An Introductory Course of Quantitative Chemical Analysis by Henry P. Talbot (superbooks4u txt) 📕

process takes place if calcium oxalate, for example, is dissolved in hydrochloric acid. The C_{2}O_{4}^{—} ions unite with the H^{+} ions to form undissociated oxalic acid, the acid being less dissociated than normally in the presence of the H^{+} ions from the hydrochloric acid (see statements regarding hydrogen sulphide above). As the undissociated oxalic acid forms, the concentration of the C_{2}O_{4}^{—} ions lessens and more CaC_{2}O_{4} dissolves, as described for the Mg(OH)_{2} above. Numerous instances of the applications of these principles are given in the Notes.

Water itself is slightly dissociated, and although the resulting H^{+} and OH^{-} ions are present only in minute concentrations (1 mol. of dissociated water in 10^{7} liters), yet under some conditions they may give rise to important consequences. The term !hydrolysis! is applied to the changes which result from the reaction of these ions. Any salt which is derived from a weak base or a weak acid (or both) is subject to hydrolytic action. Potassium cyanide, for example, when dissolved in water gives an alkaline solution because some of the H^{+} ions from the water unite with CN^{-} ions to form (HCN), which is a very weak acid, and is but very slightly dissociated. Potassium hydroxide, which might form from the OH^{-} ions, is so largely dissociated that the OH^{-} ions remain as such in the solution. The union of the H^{+} ions with the CN^{-} ions to form the undissociated HCN diminishes the concentration of the H^{+} ions, and more water dissociates (H_{2}O <—> H^{+} + OH^{-}) to restore the equilibrium. It is clear, however, that there must be a gradual accumulation of OH^{-} ions in the solution as a result of these changes, causing the solution to exhibit an alkaline reaction, and also that ultimately the further dissociation of the water will be checked by the presence of these ions, just as the dissociation of the H_{2}S was lessened by the addition of HCl.

An exactly opposite result follows the solution of such a salt as Al_{2}(SO_{4}){3} in water. In this case the acid is strong and the base weak, and the OH^{-} ions form the little dissociated Al(OH){3}, while the H^{+} ions remain as such in the solution, sulphuric acid being extensively dissociated. The solution exhibits an acid reaction.

Such hydrolytic processes as the above are of great importance in analytical chemistry, especially in the understanding of the action of indicators in volumetric analysis. (See page 32.)

The impelling force which causes an element to pass from the atomic to the ionic condition is termed !electrolytic solution pressure!, or ionization tension. This force may be measured in terms of electrical potential, and the table below shows the relative values for a number of elements.

In general, an element with a greater solution pressure tends to cause the deposition of an element of less solution pressure when placed in a solution of its salt, as, for instance, when a strip of zinc or iron is placed in a solution of a copper salt, with the resulting precipitation of metallic copper.

Hydrogen is included in the table, and its position should be noted with reference to the other common elements. For a more extended discussion of this topic the student should refer to other treatises.

POTENTIAL SERIES OF THE METALS

__________________________________________________________________ | | | | POTENTIAL | | POTENTIAL | IN VOLTS | | IN VOLTS _____________________|___________|____________________|___________ | | | Sodium Na^{+} | +2.44 | Lead Pb^{++} | -0.13 Calcium Ca^{++} | | Hydrogen H^{+} | -0.28 Magnesium Mg^{++} | | Bismuth Bi^{+++}| Aluminum A1^{+++} | +1.00 | Antimony | -0.75 Manganese Mn^{++} | | Arsenic | Zinc Zn^{++} | +0.49 | Copper Cu^{++} | -0.61 Cadmium Cd^{++} | +0.14 | Mercury Hg^{+} | -1.03 Iron Fe^{++} | +0.063 | Silver Ag^{+} | -1.05 Cobalt Co^{++} | -0.045 | Platinum | Nickel Ni^{++} | -0.049 | Gold | Tin Sn^{++} | -0.085(?) | | _____________________|___________|____________________|__________

THE FOLDING OF A FILTER PAPER

If a filter paper is folded along its diameter, and again folded along the radius at right angles to the original fold, a cone is formed on opening, the angle of which is 60°. Funnels for analytical use are supposed to have the same angle, but are rarely accurate. It is possible, however, with care, to fit a filter thus folded into a funnel in such a way as to prevent air from passing down between the paper and the funnel to break the column of liquid in the stem, which aids greatly, by its gentle suction, in promoting the rate of filtration.

Such a filter has, however, the disadvantage that there are three thicknesses of paper back of half of its filtering surface, as a consequence of which one half of a precipitate washes or drains more slowly. Much time may be saved in the aggregate by learning to fold a filter in such a way as to improve its effective filtering surface. The directions which follow, though apparently complicated on first reading, are easily applied and easily remembered. Use a 6-inch filter for practice. Place four dots on the filter, two each on diameters which are at right angles to each other. Then proceed as follows: (1) Fold the filter evenly across one of the diameters, creasing it carefully; (2) open the paper, turn it over, rotate it 90° to the right, bring the edges together and crease along the other diameter; (3) open, and rotate 45° to the right, bring edges together, and crease evenly; (4) open, and rotate 90° to the right, and crease evenly; (5) open, turn the filter over, rotate 22-(1/2)° to the right, and crease evenly; (6) open, rotate 45° to the right and crease evenly; (7) open, rotate 45° to the right and crease evenly; (8) open, rotate 45° to the right and crease evenly; (9) open the filter, and, starting with one of the dots between thumb and forefinger of the right hand, fold the second crease to the left over on it, and do the same with each of the other dots. Place it, thus folded, in the funnel, moisten it, and fit to the side of the funnel. The filter will then have four short segments where there are three thicknesses and four where there is one thickness, but the latter are evenly distributed around its circumference, thus greatly aiding the passage of liquids through the paper and hastening both filtration and washing of the whole contents of the filter.

!SAMPLE PAGES FOR LABORATORY RECORDS!

!Page A!

Date

CALIBRATION OF BURETTE No.

___________________________________________________________________________
               | | | |
    BURETTE | DIFFERENCE | OBSERVED | DIFFERENCE | CALCULATED
    READINGS | | WEIGHTS | | CORRECTION
_______________|______________|______________|______________|______________
      0.02 | | 16.27 | |
     10.12 | 10.10 | 26.35 | 10.08 | -.02
     20.09 | 9.97 | 36.26 | 9.91 | -.06
     30.16 | 10.07 | 46.34 | 10.08 | +.01
     40.19 | 10.03 | 56.31 | 9.97 | -.06
     50.00 | 9.81 | 66.17 | 9.86 | +.05
_______________|______________|______________|______________|______________

These data to be obtained in duplicate for each burette.

!Page B!

Date

DETERMINATION OF COMPARATIVE STRENGTH HCl vs. NaOH

___________________________________________________________________________
                         | |
       DETERMINATION | I | II
_________________________|________________________|________________________
                         | |
                         | Corrected | Corrected
Final Reading HCl | 48.17 48.08 | 43.20 43.14
Initial Reading HCl | 0.12 .12 | .17 .17
                         | ——- ——- | ——- ——-
                         |