If the key is released, solenoid% is actuated and provides magnetic “suction” for the seal; if the key is pressed down or tap ed, the plunger rises and the solution is ermitted to flow out of tge buret. f n preparing the assembly, it is recommended to start with the conical seal to connect the seal to a calibrated buret, and then t o slide in the plunger. The apparatus descnbed was made for a 10-ml. buret but can be adapted to larger burets.
Titration of Boric M A X HOLLANDER
AND
Acid
LITERATURE CITED
(1) Nachod, F. C.,2.physik Chem., A182, 208 (1988). PREOENTED before the Spring Meeting of the Philadelphia Section, AM~P.IOAN C ~ E M I C A S Lo c ~ mJune , 13,1945.
in the Presence of Mannitol
W I L L I A M R I E M A N 111, School of Chemistry, Rutgerr University, New Bruntwick, N. J.
IT IS
well known that the titration of boric acid with sodium hydroxide is satisfactory only in the presence of glycerol, mannitol, or some similar polyol. Although mannitol has been found most suitable for this purpose (6,8),the literature contains no satisfactory report on the optimum quantity or concentration of mannitol. Schafer (6) recommenda a mannitol concentration at least 0.2 M,but the accuracy of hia conclusion may well be questioned because he used a buret accurate to 0.01 ml. for titrations of leas than 0.9 ml. Mellon (4) recommends the use of 1 gram of mannitol, while Kolthoff and Furman (8) and Kolthoff and Sandell (3) call for 0.5 to 0.7 gram of mannitol for each 10 ml. of solution, and Hillebrand and Lundell (I) ask for the addition of 1 to 2 grams. Scott (7) calk for the use of l g a m increments of mannitol until a permanent phenolphthalein end point has been reached. This paper presents a study of the effect of varying amounts of mannitol on the titration of boric acid with sodium hydroxide to the phenolphthalein end point.
menta of sodium hydroxide were 0.10 ml. The buret readings a t the phenolphthalein end point (pH of 8.30)and a t the steepest point of the titration graph (equivalence point) were observed. The dflerence in these readings is given in Table I under the heading of “Titration Error”. The results obtained from the titration graphs are given in Table I. DISCUSSION
From Table I, it is obvious that with an initial volume of 100 ml., the proper quantity of mannitol is about 7 grams, corresponding to a ratio of 27 moles of mannitol per mole of boric acid. When the initial volume is 25 ml., 3.9 grams of mannitol, corresponding te a molar ratio of 15, will give satisfactory resulta. These conclusions show very clearly that the molar ratio of
Table 1. Titrrtion of 25 MI. of Standard Boric Acid with Sodium Hydroxide
EXPERIMENTAL
REAQENTB.Boric acid, approximately 0.057 M , was prepared by dissolving the Merck reagent in freshly boiled distilled water, Carbonate-free sodium hydroxide. aDDroximste1.y 0.045 N , was standardized with pot&i;m acid phthalate. ‘Mannitol, Eastman No. 155, was found to be neutral and used without further purification. PROCEDURE. Twenty-five milliliters of the standard boric acid solution were diluted to a given volume and 2 drops of 0.03 M phenolphthalein per 100 ml. of solution (volume a t equivalence point) were added. A known amount of mannitol was added, and the solution was titrated potentiometrically at room temperature with a Beckman pH meter, laboratory model. In the vicinity of the equivalence point, the Incre-
Volume before Titration
MZ. 100 100
100 100 100 100
loo
100
loo 25 25 25
.
w$fht
M O ~of MBoric Acid
p H of Acid
0.26 0.62 1.04 2.08 4.16 8.32 16.64
0.0 1.0 2.0 4.0 8.0 16.0 32.0 64.0
6.01 5.03 4.72 4.25 3.93 3.62 3.43 3.18
0.49 1.97 3.93
1.9 7.6 11.2
3.80 8.09 2.88
Mannitol Qramr 0.00
33.30
Of
Point
Point
Error
M1.
...
Thin quantity did not dholve entirely.
602
8%;:. :;lence sv:.“Tithtion lenoe
a.oi
10.29 9.91 9.70 9.41 8.82 8.48 8.18 7.66 7.36 10.01 8.67 8.M)
