The determination of microgram amounts of beryllium using acetyl acetone

ANALYTICA

462

THE

DETERMINATION

OF

BERYLLIUM

J. Chemical

A.

ADAM,

Inspectorate,

E.

CHIMICA

USING

BOOTH

Ministry

MICROGRAM ACETYL

AND

J.

of Supply, I.

VOL.

ACTA

D. Royal

AMOUNTS

6 (1952)

OF

ACETONE

H.

STRICKLAND Arsenal,

London

(EngCand)

INTRODUCTION

Although minute traces of beryllium are perhaps best determined spectrographically it is desirable to have an accurate chemical method as an independent check. A wide survey of the literature showed that existing methods depend on electrometric titration, fluorimetry and the absorptiometry of solutions resulting from lake formation. The electrometric methods were not sufficiently sensitive while, in our opinion, the difficulties inherent in the accurate quantitative measurement of small fluorescences discouraged further investigation in that direction. Methods involving lake formation were tried using quinizarin-s-sulphonic fi-nitro-benzeneazo-orcino12 and naphthazuins (5 : 8 dihydroxy I : 4 acid’, naphthoquinone) the last. one being the most suitable. The non-stoichiometric character of the coloured compound formed, and the rigorous control of conditions necessary to ensure reproducible lake formation, eventually proved that even this reagent was not entirely suitable for routine work. In our opinion it is always desirable to use a definite molecular species as the basis of an absorptiometric method. The only method we have found mentioned in the literature based on the formation of such a species is one using the complex formed between beryllium ions and sulphosalicylic acid4. This method is, however, less sensitive than the one described here and we have found it more subject to interference. Beryllium forms well-known chelate derivatives with @iketones. These are stable compounds and, as they are easily soluble in organic solvents, they should be suitable for analytical purposes. Although acetyl acetone, the simplest and most readily obtainable @diketone, would not be a specific reagent for beryllium it was considered that it should prove useful for separating traces of beryllium from a number of other elements. Further, the absorption of solutions of the complex for ultraviolet light might also be used for its quantitative determination. The interference of elements known to give acetyl acetonates could be eliminated by suitable complexing agents or, failing this, by the complete removal of the elements concerned. References

p.

471.

‘VOL.

6

(x952)

DETERMINATION

OF

463

BERYLLIUM

The work described in this paper is divided into three main parts, the absorption properties of the pure beryllium acetyl acetonate, the quantitative formation of this compound from pg quantities of beryllium in aqueous solution and the isolation of beryllium from large amounts of other elements. II.

A.

The absorption

of pure

EXPERIMENTAL

be~yilium

acetyl

acetonate

solutions

In Fig. I absorption curves of a 5-10-6 molar solution of the pure beryllium acetyl acetonate in chloroform and carbon tetrachloride are shown. These were obtained using the Uvispek spectrophotometer of Messrs. Hilger and Watts Ltd., fitted with a quartz prism. An absorption curve of a 10-0 molar solution of the reagent in chloroform is also included for comparison purposes. Chloroform was found to be preferable to carbon tetrachloride as a solvent. *Chloroform absorbs less light in the ultraviolet than does carbon tetrachloride and solutions of beryllium acetyl acetonate in chloroform are more resistant to washings with aqueous alkali. In order to elimate errors from stray radiation (about 0.5 per cent. at 3000 A with the particular spectrophotometer used) the determination of beryllium from the absorption of solutions of its acetyl acetone complex is best made using

Fig.

I. Absorption a. I.Io-~M b. 5.10’5M c. 5.1o-~M

Refe+ences

p. 471.

spectra

of acctyl

acetone

and derivatives

Acetyl acetone in chloroform Beryllium complex in carbon tetrachloride Beryllium complex in chloroform

464

J. A. ADAM, E. BOOTH, J, D. H. STRICKLAND

VOL. 6 (x952)

