sodium hydroxide, t h e conductometric graph obtained will be the form given in Fig. 26. T h e titer between t h e first a n d second " b r e a k s "
gives the correct a m o u n t of alkali required t o precipitate h y d r a t e d zinc oxide.
Aluminium is exceptional in t h a t its h y d r o u s oxide is t h e only one which can be proved b y conductometric titration t o be amphoteric.
Graph A in Fig. 27 illustrates t h e action of N a O H on a solution of
100 Η. Τ. S. BRITTON
aluminium sulfate. T h e first " b r e a k " corresponds t o t h e precipitation of basic aluminium sulfate, containing AI2O3, O.6SO3, while the second and more clearly defined " b r e a k " refers t o t h e formation of t h e soluble sodium a l u m m a t e , N a A 1 02, when 4 equivalents of sodium hydroxide h a v e been added. T h e back-titration graph B, with hydrochloric acid of the solution, t o which a n excess of sodium hydroxide h a d been added, again shows t h e N a A 1 02 " b r e a k " a n d an additional " b r e a k " indicating the complete formation of aluminium chloride a n d t h e appearance of free hydrochloric acid in t h e solution (64).
Britton and Young (12) h a v e shown t h a t hydrochloric acid in excess of t h a t required t o form u r a n y l chloride, U 02C 12, can be estimated con
ductometrically b y titration with alkali. T h e " b r e a k " corresponding with the complete precipitation of u r a n i u m hydroxide is slightly delayed owing t o some alkali being chemisorbed b y t h e u r a n i u m hydroxide.
(h) Barium hydroxide. H a r n e d (29) in t i t r a t i n g metallic sulphate solutions with barium hydroxide, found t h a t not only is the base precipi
tated b u t also an equivalent a m o u n t of BaSC>4, e.g., MgS04 + B a ( O H )2- > BaS04 + M g ( O H )2j
and in so doing, a very much greater angle was obtained a t the equivalence point. T h e method was satisfactory when calcium sulfate was present in t h e solution and it could therefore be applied to t h e analysis of dolomite.
T o estimate magnesia in dolomite slightly more sulfuric acid is added to the mineral t h a n is necessary t o convert it into sulfates, a n d the carbon dioxide is expelled b y boiling u n d e r reduced pressure. On cooling, it is neutralized to phenolphthalein with b a r y t a a n d again boiled to decompose a n y b a r i u m bicarbonate. T h e m o t h e r liquor is titrated conductometrically without removing t h e insoluble m a t t e r .
Conductometric titration with b a r y t a has been successfully performed with solutions of cobalt a n d nickel sulfates, though owing to the greater stability of basic zinc, cadmium, a n d copper sulfates, it is inaccurate in t h e case of these metals. B y carrying out the titration of copper sulfate just below 100° the basic sulfate is decomposed and accurate results can be obtained.
1. C O N D U C T O M E T R I C T I T R A T I O N S W I T H M E R C U R I C P E R C H L O R A T E
Only in very few instances are precipitates formed when mercuric perchlorate reacts with metallic salts, yet mercuric perchlorate is a n i m p o r t a n t conductometric t i t r a n t . I n the case of a titration involving the separation of a precipitate small changes in conductance occur because one salt in being precipitated leaves the sphere of action. N o w
CONDUCTOMETRIC ANALYSIS 101 certain mercuric salts, e.g., H g C l2, H g B r2, H g ( C N )2, H g ( C N S )2, i m p a r t to their aqueous solutions almost no electrical conductance a n d t h e little conductance t h e y do supply can be almost entirely accounted for b y their slight hydrolysis. I n general, t h e hydrolysis is less t h a n 1 % (11).
Some idea of lack of ionization of mercuric chloride in solution can be gained from the fact t h a t the specific conductance of its solutions ranging
Ν Ν
in concentration from t o — varies from 2.47 X 10~5 to 6.81 Χ 1 0- 5 m h o a t 25°C. (49). These values are extremely small a n d are of the same order as those of aqueous solutions in equilibrium with " insoluble salts."
On t h e contrary, mercuric nitrate, sulfate a n d perchlorate in solution are b o t h strongly ionized a n d hydrolyzed. Their specific conductances compare with those of solutions of ordinary metallic salts. A difficulty, however, m a y arise, through their appreciable hydrolysis, which, unless suppressed b y t h e presence of free acid, will cause basic salts to separate.
Mercuric perchlorate solutions however, despite more t h a n 10 % hydrol
ysis, can be so prepared t h a t they will remain quite clear without t h e addition of perchloric acid.
Mercuric perchlorate can be prepared b y t h e m e t h o d described b y Chikashige* (17), b y grinding a slight excess of red mercuric oxide with 2 Ν perchloric acid filtering t h r o u g h asbestos, a n d concentrating a t 40-50° under reduced pressure until crystals separate. Recrystallization from aqueous solutions u n d e r similar conditions is advisable. F o r conductometric work, it is merely necessary t o s a t u r a t e a solution of perchloric acid with pure mercuric oxide. T o ascertain whether a n y free acid exists in t h e solution, a d d an excess of sodium chloride and t i t r a t e the free acid with alkali t o m e t h y l orange.
T h e ionic mobility of t h e perchlorate ion 25° is ca. 72, so t h a t in t i t r a t ing a solution of potassium chloride with mercuric perchlorate:
2KC1 + Hg(C104) -> 2KC104 + HgCl2
the conductance during t h e replacement represented b y the above equa
tion will depend on 7C K — ZCioy, i.e., 76.6 — 72, so t h a t there will be a slight diminution. An excess of t h e t i t r a n t will cause an increase in conductance. I n a similar m a n n e r soluble bromides, thiocyanates, a n d cyanides m a y be accurately t i t r a t e d (43). I n t i t r a t i n g a solution of potassium bromide with mercuric perchlorate, a poorly-defined " b r e a k "
occurs before the final " b r e a k . " T h e first " b r e a k " shows t h e formation of K2H g B r4, viz.,
4KBr + H g ( C 1 04)2^ K2HgBr4 + 2KC104,
102 Η . Τ . S. B R I T T O N
and the second " b r e a k " refers t o t h e decomposition of K2H g B r4, t h u s K2HgBr4 + H g ( C 1 04)2- » 2HgBr2 + 2KC104
Soluble iodides m a y equally well be t i t r a t e d with mercuric perchlorate, b u t in such titrations the final " b r e a k s " will coincide with the complete precipitation of mercuric iodide. Before these are reached there will appear smaller " b r e a k s " t h a t indicate t h e formation of K2H g I4 a n d immediately afterwards mercuric iodide will begin t o separate.
Another class of mercuric salts exists which in solution h a v e low conductances. T h e y consist of t h e salts of t h e weaker monobasic acids, viz., nitrous, acetic, butyric, valeric, lactic acids. Although t h e y are very poor electrolytes, they ionize slightly in an u n k n o w n m a n n e r a n d hydrolysis plays an i m p o r t a n t role.
Conductometric titration of their respective alkali salts with mercuric perchlorate gives satisfactory end points a n d accurate titers. F o r t h e titration of alkali formates a n d lactates, t h e solutions m u s t be more con
centrated t h a n 0.01 Ν in order t o obtain good " b r e a k s " a t t h e end points.
Alkali benzoate and salicylate produce white precipitates with mercuric perchlorate. If adequate time be allowed for t h e a t t a i n m e n t of steady conductances, particularly in t h e vicinity of the end points, correct results m a y be obtained.
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