Polarographie Determ ination o f Potassium, Sodium, and Lithium

IG NA CE ZLO TO W SK I ^{A N D} I. M . K O L T H O F F , U n iv ersity o f .M in n e s o ta , M in n e a p o lis , M in n .

T

charged a t a potential ab ou t 0.2 v o lt more negative than th at of the corresponding m eth y l ion. E sp ecially w ith lithium it is very difficult to g et a well-defined diffusion current with tétram éthylam m onium hydroxide as supporting electrolyte, even in relatively concentrated alcoholic m edium .

Preliminary investigation confirmed th e observations by previous workers th a t th e com position and th e degree of pur­ (Czechoslovakia). The dropping electrode consisted of a drawn- out capillary tube connected to the mercury reservoir by means of rubber and glass tubing. Different capillaries w ith varying pres­

sures of mercury have been used in order to study the effect of the drop time on the diffusion current. The initial drop times in the authors' experiments were between 1.8 and 15.0 seconds.

An electrolytic cell with a pool of pure mercury

as anode was used in all experiments. The anode __________

potential was measured separately against an ex­

ternal saturated calomel electrode (S. C. E .) , using Ta b l e II.

an Irving and Smith (2) salt bridge filled with the

supporting electrolyte solution used in the cell. ¡vceofA

que-omce the anode potential remained constant within gUS s 0iuti0n a few millivolts during the electrolysis, the potential

of the dropping electrode was found from the ³houra

Four tetraalkylammonium products were used as supporting electrolytes in this work. Kahlbaum’s Reagent tétram éthyl­

ammonium iodide and tetraethylammonium iodide were used as the starting products. Both salts were purified by repeated recrystallizations from ethanol-water mixtures.

The tetraalkylammonium hydroxides were prepared from the iodides according to directions of Peracchio and Meloche (7).

The authors found that the freshly prepared solutions of the hydroxides yielded relatively large residual currents in the poten­

tial region where the alkali metal waves occur. These currents decreased with increasing alcohol concentration but they were still marked even in 80 per cent ethanol.

A precipitate was observed to form in both the aqueous and alcoholic solutions of the hydroxides upon standing. The resid­

ual current decreased during the period of separation of this pre­

cipitate. This phenomenon is not mentioned in the literature, but it is of great consequence in exact polarographic work.

When a solution of tetraetnylammonium hydroxide in water was allowed to stand in the dark for 5 to 10 days, the residual current of the filtrate did not decrease upon further standing. The solu­

tion remained perfectly clear and could be kept for several weeks without any change in the residual current. The alcohol-water solutions seemed to be somewhat less stable. The authors recom­

mend the preparation of a 0.5 to 1.0 iV solution of tetraethyl­

ammonium hydroxide in water, followed by filtration after about 8 days of standing. The filtrate is used for the subsequent prepa­

ration of mixtures with alcohol.

The change of the residual current at various cathode poten­

tials (vd.,.) of a 0.16 N tetraethylammonium hydroxide solution in 50 per cent ethanol with the time of aging of the aqueous solution is given in Table II.

For the lithium hydroxide solutions Merck’s Reagent was used.

Solutions prepared in 50 and 80 per cent ethanol were turbid, owing to the precipitation of lithium carbonate. The precipitate settled very slowly. After several days of standing the super­

natant liquid became clear and was filtered through hardened filter paper. When protected from the air, the filtrate remained clear indefinitely. The residual currents of lithium hydroxide solutions were markedly smaller than those of tetraalkylammo­

nium hydroxides at the same cathode potentials up to —2.2 volts.

E x p e r im e n ta l R e s u lt s

From the analytical view point it is of im portance th a t the diffusion current be proportional to th e concentration of the alkali ions. Solutions of potassium , sodium , and lithium chlorides of varying concentrations were electrolyzed in w ater, and in 25, 50, and 80 per cen t ethanol as a solven t, using tétram éthylam m onium hydroxide, tetraethylam m onium h y ­ droxide, and lithium hydroxide as th e supporting electrolytes.

