The determination of potassium by the cobaltinitrite method

VOL.

4 (1950)

ANALYTICA CHIMICA ACTA

THE

DETERMINATION

OF POTASSIUM

COBALTINITRITE

R. ~spar#ment

of

RELCHER

Chemisfry,

AND

Ths

475

BY

THE

METHOD

A.

J.

University,

NUTTEN

Bzrmingham

(England)

In the LAWRENCE SMITH method for the determination of the alkali metals in silicates sodium and potassium are isolated finally as the mixed chlorides, Potassium usually being determined as the chloroplatinate or perchlorate and sodium by difference. More recently, modifications have been introduced by MILLER AND TRAVES~, and later by HASLAM AND BEELY~, which avoid the tedious separation of the mixed chlorides. In general, these modifications consist of opening out the silicate by the conventional LAWRENCE Snfrm procedure, making up the leachings to a known volume, and determining sodium and potassium separately on suitable aliquots taken from this solution. Sodium is determined fairly rapidly by precipitation as sodium zinc uranyl acetate, but potassium is first separated by precipitation as the cobaltinitrite and is then converted to the perchlorate, as the final weighing form. These modifications effect some considerable saving of time, but, if it were possible to determine potassium as conveniently as sodium, the total time for the analysis could be reduced still further, and it would be possible to complete it in little over half a day. Since the present authors had been invited by Panel I of the British Coke Research Association to investigate the possibilities of developing a more convenient method for the determination of sodium and potassium in coal ash than that described in British Standard No. 1016 (which is based on classical procedures), it seemed probable that the modified methods for the determination of these elements in silicates could be applied with little modification. It seemed desirable, however, to investigate the possibility of developing a more convenient procedure for the determination of potassium. Potassium may be determined accurately by the perchlorate or chloroplatinatc methods, after separation of the alkali metals as the mixed chlorides, but both methods are time-consuming. The cobaltinitrite method will separate potassium fairly rapidly from most other metals, but under the usual conditions of precipitation, the precipitate is of variable composition and cannot be used as such for the weighing form. Potassium is often separated by this process and the precipitate then converted to the perchlorate as the final weighing form. Many claims References

p.

481.

R. BELCHER,

476

A. J. NUTTEN

VOL.

4 (1950)

have been advanced that a cobaltinitrite precipitate of constant composition may be obtained by adhering to a particular set of conditions, but these which have been examined have not been substantiated. Attempts have also been made to obtain precipitates of constant composition by precipitating potassium as the double lead or silver cobaltinitrite, but an examination of the results obtained suggests that a precipitate of theoretical composition is only obtained when the amount of potassium present lies between very narrow limits. The most searching examination of the cobaltinitrite method in recent times is that of PIPERS, who showed that several factors were responsible for the variable composition of the precipitate. Most noteworthy was the effect of the amount of potassium present, for the greater the amount, the greater the ratio K/Na in the precipitate. By standardising the conditions, PIPER was able to evolve an empirical formula by which the true potassium content could be evaluated from the titre obtained when the precipitate was titrated with a standard solution of potassium permanganate. Although this method appears to have given satisfactory results in the hands of later investigators, it was unattractive to us since the precipitate must stand overnight. Although potassium forms a greater number of insoluble salts than does sodium, only a few have been applied to the determination of potassium. Amongst those which have been used are the double calcium ferrocyanidebsG, the phosphotungstatec, the phosphomolybdate7, the dipicrylaminate*, the 6-chloro-s-nitrotoluene3-sulphonateD, and the periodate lo. All these methods were considered in turn, but it was concluded that, with existing reagents, the only possibility of finding a convenient and rapid method for the determination of potassium lay in examining the cobaltinitrite procedure. Two procedures, which are claimed to yield precipitates having the theoretical composition KsNa[Co(NO,),].H,O, were chosen for examination. These were the methods of HAM&~ and of WILCOX=. Although often referred to in the literature, critical examination of these two methods or observations thereon are lacking. WANDERIS has applied the WILCOX method to the calorimetric determination of potassium by allowing the precipitate to react with potassium dichromate and measuring the reduction in intensity of colour, but he prepared his standard curve from known amounts of potassium, and it is impossible to say whether the relationship was empirical or not. MILLER AND TRAVERSE applied the HAMID method to the determination of potassium in silicates, but they used it only as a means of separating potassium, and converted the precipitate to the perchlorate as the final weighing form. In the WILCOX method, precipitation is effected with trisodium cobaltinitrite in a nitric acid medium and the precipitate is allowed to stand for two hours. In HAMID’S method, a reagent prepared from sodium nitrite and a cobalt salt is used, and precipitation is effected in an acetic acid medium. The mixture is then evaporated to a paste on the water-bath, prior to filtration. It might be added References

p. 48r.

