analytical chemistry volume 28 issue 10 1956 [doi 10.1021%2fac60118a012] pflaum, r. t.; howick, l....

7/24/2019 Analytical Chemistry Volume 28 Issue 10 1956 [Doi 10.1021%2Fac60118a012] Pflaum, R. T.; Howick, L. C. -- Spec

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1542

acids, as it does in dilute aqueous solutions, that reaction could

account for the difference in the calibration line at high nitrogen

dioxide contents.

At one stage of the work the absorbances at 1.423 microns of

about

30

samples containing up t o

5

water were determined on

each of four modified Beckman DU spectrophotometers. The

variations among the instruments never exceeded 0.27, water

equivalent, and the average spread was only about 0.1 water.

Over the range of normal laboratory temperatures, the spectro-

photometric determination of water in nitric acid is not signifi-

cantly influenced by tempera ture.

Early in the work, before the techniques were precise enough

to show the nitrogen dioxide effect, the influence

of

dissolved ni-

tra tes of iron, nickel, and chromium was briefly investigated.

KO

nfluence on the water determination was detected.

It is

assumed, therefore, th at dissolved nitrates do no more th an sup-

press the extent of the self-dissociation

of

the acid.

The method has been in use in several laboratories for 2 years

or more.

ACKNOWLEDGMENT

Ruby James made the bulk of the measurements on which this

Walter B. Wade

aper is based, and most

of

the analyses.

A N A L Y T I C A L C H E M I S T R Y

assisted in developing the instrumentation and in making pre-

liminary measurements.

Twenty-three

of

the analyses used in

establishing the final calibration were performed by the Kava1

Air Rocket Test Station, Dover,

N .

J., through the courtesy of

J.

D. Clark and

H .

G .

Streim. The probable existence of the

effects

of

ionization equilibria was predicted to the authors by H.

E.

Higbie of

M .

W. Kellogg Co., New York, K .

Y.

LITERATURE CITED

1)

Dalmon,

R.

Freymann,

R.,

Compt . rend. 211,472 (1940).

(2)

Ellis,

J. W. Phys.

Rev.

38 693 (1931).

(3) Gillespie, R. J . , Hughes, E. D., Ingold, C. K.,

J. Chem. SOC.

1950, 2552.

(4)

Goulden.

J. D. S.,

Millen, D. J.

b i d . , 2620.

(5) Ingold,C. K. hlillen, D. J., b id . . 2612.

(6) International Critical Tables, vol.

111, p.

133, XlcGraw-Hill,

New

York.

1933.

(7)

Kinsey,W.

L.,

Ellis,

J.W. Phys. Rev. 36, 603 (1930).

(8) I b i d . , 51, 1074 (1937).

(9)

Lynn,

S.,

Mason, D.

M.,

Sage,

B.

H.,

I n d .

Eng.

Chem. 46

1953

(1954).

R E C E I V E Dor review September

28, 1955.

Accep ted June

21, 1956,

This

paper represents a p a r t of t he work done under Con t rac t No . AF 18(600)-53

wi th the Ai r Research and Developmen t Com mand , Wrigh t A ir Developmen t

Cen ter, Wright-Pat terson Air Force Base, Ohio.

Spectrophotometric Determination of Potassium

with Sodium Tetrapheny lborate

RONALD T. PFLAUM and LESTER C. HOWICK

D e p a r t m e n t o f Chemis t r y , S ta te Un i ve rs i t y o f lowa, lowa Ci ty,

lowa

Potassium tetraphenylborate was investigated spectro-

photometrically in an acetonitrile-water system. The

tetraphenylborate ion shows absorption maxima a t 266

and

274 m,u, with molar absorptivities of 3225 and 2100,

respectively. Beers law is obeyed over a concentration

range from 5 X 10-6 t o 7.5

X

10-1

M .

The results

obtained on the determ ination of potassium in selected

samples indicate the feasibility of employing the de-

scribed system for such determinations. Spectro-

photometric evaluations of the solubilities of tetra-

phenylborate salts in aqueous solutions were obtained.

ITHIX the past

5

years, sodium tetraphenylborate has

come into prominence as

a

precipitant for potassium.

Various methods

for

the determination of potassium based on

the insolubility of the potassium salt in aqueous solution and it s

solubility in certain organic solvents have been advanced.

Gravimetric 2,

7 ) ,

itrimetric (6, 9 ) , urbidimetric

8),

onducto-

metric

7 ) ,

and voltammetric

1 )

methods have been proposed.

