Sulphur Dioxide Gas

Fix up your apparatus as in the preparation of carbon dioxide, only set your flask on a retort-stand, so that it can be heated. Take about 20 grams of copper shavings or clippings and put them carefully into the flask, add through the thistle funnel about 60 c.c. of strong sulphuric acid, and heat up gradually until a brisk action goes on in the flask, then lower the flame. Collect three jars of the gas by downward displacement, and cover them with greased cover-glasses.

Cu + 2H2SO4 = CuSO4 + 2H2O + SO2

Sulphur Dioxide Gas Laboratory Experiment I

Bring a lighted taper into a jar of the gas; the flame is extinguished, and the gas does not take fire.

Sulphur Dioxide Gas Laboratory Experiment II

Put a few flowers (red, if possible) into another jar of the gas; the colour of the flowers after a little while is discharged, showing the gas possesses the property of bleaching.

Sulphur Dioxide Gas Laboratory Experiment III

Invert the remaining jar of the gas beneath the water in a trough, and remove the cover-glass; the water rises in the jar, showing the solubility of the gas. Place the hand beneath the mouth of the jar and remove it from the trough, then shake it briskly; again bring it under the water, and repeat this operation until the water has nearly filled the jar. Remove this jar with the water in it from the trough.

SO2+ H2O = H2SO3

Sulphur Dioxide Gas Laboratory Experiment IV

Take a few c.c. of the solution prepared in the last experiment, add a little bromine water to it, and boil the mixture;

H2SO3 + 2BrH2O = H2SO4 + 2HBr

add a few drops of barium chloride to a portion of the mixture; a white precipitate is thrown down, insoluble in hydrochloric acid, which indicates the presence of sulphuric acid.

H2SO4 + BaCl2 = BaSO4 + 2HCl

add to another portion of the mixture a little silver nitrate solution; a whitish precipitate is thrown down, indicating the presence of hydrobromic acid.

HBr + AgNO3 = AgBr + HNO3

Sulphur Dioxide Gas Laboratory Experiment V

Take a few more c.c. of the solution from Experiment III. and add a drop or two of barium chloride; a white precipitate is thrown down; add a few drops of hydrochloric acid and the precipitate dissolves. Boil a few more c.c. of the sulphurous acid solution with a drop or two of nitric acid and then add barium chloride; a white precipitate is thrown down, insoluble in hydrochloric acid, showing a change in the original solution.

H2SO3+ O = H2SO4

Sulphur Dioxide Gas Laboratory Experiment VI

Take a little potassium iodide solution in a beaker, add some water and a little starch paste, then one or two drops of chlorine water; an intense blue-coloured solution is produced. To this solution add, drop by drop, with constant stirring, sulphurous acid solution until the blue colour is entirely discharged. This bleaching is affected by the sulphurous acid taking up the oxygen from the water to form sulphuric acid, the hydrogen of the water combining with the iodine liberated from the potassium iodide, and forming hydriodic acid, which is colourless, soluble in water, and does not act on starch. Sulphurous acid, you will observe, bleaches by reduction, while chlorine bleaches by oxidation.

Sulphur Dioxide Gas Laboratory Experiment VII

Heat a few drops of sulphurous acid solution in a test- tube ; a distinct smell of sulphur dioxide is given off.

INSTRUCTIONS TO THE STUDENT

The student has now obtained a fair knowledge of simple glass-working and of the preparation and properties of the more common gases. In the following sections he proceeds to examine solids and liquids (generally solids which are put into solution). First he examines an unknown substance by ‘dry tests’ for ‘ bases ’ and ‘ acids ’; the results so obtained are then confirmed by systematic ‘ wet tests.’

The order of work laid down is to be followed with most careful and patient attention to all details; the student has then done his part, but unless this is supplemented by equal care on the part of the demonstrator, the best results cannot be expected. The demonstrator, besides supervising the actual testing, must prepare a carefully graded series of substances leading from simple salts containing one ‘base’ and one ‘ acid ’ to complex mixtures containing four or five ‘bases’ and several ‘ acids ’ (including insolubles). The substances must all be carefully selected with the definite object of teaching the student some important point in every mixture he analyses. Indiscriminate preparation of mixtures leads to waste of time and bad work.

In his first tests the student may be given salts of known composition, and his work is then checked by unknown salts from the demonstrator’s set. For instance, on the next page he may take ZnCl2, SnCl2, Pb, Bi2S3, and so on for practice, and when fairly confident, his proficiency or otherwise is checked on ‘unknown’ salts given by the demonstrator.

The demonstrator’s ‘Record Book’ should show full details of the substances given to each student, the results obtained, the time taken with the analysis, and general remarks where necessary. A somewhat similar record must be kept of the work done in Quantitative Analysis and Assaying.