T he diffusion current of sodium and potassium ions w as

474 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y Vol. 14, No. 6 can be m easured accurately in both 50 and 80 per cen t ethanol.

T he sam e is true for sodium .

From th e experience gained in a great num ber of experi­

m ents th e authors recom m end th e u se of 50 per cen t ethanol as th e m ost suitable m edium for th e polarographic determ ina­

tion of potassium and sodium . Som e results obtained w ith

Ju n e 15, 1942 A N A L Y T I C A L E D I T I O N 475 th e concentrations of both potassium and sodium ions.

A lthough th e agreem ent betw een th e --- calculated and experim entally determ ined

values is w ithin th e experim ental error, it is evid en t th a t th e polarographic m ethod cannot yield accurate results in th e indirect determ ination of potassium and sodium in

drop tim e should be greater than 3 seconds. T he residual cur­

rent of th e supporting electrolyte m ust be determ ined and sub­

tracted from th e apparent diffusion current. L ithium is deter­

mined in a m edium of 80 per cen t ethanol, the other conditions being th e sam e as m entioned above. Potassium and sodium at different concentrations of th e alkali m etals. I t was found these potentials were independent of th e ion concentration.

D etails of th ese m easurem ents w ill be given in a subsequent sodium and potassium are practically identical. Therefore, sodium and potassium , w hen present together, yield only one wave. Since th e diffusion coefficient of sodium is considerably less than th a t of potassium , th e diffusion current of sodium is notably less than th e corresponding valu e for potassium a t the same concentration. A t a given to ta l alkali m etal concentra­

tion the diffusion current of sodium and potassium should be equal to th e sum of th e diffusion currents of th e individual constituents. A lthough it is claim ed (S, 7) th a t this rule does not hold in aqueous solutions, th e authors found in 50 per cent ethanol a good agreem ent betw een calculated and experimentally determ ined values. T h e results tabulated in Table V I show th a t th e diffusion currents found in m ixtures

tration is betw een 40 and 100 tim es greater than th a t of potassium or of sodium or of the

tional to th e concentration, provided th a t th e concentration of the supporting electrolyte w as w ithin th e lim its given above.

476 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y Vol. 14, No. 6

porting electrolyte was determined as well as the constants &k + and ku * with

After subtraction of the residual current

= 0.64 microampere, the of th e concentrations of potassium and sodium and th e second to th a t of lithium . In order to obtain a well-defined lithium w ave it is desirable to work in a m edium of 80 per cen t alcohol.

A s a dem onstration Figures 5 and 6 present som e polarogram s.

In Figure 5 th e ratio of th e concentration of lithium to th a t th e supporting electrolyte and th e diffusion current of sodium and potassium a t the sam e potential. A ctually, th e diffusion current of both sodium and potassium is measured a t a poten­

tial betw een — 2 . 2 0 and — 2 . 4 0 volts. T h e corresponding authors have determ ined th e ?«2/3 i l/6 values of m ercury drop­

ping into a solution of tetraethylam m onium hydroxide in 80 tetraethylam m onium hydroxide in SO per cen t ethanol.

id {— 2 .2 5 volts) X millimole per liter) the concentration of lithium was calculated.

The authors found cu+ = 2.58/1.75 = 1.47 millimoles per liter. determ ination of lithium its concentration should n ot be more

Ju n e 15, 1942 A N A L Y T I C A L E D I T I O N 477 ammonium hydroxide as supporting electrolyte.

Initial drop tim e, 4.5 seconds. A node p oten tial, —0.330

Potassium and sodium can be determ ined

polarographi-cally w ith an accuracy w ithin found from one single current-voltage curve.

A c k n o w le d g m e n t am m onium hydroxide as supporting electro­

ly te. In itial drop tim e, 4.5 seconds. Anode- poten tial, — 0.340 v olt (vs. S. C. E .)