Under these conditions, the following ions neither gave a test which could be mistaken for antimony nor prevented the test for antimony when it was present: Li, Na, K, Cu(II), Rb, Ag, Cs, Au(III), Be, Mg, Ca, Bn, Sr, Cd, Ba, Hg(I), Hg(II), BO:-, B407-’, Al, Ga, Ce(III), Ce(IV), Ti(IV), G~OS-’,Zr(IV), Sn(II), Sn(IV), Pb, Th, NH(+, NO’-, NOS-, Pod-’, VO+’, OS-, As(III), As(V), Bi(III), S-’, SZOS-~,SOS-’, SO4-’, Cr(III), Mo(VI), UOZ+~, uO4-’, F-, C1-, c104-, Mn(II), Br-, BrOs-, I-, Ios-, Re(III), Fe(II), Fe(III), Co, Ni, CN-. A few ions gave interferences. Their nature is described in Table V.
The oxidizing agents, NOz-, Cr(VI), c103-, Br03-, IOs-, and Fe(III), do not interfere when present as 100 pg. with 1 pg. of Sb(III), but, because they liberate iodine and may prevent complete complexation of larger amounts of Sb(III), they should be destroyed. Prior treatment of the test solution with hydrazine sulfate is recommended, except for NOz- which can be treated with urea. LITERATURE CITED
(1) Fairhall, L. T., Hyslop, F., U. S. Public Health Repts., Supplement 195 (1947).
(2) Feigl, F., “Qualitative Analysis by
Spot Tests,” 3rd ed., p. 4, Elsevier, New York, 1946;( (3) Jacobs, M. B., The Analytical Chemistry of Industrial Poisons, Hazards and Solvents,” 2nd ed., p. 253, Intencience, New York. 1949. (4) Knodel, ’W,, Weisz, H., Mikrochim. Acta, 1957, 417;(
(5) Vogel, A. I., Quantitative Inorganic Analysis,” 2nd ed., p. 376, Longmans, Green and Co., New York, 1967. (6) West, P. W., J . Chem. Educ. 18, 528 (1952). ( 7 ) West, P. W., Hamilton, W. C., ANAL. CHEM.24, 1025 (1952). RECEIVED for review October 30, 1961. Accepted January 30, 1962. Work was supported by the National Institutes of Health, Grant No. 7481.
Estimation of Beryllium with Eriochrome Cyanine R Using the Ring Oven Technique PHILIP W. WEST and PATRICIA R. MOHILNER Coates Chemical laboratories, Louisiana State University, Baton Rouge, l a .
A method for the microdetermination of beryllium using Eriochrome Cyanine R and the ring oven technique is presented. Determinations can b e made on as little as 0.01 mg. per ml. (0.05 pg.) of beryllium with an average error of 7%. Of the elements likely to b e of significance in air pollution studies, none was found to interfere when present in 10-fold excess, and only Mg, Th, AI, and Cr interfere when present in 1 00-fold excess.
T
of beryllium and its compounds has emphasized the need for improved methods for determining this metal in air pollution studies. This work has been carried out with a view to developing a rapid, reliable, semiquantitative procedure for the determination of beryllium in atmospheric samples utilizing the simplicity of the ring oven method (3) in an adaptation of the spectrophotometric method employing Eriochrome Cyanine R ( I ) . The method presented here permits a rapid evaluation of the concentration of a solution, such as may be prepared from an air sample suspected of containing beryllium as a pollutant, and is applicable to readily soluble forms of beryllium collected from air samples. HE INCREASING USE
EXPERIMENTAL
Reagents. Eriochrome Cyanine R. Solution is prepared by triturating the 558
ANALYTICAL CHEMISTRY
solid material (Matheson, Coleman, and Bell, dry stain) with 2 t o 3 ml. of water. One milliliter of the saturated solution is decanted and diluted to 5 ml. This solution should be prepared fresh daily. The buffer solution is prepared by dissolving approximately one mole of ethylenediamine in 500 ml. of water, adding HC1 to bring the pH to 9.8 as measured with a pH meter, and diluting to 1 liter. The acid solution is approximately 0.1N HC1, while the masking agent is a 2% aqueous solution of sodium nitrilotriacetate (NaNTA) (LaMotte Chemicals, Dallas, Tex.). Paper. Whatman No. 41 filter paper. Circles of 55-mm. diameter are convenient for the ring oven. Collection and Preparation of Sample. The sample of air to be examined for beryllium content may be collected in any of the customary ways and placed in solution by any method which avoids the use of fluorides, which mask the reaction if present in gross excess. Clearly, only the more soluble forms of beryllium can be detected and estimated by this technique. Procedure. Add to the center of the piece of Whatman No. 41 filter paper on the ring oven the following, in order, taking care to wait between additions for a sufficient period of time (20 to 30 seconds) to prevent flooding of the ring, 10 pl. of 2% aqueous NaNTA, sample, 10 pl. of 2% aqueous NaNTA, 3 10-pl. portions of 0.1N HCl, 10 pl. of buffer, 10 pl. of Eriochrome Cyanine R solution, and 10 10-p1. portions of buffer.
