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Art. V.—On the Velocity of Evolution of Oxygen from Bleaching-powder Solutions in Presence of Cobalt-nitrate, and the Modifications produced by the Addition of various Compounds.
[Read before the Philosophical Institute of Canterbury, 2nd November, 1910.]
In each experiment measurements of two kinds were made :—
1. The evolved oxygen was collected in a burette and the volume read at definite intervals.
2. The evolved oxygen was calculated by titrating the bleaching-powder with N/10 arsenious acid before and after the experiment. The results of the two methods agreed well.
The concentrations obtained from the two titrations inserted in the equation for a mono-molecular reaction gave the velocity.
The reactions were carried out in a 100 c.c. glass flask, the rubber stopper of which carried a short wide tube, conical at its lower part, and closed near the bottom by a glass-rod stopper ground in. This rod stopper passed through and was connected by a rubber-tube joint with a short glass guide-tube carried by a cork at upper end of the outer glass tube. The accompanying diagram shows the complete apparatus.
The evolved gas was carried off by a narrow glass tube over a meter long, arranged to condense and carry back into the flask any water-vapour formed. The flask was charged with 25 c.c. of bleaching-powder solution,
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unfiltered, and containing 16 grammes per litre. The stoppered tube carried 5 c.c. cobalt-nitrate solution, 0·005 N to 0·00125 N, and in the first series of experiments 5 c.c. of water, in the second series 5 c.c. of the compound under examination. The flask and wide tube were immersed in a large thermostat with regulator and centrifugal stirrer. When the temperature became uniform the rod stopper was raised and the catalytic agent run in. Each experiment in the first series lasted about forty minutes. Somewhat over 400 experiments were carried out, apart from all preliminary work.
Part I.
Specimen Experiment: Measurement of Gaseous Oxygen evolved.
Taken, 25 c.c. bleaching-powder solution, turbid, 32 grammes per litre; 5 c.c. cobalt-nitrate, 0·00125 N; 5 c.c. water.
| Time in Minutes. | Cubic Centimetres of Oxygen. | Volume. | Temperature. | Barometer. | |
|---|---|---|---|---|---|
| Time | Burette. | Thermostat. | |||
| 4 | 3·1 | 0·73 | 10·1° | 78·3° | 761 |
| 8 | 5·9 | 0·67 | 10·1° | 78·3° | 761 |
| 12 | 8·0 | 0·49 | 10·1° | 78·3° | 761 |
| 16 | 9·4 | 0·33 | 10·0° | 78·3° | 761 |
| 20 | 10·8 | 0·33 | 10·0° | 78·3° | 761 |
| 24 | 12·0 | 0·28 | 9·9° | 78·3° | 761 |
| 28 | 12·9 | 0·21 | 9·9° | 78·3° | 761 |
| 32 | 13·8 | 0·21 | 9·9° | 78·3° | 761 |
| 36 | 14·4 | 0·14 | 9·9° | 78·3° | 761 |
| 40 | 15·2 | 0·19 | 9·9° | 78·3° | 761 |
Summary of Results with Bleaching-powder and Cobalt-nitrate.
1. The velocity is proportional to the concentration of the cobalt solution.
2. The measurable velocity proceeds as for a unimolecular reaction.
3. Increase of temperature increases the velocity. A rise of 10° C. almost triples the initial velocity.
4. Reversing the order of addition of the components alters the reaction-velocity.
5. Shaking alters the reaction-velocity.
The velocity varied slightly with different batches of solution.
For a solution containing 25 c.c. bleaching-powder solution, 16 grammes per litre; 5 c.c. cobalt-nitrate solution, 0·0025 N; and 5 c.c. water, K = 0·01993 (c.c. O per minute).
Part II.
In this part various additions were made to the cobalt-nitrate solution, and their effect on the reaction-velocity determined. A bleaching-powder solution refiltered after twenty-four hours, of which 10 c.c. = 17·5 c.c. N/10 arsenious acid was used, and was found to keep for weeks without change in titration-value, although the reaction-velocity diminished very slightly.
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10 c.c. bleaching-powder solution; 5 c.c. cobalt-nitrate, 0·000156 N; and 5 c.c. aqueous solution of the compound investigated.
The experiments in most cases lasted twenty minutes. Twenty-seven compounds, including acids, alkalies, oxidizing and reducing agents, and salts, were tried.
Summary of Results.
1. Acids accelerate the evolution of oxygen, and this effect is proportional to the concentration of the hydrogen ion.
Example: 5 c.c. of 0·02 N hydrochloric acid added to the cobalt-nitrate solution doubled the velocity (from K = 0·0063 to K = 0·0141). In case of oxalic acid, 5 c.c. of 0·01 N acid had a slight retarding effect, whereas 5 c.c. of 0·02 N acid doubled the velocity as with other acids. This suggests some changes in the oxalic acid at great dilution.
2. Alkalies retard. The action is proportional to the concentration of the hydroxyl ions, and is forty times as great as that of the acids. 5 c.c. of 0·0005 N alkali halves the velocity.
3. Salts of the alkalies were practically without effect, even in relatively strong solutions—e.g., 5 c.c. of N potassium-chloride had no very appreciable effect.
4. Salts of the heavy metals either accelerate or retard.
5. Copper-sulphate accelerates greatly. 5 c.c. of N copper-sulphate evolves chlorine plentifully. 5 c.c. of 0·001 N solution doubles the velocity.
5. Reagents such as sodium-sulphide, which, like the alkalies, precipitate the cobalt, retard the evolution of oxygen.
6. Oxidizing agents, such as ferric chloride and potassium-dichromate, gave no uniformity of action; their effects, also, were not proportional to their concentrations.
7. Nickel and iron salts are similar in their action on bleaching-powder to cobalt-salts, the effect of nickel being about equal to that of cobalt, while iron has only about 1 ½ per cent. of the effect of cobalt.
8. It is impossible to judge from the chemical analogues of a particular metal whether it will accelerate or retard the cobalt reaction.
No record of previous quantitative work bearing on this subject could be found in the “Journal of the Chemical Society,” “Comptes Rendus,” “Zeitschrift für Physikalische Chemie,” and “Journal of Physical Chemistry.”