calibration curves. After allowing fof stray radiation in our instrument it could be shown that the BEER-LAMBERT law was strictly obeyed. The molar extinction coefficient of the pure complex in chloroform at 2950A was found to be 3.~6~~04, B. ‘The quantitative formation of micro-g qzcantities of bcryWum acetyi? acetonate I. The removal of excess reagent Excess’ reagent is necessary for the quantitative formation of the beryllium complex and, as the reagent is extracted into chloroform and absorbs light at about the same wavelengths as the complex, it is essential to ensure that it is completely removed. Chloroform solutions of the complex were found to be unaffected by aqueous alkali extractions provided the extraction times were not excessive (over 0.5 I minute). The excess reagent, on the contrary, was found to extract’itito the aqueous phase, presumably as the enolate ion. The removal of excess reagent was checked by the following experiments. 25 ml of a 0.2 per cent. solution of acetyl acetone in chloroform were extracted with successive 50 ml portions of alkali for about a minute. The extinction of the chloroform solution was measured in a I cm cell after each extraction. The results of these experiments showed that with ammonia a small residual blank of extinction 0.x to 0.15 was always obtained, which could not be significantly reduced. Two successive extractions with o.~iV sodium hydroxide, however, reduced the blank extinction to about o,os and this could not be conveniently further reduced, 2. T&z effect of reagent concentvatiolt and reaction time ott the formation of tk com$dex from traces of berytlium As beryllium acetyl acetonate is known to be hydrolysed by strong bases or acids, it was expected that the quantitative formation of the complex would occur most readily in an approximately neutral solution. The following experiments were therefore carried out at a pa of 5.5 to 6.0. To avoid difficulties owing to the emulsification of the aqueous and chloroform phases during the extraction procedure, the aqueous phase contained some electrolyte. IO $g of ‘beryllium were added as a standard beryllium sulphate solution to 25 ml of 0.02 molar sodium chloride. The acetyl acetone was then introduced as a 0.1 or I per cent. solution, as convenient, and the pR adjusted to 5.5-6.0 by the addition of dilute alkali. After allowing the appropriate reaction time to elapse, the complex formed was extracted into chloroform and estimated by the technique described in detail later. The extinction of the blank was about 0.07 and did not appreciably alter throughout the experiment, the results of which are given in Table I. References p. 47x.

VOL.

6 (1952)

DETERMINATION

OF

TABLE I.I*IO~

Moles Acetyl Acetone x 100

ml

SOLUTION

Percentage Formation

IO

2

2

IO

IO IO IO

a

IO 100 IO0 500

less

so0 500 so0

effect of fiH

25

Time of Standing Minutes

4

3. The

I

BE IN APPROXIMATELY

MOLES

465

BERYLLIUM

on the formation

tdn

;z 100 IO0

86

0.5

;

;:

IO

100

of the cotnfilex

From a consideration of the data recorded in Table I, it was decided that 2.0 ml of 1 per cent. reagent in 50 ml (xoo-Io’6 moles in 25 ml) and a reaction time of 5 minutes would be satisfactory. Similar experiments to those described above were then carried out at varying pH values of the base solution. The results are shown graphically in Fig. 2 together with the titration curves of 0.25iIf acetyl acetone and 1.2=10-3M beryllium sulphate, the significance of which will be discussed later. 100

Pprc~rqe

bwyllium 75

rwtracled as complex so

25

UC

pefcont

extracted

beryllium

tltratlon

4

0.5

Equlvaletis Fig.

References

#. 471.

2.

Effect

Clli’per

“Zole of

1.5 2.0 Be3 of per mole of acetyl acetone

of prr on the formation

of the

complex

I

466

J.

A.

ADAM,

E.

BOOTII,

J.

D.

H.

STRICKLAND

VOL.

6

(1952)

In all subsequent experiments 2.0 ml of I per cent. reagent and a reaction time of 5 minutes at PH 6-8 (neutral tint of bromo-thymol blue) have been used for the formation of the complex in 50 ml of aqueous solution. 4. Interference

by other iolts

a. Cations. I mg quantities of the following elements, uranium, lead, silver, cerium, copper, sodium, potassium, calcium, strontium, barium, molybdenum, zinc, chromium and manganese were added to a base solution of sodium chloride and the normal procedure for the determination of beryllium carried out. In no case were extinctions obtained which were equivalent to more than 0.~25 pg beryllium. The presence of the ammonium ion, however, invariably increased the blank and it is inferred that some extractable ammonium compound with acetyl acetone is formed. Aluminium and iron interfered seriously and in their presence beryllium could not be determined. Anions. No interference was observed from large quantities of sulphate, b. nitrate, chloride and perchlorate. I mg quantities of fluoride, phosphate and acetate also had no affect on the recoveries. The presence of I mg of citrate, however, reduced the recovery of beryllium to 60 per cent. while in the presence of IO mg of citrate the recovery was only 30 per cent. Large amounts of acetate (exceeding about o.zjW) caused low erratic recoveries and the presence of acetate should be avoided if at all possible. 5. The use of ethylene diamine tie&a-acetic acid (com+Zexone) from other cations

