CHM171L Physical Chemistry 2 Laboratory 1st Quarter SY 2014-2015
Transference Number Marquez, Ariziel Ruth 1, Goce, Ivan Rei. 2, Lagamayo, Linear. 2, Landingin, Junard 2, Lau, John Victor 2 Professor, School of Chemical Engineering, Chemistry an d Biotechnology, Mapua Institute of Technology; 2Student (s), CHM170L/B41, School of Chemical Engineering, Chemistry and Biotechnology, Mapua Institute of Technology
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ABSTRACT Transference/transport number is a dimensionless parameter which informs about the contribution of the particular charged species present in the electrolyte (ions and electrons) to the overall charge transport across the cell by the process of electrolysis. Applying electric current in the solution of electrolytes can cause movement of the anions and cations with unique speeds towards the cathode and anode. Such speeds vary with respect to the magnitude of the applied voltage, temperature, and nature of the individual ions. During the experiment, nitric acid was placed in a W tube and was subjected for a certain voltage for one hour. Aliquots of this charged solution, 5 mL each from the left and right of the W tube, was collected and titrated by using sodium hydroxide for the determination of end point. The results show that the transference numbers of the nitrate and hydrogen ions are a re 0.8175 and 0.1825, respectively. Keyword: Transference Number, Electrolysis, Overall Charge Transport
INTRODUCTION If an electric current is passed through a solution of an electrolyte, the anions and cations move with characteristic speeds toward the anode and cathode. These speeds vary with magnitude of the applied voltage, voltage, temperature temperature and nature of individual ions. ions. Among the factors associated with the nature of the ions are its charge and size. size. One type of ion does not affect the velocity of another type under a given set of conditions unless the concentration is sufficiently great to result in appreciable interionic attraction effects. The mobility of an ion is defined as its velocity in the direction of an electric field of unit strength (i.e. V/m in SI units). The mobility of ions is generally quite small (in the order of 6x10-8 m2.s1.V-1). The exceptions are hydrogen and hydroxyl ions with the abnormally high mobilities of 36.3x10 8 and 20.5 x 10 -8 m2.s-1.V-1, respectively. r espectively. Ionic mobilities are important factors in many electrochemical phenomena. However, it is more convenient to use a quantity called the transference number of an ion rather than the ionic mobilities in the formulas applicable to such phenomena. The transference numbers numbers of the cation and anion are simply the numbers which gives the fraction
Experiment 03│ Group No. 4│ September 03 , 2014
of the current carried by each of the ions. Hittorf’s Method has been generally employed for the experimental determination of transference numbers and involves measurement of changes of concentration in the vicinity of the electrodes.
Current carried by the cations;
where; cations = number of cations = velocity of the cation in the solution [cm/s] or mobilities of the cation = charge of the cation = quantity of electricity associated with a unit charge [volts] = separation distance of the two plates Following a similar definition, current carried by the anions;
(1)
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CHM171L Physical Chemistry 2 Laboratory 1st Quarter SY 2014-2015
Total current carried by both ions,
(3)
(4)
(5)
(6)
Thefore, the fraction of the total current carried by the ions are directly proportional to their velocities.
(8)
It is clear that the sum of the transference numbers of the ions of a particular electrolyte equals to unity since each is a fraction of a unity.
(9)
METHODOLOGY
The reagents used were the nitric acid, NaOH, acetone and Copper sulfate.
The fraction of the total current carried by anions,
The laboratory apparatus used were beaker, pipette, burette, Erlenmeyer flask, Phywe Transference Number Apparatus and the W tube.
The fraction of the total current carried by cations,
Although the speed of a particular ion does not dependent on the nature of the ion, the transference numbers depends on each other.
Thus, the total current becomes
(2)
The electroneutrality principle expresses the fact that all pure substances carry a net charge of zero;
(7)
In summary, the transference numbers of the ions in a particular electrolyte are defined as follows:
Experiment 03│ Group No. 4│ September 03 , 2014
The two electrodes were measured using analytical balance. Nitric acid was transferred in the W tube. The heights in each column were equal when the Phywe Transference Number Apparatus were inserted in the right and left column of the W tube. The two electrode were dip in a solution of copper sulfate and the current flowing in the system were adjusted. The set-up was left for 1 hour and after that, the two electrodes were washed by acetone and water and their weights were measured again. Five milliliters were withdrawn from the right and left column and were titrated by NaOH solution. Five milliliter of the original nitric acid was also titrated with the use of NaOH solution. The indicator used in this titration was the methyl red and the solution turned pink after dropping a methyl red. The titration stopped when the solutions turned to light yellow. The volume of NaOH solution was recorded as well as the initial and final weights of the electrodes. These data were used to compute for the transference number of hydrogen and nitrogen ion. RESULTS AND DISCUSSION
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CHM171L Physical Chemistry 2 Laboratory 1st Quarter SY 2014-2015
Table 3.1 The following table shows the mass of the copper electrodes before and after electrolysis. W1 (mg) W2 (mg) Left 1645 1640 Right 1758 1761
and right solutions, respectively. From the data in table 3.2, it can be seen that the amount of NaOH needed to neutralize the solution is higher in the right as compared to the original solution and, it is lower in the left as compared to the original solution.