VOL. 4

DETERMINATIONOF

(1950)

POTASSIUM

477

that it seems to us that this procedure is exactly the same as that of DRUSHEL~~ published years earlier and to whom HAMID does not refer. It is claimed that this treatment not only gives a precipitate of constant composition but yields a precipitate which is easier to filter. Poor results were obtained by the WILCOX method. The results not only differed markedly from those required, but the reproducibility was poor. Furthermore, the precipitate was difficult to filter, tending to pass all but the finest filters. The HAMID method gave much better reproducibility and the precipitate obtained was easier to filter, but the results did not accord with those required. It was noted, however, that a fairly constant relationship existed between the determined and the required figures; and it seemed likely that by using an empirical factor, the true potassium content could be calculated with fair accuracy. The results were examined by the Method of Least Squares, and it was found that the determined values, as calculated from the ideal formula KaNa[Co(NO,)J I-&O gave the closest fit when multiplied by the factor o.g7_ As our purpose in seeking a rapid method for the determination of potassium was to apply it to the analysis of coal ash, it is considered that the results obtained are sufficiently accurate for an analysis of this type. Whilst the use of an empirical factor is generally considered undesirable, it should be pointed out that the chloroplatinate method, accepted as a standard method, also involves the use of an empirical factor. Where greater accuracy is demanded, the method cannot be recommended, for the reproducibility is not good enough. It seems unlikely to us that any modification of the cobaltinitrite method will supply a precipitate having the exact composition KaNa[Co(NO,),].I&O; and that a satisfactory method for the rapid determination of potassium will only be achieved by the development of new reagents. Most investigators who have examined this problem consider that the precipitate varies in composition according to the formula Ks_,Na,+,[Co(NO&JI&O. where x varies between o and I. Under certain conditions, such as those in HAMID’S method, x is of the order 0.x or less, but the value x = o appears to be unattainable. Another way of looking at it is to consider that the z compounds are formed in varying amounts. and K.Naa[Co(NO,),].HeO KBNGWWJd.~O Indeed, the latter conception is to be preferred, otherwise we have to accept the existence of a limitless number of coumponds containing varying amounts of sodium and potassium. It seems inconceivable to us that the formation of only these z insoluble salts can occur; one could reasonably expect some of the third insoluble salt of this series, the tripotassium salt, to be formed. We now advance a third possibility. Although the compounds &Na[Co(NO&]. are often referred to in the literature, we have Hz0 and K.Na.JCo(NO,),].I&O been unable to find any evidence that these compounds have been isolated as ’ such. The only two compounds of this type, the existence of which can be accepted with any certainty, are the tripotassium salt and the trisodium salt. It may References

p.

481.

R. BELCHER, A. J. NUTTRN

478

VOL. 4 (1950)

be that the variable composition of the precipitate is due to mixed crystal formation between these two compounds, and it is then unnecessary to assume the formation of other compounds whose existence has yet to be proved. In an attempt to obtain further information on this problem, an X-ray investigation of these compounds has been started, the findings of which will be communicated at a later date. A preliminary X-ray examination of precipitates obtained by the WILCOX and the HAMID methods bears out the results recorded in Tables I and II. Those obtained by the WILCOX method show different displacement of the diffraction pattern, whereas the variation in the precipitates obtained by the HAMID method is much less marked. EXPERIMENTAL Tlro Detcvmination

of Pofassizrm

by the WsCco.r Method

and filtered Pvocedwo : I ml of I N nitric acid and 5 ml of freshly-prepared trisoclium cobaltinitrite solution (containing I g of the salt) were added to IO ml of the solution containing potassium (as the chloride). The mixture was stirred, and allowed to stand for 2 hours. The preci itatc was then filtered on a porcelain filter crucible using 0.01 N nitric acid to ef Pect the transference, washed IO times ump. with 2 ml lots of the same acid and dried at the The precipitate was washed into a 250 ml beaker, tK e crucible placed in the beaker and the washings made up to approximately IOO ml with water. 20 ml of 0.5 N sodium hydroxide were added and the solution boiled for 3 minutes. A slight excess of 0.05 N potassium permanganate was withdrawn into another bealcer and diluted to about 30 ml with water. 5 ml of concentrated sulphuric acid were added and the solution cooled. The hot cobaltinitrite solution was then poured into the cold permanganatc solution, the crucible transferred and the beaker washed wrth a little water. An excess of standard sodium oxalate solution 0.1 N) was added, the solution boiled, and the titration complctcd with the Stan 6 ard permanganatc. TABLE THE IC