to @went

interfmence

It was found that roe per cent. recoveries of beryllium could be obtained from 50 ml of a solution containing 5 per cent. sodium nitrate, 6.8-1o-~iV (I ml of z per cent.) !ethylene diamine tetra-acetic acid and IO pg of beryllium using the were then made under these normal procedure. A number of determinations conditions in the presence of IOO pg of iron and aluminium and full recoveries of beryllium were again obtained with no significant increase in the blank. The concentration of the complexone was then increased tenfold and a blank determination made in the presence of I mg of each of the following elements iron, aluminium, chromium, zinc, copper, manganese, lead, silver, cerium and uranium, when the’blank obtained was found to be equivalent to less than 2.5 pg of beryllium. ‘, C. The fireliminary

concentralion

of beryllium

The separation of beryllium from most materials, except those containing large amounts of aluminium, is in general a fairly simple matter. Many cations such as zinc or copper can be separated by electrolysis while suitable extraction procedures can be used for the bulk removal of such metals as iron, Refevenccs

$. 47x.

VOL.

6

(1952)

DETERMINATION

OF

BERYLLIUM

467

After such preliminary separations, however, milligram quantities of many metals may still remain and if necessary the solution can be further freed from interfering elements by, for example, an oxinate extraction or a hydrogen sulphide precipitation. I. The recovery

of beryllium

from

a sodium

nitrate-acetate

sol&on

As a result of possible preliminary separations of other cations from a sample the beryllium may remain in a solution containing comparatively large amounts of ammonium, potassium or sodium salts together with a few mg of other metals. A typical example might be 50 ml of solution 5 per cent. with respect to sodium nitrate, I per cent. with respect to sodium acetate and containing about I mg each of aluminium, iron, copper, cobalt and manganese. Even in the presence of excess complexone (ethylene d&nine tctra-acetic acid) the beryllium cannot be determined directly in such a solution, since erratic results are obtained owing to traces of iron and aluminium etc., being carried through to the absorptiometric finish. Other impurities such as oxine and its decomposition products might also be present as the result of preliminary analytical separations. We have shown, however, that this first chloroform extract can be wet oxidised, without loss of beryllium, by a procedure such as that outlined in III below, and that the beryllium can then be easily estimated in the residue, the concentration of the interfering elements having been reduced to such an extent that their effect is completely masked by a second addition of complexone. From determinations of the recoveries of IO (ug amounts of beryllium from the base solution described in the first paragraph above, using various amounts of ace+1 acetone, it was found that 5 ml of 5 per cent. reagent is a safe minimum quantity to use for ,ug quantities of beryllium in 50 ml of the base solution and this amount per 50 ml is used in all subsequent experiments. 2.

The limit of detection

of beryllium

from

a base sol&on

For statistical reasons we consider a safe upper limit of detection to be a quantity of beryllium equal to three times the standard deviation of its determination. In order to find approximate values for the standard deviations three sets of eight determinations were made on 50 ml of a solution of the same composition as the “base” solution described above except that the sodium acetate was omitted. One set contained no beryllium, the other two 0.25 and 0.5 pg respectively. A blank determination was made alongside each of the 0.5 and 0.25 pg determinations and used to correct the extinctions obtained. The results are collected in Table II. Relevemes

p.

471.

J. A.

468

ADAM,

E. BOOTH,

J. D.

TABLE Extinction Bisnk

of

0,ozG

0.026

0.054 0.064 0.065

0.025

0.060

0.032 O.OY4 0.04s o-033

0,081 0.046 0.043 Mean s

0.062 0.014

6

(1952)

IX

0.042

CLOG2

S

VOL.

Extinction of 0.25 c/g Extinction of 0.35 /rg -Extinction of -Extinction of corresponding blank corresponding blank

0,058 0.069 0,056 0**79

Mean

H. STRICKLAND

0.080 0.075 0.069

0.076

0.030 0.010

Mean S

o.oG8 0.008

It will be seen from this table that three times the standard deviation of the determination of 0.25 pg is about equal to 0.~5 pg. This weight of beryllium will therefore be considered as the limit of detection. Even allowing for any reasonable skewness of distribution a quantity of beryllium equal to or greater

than 0.25 pg should nineteen times out of twenty be detectably greater than a parallel blank determination. 3. The @~centuge uecooevz’es of ~~~~~~~~rn from a eebasc*’ solution Various amounts of beryllium were added to 50.0 ml of the base solution used above and the beryllium was then determined by the full method given below. The results of these determinations are shown in Table III and indicate that within the limits of experimental error full recoveries of beryllium can be obtained. TABLE fig Be added

Extinction blank

0.5 I.0 2.0

0.x2

0.08 0.09 O.OQ 0.0s) 0.09

8::

9.0 x0.0 X0.0 Rcfevences 9.

