http://www.chemistrymag.org/cji/2000/027031pe.htm |
Jul.1, 2000 Vol.2 No.7 P.31 Copyright |
Cao Chengxi, Zhou Shulin
,# He Youzhao, Zhou Tianchen,## Qian Yitai( Dept. of Chemistry, University of Science and Technology of China, Hefei 230026; # Institute of Allergy Reaction, Wannan Medical College, Anhui Wuhu 241001;## Dept. of Light & Chemical Industry, Wuhu Professional Technology University, Anhui Wuhu 241002)
Received Feb. 16, 2000; Supported mainly by the National Natural Scientific Foundation of China (No. 29775014, 29975026), partly by the Health Committee of China (No. 98-2-334))
Abstract The mechanism of "pre-precipitate zone" observed after
the experiments of moving chemical reaction boundary (MCRB) formed with cobalt and
hydroxyl ions in agarose gel is still unclear. In order to elucidate the mechanism, two
methods, viz., the "cutting gel after run of MCRB" and "ionic
diffusive reaction in gel", are designed by the authors. The experimental results of
the former method showed no "pre-precipitate zone", and the latter method formed
ionic diffusive reaction zone that is very similar to the "pre-precipitate
zone". The results directly demonstrate that the "pre-precipitate zone" is
produced by the diffusive reaction between cobalt and hydroxyl ions remaining in the gel
after the run of MCRB. In addition, the procedure of ionic diffusive reaction may be of
significance for the study on ionic diffusive, diffusive reaction and diffusive reaction
boundary.
Keywords ionic diffusion, diffusive reaction, electrolyte, moving chemical reaction
boundary (MCRB).
In 1971, Demen and Rigole[1,2]
firstly advanced the idea of precipitate reaction front of electromigration (viz.,
moving chemical reaction boundary, MCRB) between reaction ions, such as hydroxyl and
cobalt ions, and performed the experiments of electromigration reaction formed by cobalt
and hydroxyl ions. In 1993, Bocek's group[3,4] advanced the concept of
stationary neutralization reaction boundary and used this concept to study the mechanism
of isoelectric focusing(IEF)[5,6].
Recently, some of our papers advocated the concept of MCRB[7-10],
showed the relations between MCRB and IEF[11-13] and those between Kohlrausch'
regulating function and MCRB[14,15], and performed the experiments of MCRB,
which directly proved the validity of the theory of MCRB[16-18]. During the
experiments of MCRB, the blue "pre-precipitate zones" gradually formed after the
runs of MCRB was observed (see Fig. 4 in Ref. 11, Fig. 1-2 in Ref. 18 and Fig. 1 here).
The reasons that cause the "pre-precipitate zones" may be the diffusive reaction
between cobalt and hydroxyl ions remaining in gel after the run of MCRB[11] or
the electromigration of very fine colloids of Co(OH)2 formed during the run[1,18].
The reason that causes the pre-precipitate zone is still a unsolved problem so far.
Therefore, the authors try to elucidate the mechanism of the
"pre-precipitate zone" by some experiments of MCRB and of the diffusive reaction
between cobalt and hydroxyl ions in agarose gel.
1. EXPERIMENTAL
1.1 Regents
CoCl2ยท6H2O, NaOH and KCl are AR grade(Shanghai Chemical Reagents
Co., Shanghai, China). The agarose used to prepare the gel is a biochemical
reagent(Shanghai Huang-Hua Pharmaceutical Factory, Shanghai, China). The agarose gel
produced from agar is better than agar gel, because the former is almost free of
electro-osmostic flow (EOF), but the latter does obviously due to the charged groups in
agar gel.
1.2 Methods
1.2.1 MCRB experiment
The MCRB experiment was designed created with cobalt and hydroxyl ions according to
Ref. 17. To understand the mechanism of "pre-precipitate zone" better, the
diagrammatic sketches of the MCRB runs and the "pre-precipitate zone" developing
after the MCRB run was given in Fig. 1.
1.2.2 Experiment of cutting gel after run
Just after the MCRB run, as shown in Fig. 2, the gel is pressed out off the tube partially
and is cut just ahead the boundary formed at the end of MCRB. Then the left of gel in the
tube is returned to its original place. After this, one could observe and record the
development of the "pre-precipitate zone".
Fig. 1. The diagrammatic
sketches of the MCRB experiment formed with cobalt and hydroxyl ions and of the
"pre-precipitate zone" developing after the MCRB run. A) the beginning of
MCRB created with cobalt and hydroxyl ions, B) the end of MCRB, and C) the
"pre-precipitate zone" developing after the run of MCRB.
Fig. 2. The diagrammatic
sketch of observing the "pre-precipitate zone" after cutting the gel just ahead
the boundary formed just after MCRB run.
1.2.3 Experiments of ionic
diffusive reaction in gel The diffusive reaction between cobalt and hydroxyl ions was designed as following. As shown in Fig. 3, the tube was filled with 1% agarose gel containing 0.1 N background electrolytes KCl and 0.01 N CoCl2. Then one end of the tube was covered with plastic film, after that, the tube was inserted into rube plug. Finally, the tube was placed into a larger tube containing 5.0 ml solution of 0.1 N KCl and 0.01 N NaOH, the time began.