A

spectrophotometric method is a logical and useful extension to

this existing list of measurements.

Tetraphenylborate salts are soluble in certain organic solvents.

Dissolution in acetonitrile leads to

a

solvent medium that is

especially well suited for spectrophotometric measurement.

This work is concerned primarily with an investigation of the

potassium salt in such an acetonitrile medium. I t was under-

taken in order to elucidate the feasibility of a spectrophotometric

determination of potassium.

APPARATUS AND REAGENTS

All spectrophotometric measyements were made at room

C.) with a Cary Model 11

emperature (approximately 25

Table I. Summary of Potassium Determinations

Potas s ium, Mg.

Sample Presen t F ound

Table

11.

Solubilities of Tetraphenylborate Salts in

Pure Water at 25C.

Solubi l i ty , X 106

Sal t Exper imen tal L i t e ra tu re

4

1 0 . 7

c s 2 . 7 9 3 . 2 8

20 C.) 4)

K 1 7 . 8 1 8 . 2 (IO)

R b

2 . 3 3 4 . 4 1 20

C.) 4 )

T1

5 . 2 9 2 . 9

5 )

recording spectrophotometer, using 1-cm. matched silica cells.

A Beckman Model G pH meter was used for all pH values.

Sodium tetraphenylborate was obtained from the J. T. Baker

Chemical Co.

It

was used as received for all work except for

obtaining the absorption curve

of

the reagent. In this case,

reagent recrystallized from an acetone-hexane mixture was used.

Crystalline salts of ammonia, cesium, potassium, rubidium, and

thallium(1) were prepared by reaction of the respective chlorides

with the reagent in aqueous solution. Recrystallization of the

precipitated material was effected from an acetonitrile-water

system.

Acetonitrile was obtained from the Matheson, Coleman Bell

Division

of

the Matheson Co. Purification was effected by

treating with cold saturated potassium hydroxide, drying over


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V O L U M E 28, N O . 10 OCTOBER

1 9 5 6

1543

2.c

I

W

z

m

a

a

8 1s

m

U

O.

Figure 1.

Absorption spectra

of

tetraphenylborate

salts in acetonitrile

Curve

1

2

3

4

5

C o n c n . ,

. f x

10-4

1 . 2

2 . 4

3 . 6

4 . 8

6.0

Salt

S

I

Rb

Na

K

anhydrous potassium carbonate for

24

hours, refluxing over

phosphorus pentoxide for 2 to 3 hours, then distilling from

phosphorus pentoxide in an all-glass system. The fraction

boiling a t 81-81.5' C. was used as the pure solvent.

All other chemicals used were of reagent grade quality.

SUGGESTED METHOD

After appropriate preliminary treatment of t he sample to yield

an aqueous solution of potassium ion, adjust the pH

of

the

solution to 4.0 to 5.0 with dilute sodium hydroxide and dilute

sulfuric acid.

Prepare a stock solution of sodium tetraphenylborate by

dissolving 1.0 gram of t he reagent and 0.5 gram of aluminum

chloride hexahydrate or aluminum nitrate hexahydrate in

100

ml. of water. Filter the solution to remove any turbidity th at

mag develop.

Add 5 ml. of the reagent solution to 5 ml. of the sample solution

in a 15-ml. graduated centrifuge tube. Centrifuge for 3 minutes

in a high speed centrifuge and remove the supernatant liquid by

pipet. Kash the precipitate twice with 3 ml. of a cold saturated

solution of the potassium salt, again removing liquid by pipet.

A constant volume of liquid (0.5 ml.) is left with the precipitate.

Dissolve the precipitate by adding 5 ml. of

a

mixture of 7573

acetonitrile and 25% water. Transfer to a 25-ml. volumetric

flask, rinse the centrifuge tube with additional solvent, and dilute

the sample to 25 ml.

solvent mixture to yield the 25-ml. volume.

Prepare a blank solution by an identical procedure, using the

Measure the absorbance at 266 mp. Determine potassium

concentration from the absorbance value and a prepared cali-

bration curve.

RESULTS

The results obtained with this method are shown in Table

I.

The values given are the average of multiple measurements on

the various samples. The analyses indicate tha t potassium can

be determined with an accuracy within that usually assigned to a

spectrophotometric method. The method is applicable to the

determination of

2

to 30 p.p.m. (5 X 10-6 to 7.5

X

10-4M) of

potassium in the measured sample with an accuracy of 2 .