0.06

0.08 0.09

0.09

::::

0.09 0.06

471.

Corrected Extinction of Determination

0.08 0.08

;:z

of

III pg Be found

o-45 0.91

Percentage Recovery

;:

2.05

LO2

2.52

101

0.65 0.80

Z:,“,

IO1

0.89

g*;s

97 100

0.27 0.33 0.5X

I .06 r.16 X.30 1.26

3.86

8:8 9.8 9.5

;:

;: 95

(I

VOL.

6 (x952)

DETERMINATION III.

N.B.

-

DETAILS

OF BERYLLIUM OF

THE

469

METHOD

To avoid interference in the absorptiometric finish, the stopcocks of separating funnels should be lubricated with a siIicone type grease and not with Vaseline.

To 50 ml of the solution containing up to IO pg of beryllium, (neutralise any excess acid so that the pn is about 0.5-1, but not greater than z) add 2.0 ml of xo per cent. complexone and adjust the PH of the solution to about 7-8 b the addition of o,xN sodium hydroxide (greenish blue tint of bromo-thymol b Yue). Add 5 ml of s per cent. aqueous acetyl acetone and once more adjust the pxi to 7-8. Allow the solution to stand for five minutes and then extract it with three x0.0 ml portions of chloroform. Combine the extracts in a platinum dish. T?re wet &es&u&on

of the cMovo/ortn

extracts

Cover the chloroform extracts with about 13 ml of water, 2.0 ml of concentrated nitric acid (16.M) and 2.0 ml of 6o per cent. perchloric acid. Evaporate off the chloroform on a water bath and then transfer the dish to a hot plate and continue the evaporation to dryness, Repeat the evaporation with a further 2.9 ml of nitric and perchloric acids after which no organrc material should remain. Tlhe absovpciometvic

fin&&

Dissolve the residue from the wet oxidation in about 1s ml of o.rN nitric acid and transfer the solution to a separating funnel. Add a few drops of bromo-thymol blue indicator followed by I ml of z per cent. complexone reagent. Adjust the of the solution to about PH 7 by the addition of o.xN sodium hydroxide. Ad 8: ml of I per cent. aqueous acetyl acetone and readjust the pli to 7. Allow the solution to stand for 5 minutes and then extract it with three 8 ml portions of chloroform, the extracts being collected in a 2s ml measuring flask. Adjust the volume to 25 ml by the addition of chloroform and transfer the whole (without further addition of chloroform) to a separating funnel containing 50 ml of O.IN sodium hydroxide. After shaking the solutions vigorously for 30-60 seconds allow the twp layers to separate and repeat the aqueous alkali extraction with a further so ml of o.xN sodium hydroxide. Finalfy run the chloroform through a small filter paper into a x ofn quartz cell and measure the extinction of the solution at a wavelength of zg3o A against a blank of pure chloroform. Calibration

To 25 ml of 0.02 M sodium chloride solution add known quantities of beryllium, Continue the determination as described in the last paragraph above and plot a calibration curve of c(g beryllium against extinction. IV.

DXSCUSSION

Acetyl acetone has proved to be a useful reagent both for the concentration and the absorptiometric determination of traces of beryllium. Other j?-diketones would probably be equally effective but they are unlikely to be much more sensitive or specific and are less readily available. Beryllium acetyi acetonate has a high molar extinction coefficient in the region of 2950 A and this makes the method as sensitive as any to be found in the literature. The sensitivity of the method also relies to some extent on the small equivalent Refcvemcs

p. 471.

470

J.

A.

ADAM,

E. BOOTH,

J. D.

Ii. STRICKLAND

VOL.