The length of diffusive reaction zone can directly be determined and photographed at any time during the diffusive reaction between cobalt and hydroxyl ions.
Fig. 3. The diagrammatic
sketch of procedure of diffusive reaction between cobalt and hydroxyl ions in 1%(w/v)
agarose gel. For more details, see the text.
2. RESULTS AND DISCUSSIONS
The MCRB experiments were performed with the novel apparatus by continuous flow of the
catholyte and anolyte[17]. With the apparatus, the experiments were easily
observed during the runs. After the MCRB runs, the "pre-precipitate zones" were
also developed. The "pre-precipitate zone" occurring in the apparatus is very
similar to those observed in our prior experiments in the disk electrophoretic apparatus
(comparing Fig. 4 here with Fig. 4 in Ref. 11 and Fig. 1-2 in Ref. 18).
As analyzed in the introduction, there are two reasons that may cause
the "pre-precipitate zone" after the MCRB runs. One is the diffusive reaction
between cobalt and hydroxyl ions, which remain in the agarose gel and the precipitate zone
respectively[11,18], the other is probably due to some of the precipitates (viz.,
the colloids of Co(OH )2) formed during the run. The precipitates are very fine
and charged, and can penetrate the agarose gel and move ahead the boundary. After the run,
the fine precipitates of Co(OH)2 aggregates to form the blue
"pre-precipitate zone" gradually due to the existence of high concentration of
KCl in gel[1,18].
Fig. 4. The
"pre-precipitate zone" developing after the MCRB experiments created with cobalt
and hydroxyl ions in 1% (w/v) agarose gel. The arrows indicate the boundaries formed just
after the end of runs, the symbols + and - imply the cathodic and anodic ends
respectively. Those blue-yellow zones from the anodic ends to the arrows are the
precipitate zones formed during the runs, those blue zones under the arrows are the
"pre-precipitate zones" developing after the runs of MCRB. Conditions: [CoCl2]
= 0.01 N; [NaOH] = 0.01 N; 0.1 N background electrolyte KCl; length of tube = 90 mm; run
time = 20 minutes; A) constant voltage = 260 V, ID = 5.7 mm; B) 260 V, ID=
5.8 mm; C) 300 V, ID = 6.0 mm.
If the former reason
causes the "pre-precipitate zone", there would not be "pre-precipitate
zone" to be observed after cutting the gel just ahead the boundary formed after the
run (see the method 2 in Fig. 2). If the latter leads to the "pre-precipitate
zone", there will be "pre-precipitate zone". In order to test the above
deduction, we performed the MCRB experiments described in Fig. 1 at first, and after the
completion of run we carried out the experiment in accordance with method 2 shown in Fig.
2. The results are shown in Figs. 5 that there is no "pre-precipitate zone" to
be observed. Therefore, it is the former that causes the "pre-precipitate zone".
In order to further demonstrate the "pre-precipitate zone" is
created by the diffusive reaction between cobalt and hydroxyl ions remaining in the gel
after the MCRB run, we also performed the experiments in accordance with method 3
as shown in Fig. 3. The results are given in Fig. 6. The results show that the diffusive
reaction between cobalt and hydroxyl ions occurs and the zone of diffusive reaction is
very similar to the "pre-precipitate zone" developing after the MCRB run. In
addition, the results also show a series of precipitate disks which are the same as those
observed near the end of "pre-precipitate zone"(see Fig. 1 here, Fig. 4 in Ref.
11 and Fig. 1-2 in Ref. 18). The results further verify that the "pre-precipitate
zone" is caused by the diffusive reaction between cobalt and hydroxyl ions remaining
in the gel after the MCRB run.
Fig. 6. The precipitate
boundaries and zones developing during the "experiments of diffusive reaction"
created with cobalt and hydroxyl ions in accordance with method 3. The arrows indicate the
boundaries. A) No any precipitate boundary and zone are observed before the
experiment with two tubes. B) The photograph after the experiments about 32 hours,
the length of the zones is about 26 mm as shown by the rule. C) the photograph
after the experiments about 56 hours, the length of zones is about 30 mm, there are a
series of blue disks near the arrows, after this time, no growth of the precipitate
boundary occurs. Conditions: 1% agarose gel; [CoCl2] = 0.01 N; [NaOH] = 0.01 N;
0.1 N KCl; length of tube = 90 mm, ID = 5.7 mm.
From the above results and discussions, one can conclude that the pre-precipitate zone is caused by the diffusive reaction between cobalt and hydroxyl ions remaining in the gel after the MCRB run, but not the existence of very fine precipitate of Co(OH)2 formed during the run and moving ahead the boundary under the electric field. In addition, the procedure of diffusive reaction in Fig. 3 may be of significance for the studies on the ionic diffusion, diffusive reaction and diffusive reaction boundary[19,20].
Acknowledgment The authors sincerely wish to thank Dr. Ge Pinshi and Lin Xuodong for their aids to obtain the photographs.
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