DISCUSSION

Spectrophotometric Studies. l spectrophotometric investi-

gation

of

various systems was undertaken as an initial step in

this study. Absorption curves were obtained for weighed

amounts of the various pure tetraphenylborate salt s dissolved

in pure acetonitrile. Curves of ammonium, cesium, potassium,

rubidium, and sodium tetraphenylborate are shown in Figure 1

from n-hich it is evident th at all of these salts have the same

absorption characteristics. I t

is

concluded, therefore, that the

absorption maxima of the tet raphenylborate ion occur at 266

and 274 mp. The molar absorptivities at these maxima are

3225 and 2100, respectively. It was found, in addition, that

identical results were obtained when technical grade solvent was

used without purification.

Solutions containing varying concentrations of the potassium

salt (5.0 X 10-6 to 7.5

X l O - * X

were prepared and measured

in order to determine the conformance to Beer's law.

It

was

found that Beer's law was obeyed over the concentration range

studied. Moreover, the solutions show a high degree of stabi lity

with no apparent changes even after

5

days.

The effect of the addition of water t o acetonitrile solutions of

the salts

was

also investigated. Varying amounts

of

water were

added to constant amounts of potassium tetraphenylborate in

acetonitrile, with subsequent dilution to constant volume with

the organic solvent. Absorption curves for a series of solutions

prepared in this manner are shown in Figure 2. The curves have

been displaced vertically, inasmuch as very little change occurs

in the absorptivity of th e absorbing ion. However, a loss in the

definition of the curves results from the addition of increasing

amounts of water to the system. There is no apparent change in

the shape of the absorption curve up to

40%

of water by volume.

Thus, analytical measurements could be carried out in a solvent

mixture of these proportions.

A study of the solubility characteristics

of tetraphenylbora te salts was carried out. The recrystallized

ammonium, cesium potassium, rubidium, sodium, and thallium

(I)

salts were studied. With the exception

of

the sodium com-

pound, all are insoluble in water. All dissolve in acetone, aceto-

nitrile, dimethylformamide, and dioxane. The thallium sal t is

the least soluble in these solvents. All are insoluble in benzene,

carbon tetrachloride, and chloroform.

4 study of the solubility

of

the salts in aqueous solution was

carried out by saturating conductivity water with the respective

salts at 25 C. After equilibrium was attained, portions of the

solutions were analyzed for tetraphenylborate

ion

content by

spectrophotometric measurement a t 266 and 27-1 mp. The

results of this study are presented in Table

11

d comparison

with reported literature data indicates that solubility values

from spectrophotometric measurement are comparable to those

obtained by conductometric (IO) and radiometric measure-

ments 4 ) .

A study was carried out of the pre-

cipitation reaction of potassium and th e reagent in aqueous

media, as well as the effect of pH.

For

these purposes

a

stock

solution of potassium ion, 2 X 10-2J4, was prepared by dissolving

potassium chloride in conductiv ity water. A stock solution 4

X 10-*M in sodium tetraphenylborate was prepared by dis-

solving the reagent in water. Turbidity in th e solution

was

removed by filtration. Portions

of

5 ml. of t he potassium solu-

tion were added to 5 ml. of the reagent solution. Changes in pH

were made with dilute sulfuric acid and sodium hydroxide.

It

was found th at qu anti tati ve precipitation of potassium

occurred in a pH range from 1 to 9. Precipitation in cold solu-

tion is recommended for the pH range from 1 to 3 in order to

prevent the decomposition of the reagent (6). The reagent shows

a high degree

of

stability in neutral

or

basic solution.

Solubility Studies.

Precipitation Studies.


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1544

A N A L Y T I C A L C H E M I S T R Y

WAVE

LENGTH, rnfi

Figure 2.

Effect of addition of water to potassium tetra-

phenylborate in acetonitrile

C u r v e

1

2

3

4

5

Water

0

20

40

60

80

The insoluble potassium salt can be quantitatively separated

from the aqueous phase by filtration with a fine-porosity sintered-

glass filter. Separation can also be effected by centrifugation in

a high speed centrifuge. The presence of aluminum ion in the

solution, as proposed by Kohler

(6)

and Findeis and De Vries

I ) ,

is helpful in causing the separation of the precipitate. Alu-

minum ion is apparently precipitated together with the potassium

salt and is not soluble in pure acetonitrile. The mixed precipi-

tate dissolves completely in an acetonitrile-water medium.