DETERMINATION

present (mg) 10.00 10.00 10.00 10.00 10.00

OF

POTASSIUM

I BY

The Determanation

WILCOX

METHOD

I< found (m6)

I< present (mg)

I< found (mg)

IO.54

5.00 5.00 3.00 3.00

4.92 4.78 3.51 3.28

;*“g; . IO.37 9.17 5.04 5.25

5.00 5.00

THE

of Poiassswn

2.00 2.00

I .oo I .oo

by the Hamid

I.91

2.14 I.12 1.15

Method

Praparation of the Reagent: A solution containing 220 6 of sodium nitrite in 400 ml of water was added to 113 g of cobalt acetate dissolved in 300 ml of water and IOO ml of glacial acetic acid. The mixture was shaken, warmed, and nitrogen oxides removed by evacuation at the water-pump. After standmg overnight the mixture was filtered and diluted to IOOO ml with water. References

9.

@r.

VOL 4 (rgso)

DETERMINATION

OF

POTASSIUM

479

Procsduvs: I ml of reagent was added to IO ml of the standard potassium solution previously acidified with I or 2 drops of glacial acetic acid, and the mixture evaporated to dryness on the water-bath. The residue was transferred to a porcelain filter-crucible using 5 per cent acetic acid, and finally washed with water. Some determinations were completed titrimetrically as in the previous method; others were completed gravimetrically by washing with five 2 ml lots of ethanol after the precipitate had been washed free from acetic acid, then with five 2 ml lots of ether, and dried in an oven at 100“ C (for not longer than 20 minutes) and weighed. The results are recorded in Table II. Table III includes the same figures after being multlplied by the tactor 0.97. TABLE

II

THE DETERMINATION OF POTASSIUM

BY

THE HAMID METHOD

K found.

K present mg

Gravimctric

Method

mg -

KC1 20.56 20.66 IO.30 10.28 10.30 5.15 5*‘5 2.oG 2.12 1.10 I .05 1.06 o-53 0.54 0.12 0.12

20.00 10.00 5.00 2.00 I .oo 0.50 0.10

KBr 10.32 10.28 30.35 5.12 5.18 2.10 2.00 2.11 I.10 I .og I .oG -

K,SO,

I

10.38 s-19 -

2.10 1.08 I .05 I

-

-

I

K found.

Titrimetric

Method

mg KC1 IO.33 70.38 IO.40 5.16 5.15 5.12 2.1 I 2.08 1.06 I .03 -

--

KBr 10.36 10.28 5.15 5.17 5.12 2.08 1.08 1.05 -

I&SO, 10.25 10.38 5.*5 5.14 s-=5 2.10 2.09 I .og 1.10 -

In the method we finally adopted for the determination of sodium and potassium in coal ash, (details of which will be published when the results are available from several co-operating laboratories who are comparing the method with the British Standard method), 1.0 g of ash is treated according to the LAWRENCE SMITH fusion procedure and the leachings made up to IOO ml. 25 ml of this solution are then taken for the determination of potassium. To express the results obtained as a percentage on the ash it is necessary to multiply the number of mg found by 0.4. The results to be expected under these conditions calculated from those in Table III by multiplying by 0.4 are recorded m Table IV. The results have been considered as a whole, regardless of which salt of potassium was used, or whether the determination was completed gravimetrically or titrimetrically. In practice it would be uncommon to find a potassium content as high as 4.0 per cent in coal or coke ash, hence only two determinations were made at a potassium concentration equivalent to 8.0 per cent (20 mg). For amounts of potassium less than 0.4 per cent, both weighing and titration errors would be appreciable and only four determmations were carried out below this level. It will be seen that for an analysis of this type the results are reasonably satisfactory. Rejevences

p.

48x.