6 (rgsz}

weight of beryllium, the correspondingmolar extinction coefficients of the iron and aluminium complexes being of about the same order. Although many metals form acetyl acetonates the conditions of formation used here appear to make the reagent fairly specific for beryllium, only iron and aluminium of the more common elements giving serious interference. The use of complexone (ethylene diamine tetra-acetic acid), however, makes acetyl acetone virtually specific for beryllium which is only weakly .complexed while even mg quantities of other metals are complexed to such an extent that they no longer react. Should the amount of iron and aluminium exceed about 5 mg however it would probably be advantageous to remove these elements by an oxine extraction. Most anions give no interference but the presence of citrate causes low recoveries of beryllium and should be avoided. Large concentrations of acetate also have an adverse effect and should if possible be absent. Although the limit of detection of beryllium may vary considerably depending on the amount and number of other cations present in solution the figures shown in Table II are considered to be fairly representative of results obtained from a typical residue in which the beryllium is to be determined. From these data the statistical limit of detection’evaluates as about 0.25 pg of beryllium and it is unlikely that this could be lowered appreciably by using longer cells or more concentrated extracts. There is no apparent bias in the recoveries of added beryllium to the typical base solution and the data in Table III indicate a precisionof about & 5 per cent. at a x0 pg level. The mechanism of the reaction between beryllium and acetyl acetone is to be the subject of a further full study, but it will be seen from Fig, z that full recoveries of beryllium are obtained over a PH range between that required to give the first inflection in the titration curve of Be+2 and the inflection in the titration curve of acetyl acetone. The first inflection of the titration curve of Be+a with alkali in dilute solution corresponds to the formation of a Be(OH) f ion which subsequently gives an uncharged beryllium hydroxide Be(OH), molecule at the second inflection point. The inflection in the reagent titration curve corresponds to the ionisation of one enol group in the ketone. CH,CO-CH = C(OH)-CH,

G-2

C&,CO-Cl-I = C(0) --CH,

+

H+

As this ionisation becomes complete it will be seen that rio beryllium acetyl. acetonate is formed. It must be assumed, therefore, that the reaction forming the beryllium acetyl acetone complex isbetween a basic form of beryllium (both Be(OH)+ and Be(OH),) and the unionised form of the reagent. It has been confirmed by us, experimentally, that the reaction with Be(OH)s gives rise to no PN change (as would be expected with a simple water elimination Refevcmces p. 471.

VOL. 6

DETERMINATION

(x952)

OF BERYLLIUM

combines,

reaction) whereas, when the species Be(OH)+ liberated. In the latter case the reaction must be, kH,

471 one hydrogen

ion is

CH, I

LO\ /

Be+

HC

\ \c

-OH

bH

_--+

P-Ok

Be-OH

HC

\ \c

-0

+

H+

/

followed by a further condensation of acetyl acetone with the beryllium acetonate hydroxide to form the final beryllium complex.

acetyl

ACKNOWLEDGEMENTS This paper is published by permission of the Chief Scientist, British Ministry The authors wish to acknowledge the assistance of Mr. D. A. CHARMAN of Suppl during t l?e earlier stages of the work. SUMMARY This paper describes the development of a method for the determination of This reagent is first used to of beryllium using acetyl acetone. pg quantities isolate the beryllium, after interfering elements have been complexed with ethylene &amine tetra-acetic acid. The beryllium is then determined absorptiometrically as the acetyl acetone complex using radiation of wavelength 2950 A. The method will detect 0.25 pg of beryllium with certainty and determine larger amounts with a precision of about f 5 per cent. RESUME Les auteurs d&rivent un microdosage de glucinium au moyen d’acdtyl-acdtone. Ce reactif permet de s&parer le glucinium, a r&s masquage des ions genants & l’aide de l’acide ethylene-diamine-tdtracetique. E e glucinlum est alors dose absor tiometriquement B l’etat de complexe-acdtylacetone, longueur d’onde 2950 A. z ette methode permet de deceler 0.25 pg de glucinium et d’obtenir, pour des teneurs plus fortes, une pr&ision de & 5 O/.. ZUSAMMENFASSUNG Die Verfasser beschreiben eine Mikro-Bestimmungsmethode des Berylliums mit Hilfe von Acetylaceton. Mit Hilfe dieses Reagens wird das Beryllium, nach Masisoliert. Hierauf kierung der sttirenden Ionen, mit Athylendiamin-tetraessigs%ure, mit Acet laceton, bei 2950 A wird es absorptiometrisch, als Kom lexverbindung bestimmt. Mit Hilfe diesel- Methode l?ann man bis zu 0.25 pg B eryllium nachweisen und grossere-Mengen mit einer Genauigkeit von & 5 ejo bestimmen.

1 s a 4

REFERENCES M. W. Cuccx, W. F. NEUMANN AND B. J. MULRYAN, Anal. Ghem., 21 (1949) W. STROSS AND G. H. OSBORN, J. SOG. Cirem. Ind., 63 (x941) 249. A. L. UNDERWOOD AND W. F. NEUMANN, AndZ. Chem., 21 (1949) 1348. H. V. MEEK AND C. V. BANKS. AnaZ. Chem., 22 (x930) 1512. Received

November

23rd,

1358.

x93x