The effects of the presence

of

diverse ions on the precipitation

reaction are summarized in Table

111.

The data were obtained

at pH 3.0 to

4.0

for solutions 4 X 10 -3J I in potassium ion and

8

X 10-3M

in reagent. Diverse cations vere added as the

chloride or perchlorate salts.

Sodium salts

of

the anions were

employed.

Table 111.

Effect of Diverse Ions

A m o u n tPermissible,

I o n P.P.M.

C2H802-

A l + + +

NHI

Gal-

c s

C o + +

C u t +

F e + *

L i t

R l g

2000

2000

0

2

0

1000

1000

1000

2000

2000

I o n

+ +

3+

XO:-

R b

SO,

- -

X-(Br- ,

C1-,

I - )

A m o u n t

Permis s ib l e ,

P.P.hI.

0

1000

2000

0

0

2000

0

2000

2000

Only a few ions interfere in the system.

Interferences result

from the interaction

of

the particular ion with the reagent.

Of

the ions interfering, silver, mercury(II), and thallium(1) can

readily be removed by simple preliminary treatment of the

sample. Certain of th e amines and ammonia are likewise easily

removed through preliminary treatment . Cesium and rubidium

ions, although usually present only as traces in a sample, are

precipitated together with the potassium. Certain amines,

cesium, potassium, or rubidium can be quantitatively deter-

mined with sodium tetraphenylborate in the absence of

the other

species.

LITERATURE CITED

(1)

Findeis,

A .

F.,

De

Vries,

T.,AXAL. HEM.8, 209 (1956),

(2)

Flaschka,

H., 2

anal.

Chem. 136, 99 (1952).

(3)

Geilmann,

W., Angew. Chem. 66,454 (1954).

(4)

Geilmann,

W.,

Gebauhr,W., 2

anal. Chem. 139, 16 1 (1953).

5) Hahn,

F. L.,

Ibid. 145, 97 (1955).

(6)

Kohler, M.,

Ibid. , 138, 9 (1953).

7)

Raff,

P.,

Brotz,

W. Ib id . , 133,

241

(1951).

(8) Rubia Pacheco, J. de la, Blasco Lopez-Rubio,

F.,

Chemist

(9)

Rudorff,

W.

Zannier,

H., 2

anal.

Chem. 140,

1

1953).

(10)

Rudorff,

W.,

Zannier, H., 2 Naturforsch. 8b,

11 (1953).

RECEIVED or r e v i e w M a y 5 , 1956.

Analy t i ca l Chemis t ry , 129 th meet ing . I C s . Dal l as , Tex . , A pr i l 19;1G.

AnaEyst 44,58 (1955).

Accepted July 18, 1956. Division

of

Spectrochemical Analysis of Thermionic Cathode Nickel Alloys

by a Graphite to Metal Arcin g Techniq ue

EDWIN

K J A Y C O X

A N D

BETTY

E.

PRESCOTT

Bel l Telephon e Laborator ies,

I n c ,

Mur ray H i l l , N.

A

technique i s described for the determination of alumi-

num, cobalt, chromium, copper, iron, magnesium,

manganese, silicon, and titanium in thermionic cath-

ode nickel alloys, in the general concentration range

from 0.003 to 0.2 for each element. In this procedure

10 mg. o f nickel metal is placed in the crater of a graph-

ite cup and burned to extinction in the direct current

arc. The precision is adequate for the determination

of

the metals listed in the concentration ranges nor-

mally encountered in cathode nickel alloys. The speed

of the analysis is considerab ly increased over tha t

of

the dry oxide powder technique.

HE

chemical composition and the analysis of the nickel

T

lloys used for the thermionic cathodes in electron tubes has

long been of primary concern to the manufacturers of these

devices. The ease

of

activation, the ultimate degree

of

therm-

ionic activity , and the emission life of thermionic cathodes

are markedly dependent upon trace constituents present in the

nickel base

to

which the alkaline earth emitter

is

applied. Trace

constituents, particularly magnesium, silicon, aluminum, and

titanium, react with the alkaline earth compounds (barium,

strontium, and calcium oxides) of t he coating to produce free

alkaline earth metal thought to

be

essential

for

high thermionic

activity. Other elements such as iron and manganese may

analytical chemistry volume 28 issue 10 1956 [doi 10.1021%2fac60118a012] pflaum, r. t.; howick, l....

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