The setup in the experiment is an example of a galvanic cell. In such a cell, the polarity is determined based on which node is connected to the positive and negative terminal of a voltage source. In this case, the right hand side is the positive (cathode) and the left hand side is the negative (anode). Theoretically, the anode will spontaneously undergo oxidation converting Cu(s) to Cu2+ and, the net electrons would travel from the anode to the cathode where it would reduce Cu2+ to Cu (s). From this, it is expected that the left electrode would decrease in mass while the right electrode would increase in mass. Looking at the table 3.1, it can be seen that this is exactly what was observed. The copper on the left side decreased in mass from 1645 mg to 1640 mg. On the other hand, the copper on the right side increased in mass by roughly the same magnitude from 1758 mg to 1761 mg. This observation is consistent with the theory discussed, thus confirming the polarity of the galvanic cell in the experiment.
Table 3.3 The following table shows the experimentally determined transference numbers. Nitrate Ion 0.8175 Hydrogen Ion 0.1825
Table 3.2 The following table shows the amount of titrant needed to neutralize the corresponding nitric acid solutions. Volume NaOH Volume W-tube (mL) (mL) Left 22.35 23.5 Right 22.75 24 Original 22.68 The process described in the previous paragraph would logically produce an excess of positive ions in the left hand column and an excess of negative ions in the right hand column. The W-tube used in the experiment possesses semi-permeable membranes in between all three glass columns. These membranes inhibit the passage of certain ions, thus preventing the three separate columns from reaching electric equilibrium with one another by means of diffusion. The end result is that the three columns would possess varying acidities as confirmed by the data in table 3.2. Due to the chemical reaction made spontaneous by the electrolysis process, the solution in the right hand column would become more concentrated in nitric acid whereas the solution in the left hand column would become less concentrated in nitric acid. The reason behind this is the presence of excess positive and negative ions in the left
Experiment 03│ Group No. 4│ September 03 , 2014
The transference number of a particular species depends on multiple parameters one of which is the ionic mobility. The ionic mobility of an ion varies inversely with the size of the ion. Larger ions tend to be less mobile while smaller ions tend to be more. The relationship to be considered is shown below:
From this relationship, it can be deduced that the transference number is directly proportional to the ionic mobility and inversely proportional to the size of the ion. Since hydrogen ions is much smaller than nitrate ions, it is expected that hydrogen ions would have higher mobilities and, therefore, higher transference number. On the other hand, it is expected that nitrate ion would have the lower transference number among the two. However, as seen in table 3.3, the transference numbers obtained from the experiment are quite the opposite of what is anticipated. From the reference materials used, it was found that the transference number data might have been switched as it would have otherwise been quite accurate if they were. The trends observed from the data in table 3.1 and 3.2 were accurate and consistent with the theory. The minor inconsistencies observed in the amount of NaOH used to neutralize the three nitric acid solutions may have been caused by the inaccurate concentrations of the solutions used. It was undetermined how long the acid and base were put in storage and, the presumed 0.1 molar might have change somewhat throughout the storage duration. Other sources of error possibly include human error, particularly in the titration process. The titration endpoint was determined to as high a degree as possible but, due to the limitation of the apparatus, the obtained data may still deviate about ±1 mL from the true value. Lastly, for table 3.3, the error committed is mostly likely to be a gross error. The transference number for the nitrate and hydrogen ions
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CHM171L Physical Chemistry 2 Laboratory 1st Quarter SY 2014-2015
may have possibly been switched but, otherwise, they were quite precise. CONCLUSIONS AND RECOMMENDATIONS The experiment was quite a success because the percent error computed by the group was below 10% which means the group performed the experiment well with only few errors. After the electrolysis, the final weights of one electrode decreases while the other one increase. The left electrode was expected to decrease because it is the anode part and the right side is the cathode so it was expected to increase. This proves the theory when the group final weights were consistent in the theory. Another theory proven was the decreasing concentration of the left column of the W tube and the increasing concentration of the right column. After titration it was observed that the concentration in the right side of the W tube increases because the NAOH required for its titration is greater than the NaOH required in the titration of the original nitric acid solution. The left side was decreasing because of the observed NaOH solution required solution was less than the original nitric acid solution.
[1] Castellan, G.W., Physical Chemistry 3 rd edition, 1983. [2] Atkins, Peter., de Paula, Julio., Physical Chemistry 9 th editions, 2010 [3] Levine, I.N., Physical Chemistry 6 th edition, 2009 [4] Perry, Robert., Perry’s Chemical Engineer’s Handbook 8 th edition, 2008 [5] Alberty, R., Physical Chemistry 4 th edition, Wiley Publishing Co. [6] Klein, David., Organic Chemistry 2 nd edition, 2010. [7] C.W. Garland, J.W. Nibler and D.P. Shoemaker, Experiments in Physical Chemistry, 4 th edition, 2004.
Performing the experiment, the group understands the effect of the electrolysis in each electrode, the cathode and anode. Anode part will undergo oxidation while the cathode part undergoes reduction. The transference number was determined and the relationship of it to the ionic mobility and ion size was also deduced. The group deduced that the transference number is directly proportional to the ionic mobility and inversely proportional to the size of the ion. Because of these, the objectives of the experiment were met thus the experiment was a success. The flaw or error that the group committed maybe in the titration part were NaOH was used the solution to titrate the three solutions. The volume of the NaOH used maybe not exact because the color of the solution was not that light thus the NaOH used was excess. Drop of NaOH make a big difference in color because every drop counts in every titration so it was very hard to get an exact volume of NaOH. REFERENCES
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