Remove the paper from the ling oven and immediately ITash with continuous agitation in distilled water until all traces of yellow, browi, orange, or red color disappear. The washed ring is then dried over an electric hair drier. Analysis of an Unknown. The above procedure is followed for the preparation of each ring. A standard scale is conveniently prepared by making rings where the sample in each case is 1, 2 , 4, 6, 8, or 10 portions containing 5 or 10 pl. of a standard solution containing 0.01 pg. of Be per pl. The size portion chosen for the standard scale should also be chosen for making the rings of the unknown. Generally 3 rings made from different numbers of portions of the solution are sufficient for each unknown. Each of the three rings is then compared visually to the standard scale, and it is decided if the ring matches one of the standard scale or falls between two members. Thus, to each of the three rings made from the unknown solution is assigned one of the numbers: 1, 1.5, 2 , 3, 4, 5, 6, 7, 8, 9, 10. To compute the ratio of the concentration of the unknown to that of the standard solution, the sum of the numbers of the three matching rings of the standard scale is divided by the sum of the number of portions of unknown solution used in making the unknown rings. The concentration is then determined by multiplying the quotient found in the preceding step by the concentration of the solution used to prepare the standard scale. Once the stable standard scale is prepared, the analysis of an unknown, including the preparation of the three
rings, matching and calculation, should take less than an hour. The best results are obtained when the ratio of the concentrations of the unknown to the standard solution is as near unity as possible. For solutions more dilute than the standard, larger numbers of portions may be taken, although this introduces more opportunities for error. K i t h more concentrated unknown solutions, simple dilution will permit optimum operation. DISCUSSION
Reagent Concentration. A wide variety of concentrations of aqueous Eriochrome Cyanine R gives a characteristic bluc-yiolet ring n-ith beryllium in basic media. A large ewess of reagent is required in order to obtain favorable equilibriurn for the formation of the colored species and to combine n i t h any other reacting ions that may be present. On the other hand, an excess of Eriochrome Cyanine R, colored yellon-brown in basic media, masks the color of the beryllium complex Washing the final ring in water n ill ordinarily remove the excess reagent unless too much is present. When 10 p l . of a saturated aqueous solution of Eriochrome Cyanine R per ring was used, not all the excess reagent could be washed off the paper nithout resorting to strenuous conditions which also dislodged some of the beryllium-containing species. A reasonable compromise between these conflicting requirements was found with a saturated aqueous solution of reagent diluted fivefold with water. Masking. A number of ions react n i t h Criochronie Cyanine R to produce colored products, not all of which :ire soluble in water. To make the reaction a' selective as possible, n r i o u s masking agrnts were investigated. Reagents tested for poqsible maqking ability includPd acetate, citrate, oxalate, thiocysnate, thiosulfate, phosphate, tliioglycolic acid, (ethylenedinitriloi tetraacetic acid (EDTA), [ (carboymethylimino) bis(ethy1enenitri1o)ltetraacetic acid (available from LaMotte as diethylenetriaminepentaacetic acid), iY- (carboxymethyl)-N'-2hydroxyethyl - X,-V' - ethylenediglycine (LahIotte : hydroxyethylethylenediaminetriscetic acid), (1,2-cyclohexylenedinitri1o)tetraacetic acid (LaMotte: diaminocyclohexanetetraacetic acid), [ethylencbis(oxyethylenenitrilo)] t e t r a acetic acid [ Lallotte : ethylencglycol bis(b-aminoethy1ether)-iV,fi'-tetraacetic arid], and sodium nitrilotriacetate. With the exception of thioglycolic acid and EDTA and its derivatives, the masking agents tried reduced the sensitivity of the beryllium-Eriochrome Cyanine R color markedly. Thioglycolic acid produced a strong blank. EDTA
and its derivatives reduced the sensitivity of the reaction to varying degrees, although none of them as markedly as the other reagents investigated. A comparison of rings formed with equal amounts of beryllium under similar conditions, but using the various EDTA derivatives as masking agents, showed that NaKTA reduced the sensitivity of the reaction the least. Accordingly, it was chosen as the masking agent in this work. The concentration of solution used was a compromise between maximum sensitivity for beryllium and minimum interference by other ions. A satisfactory system seemed to result from the use of a 2% aqueous solution of Xah-TA. Control of pH. Hill (1) points out that only a t p H 9.8 does the intensity of the beryllium-Eriochrome Cyanine R color remain independent of the amount of other ions present when E D T A is used for masking. Similar criteria apply t o NaXTA, although, as shown later in the discussion of interferences, the color is independent of certain other ions only within a limited range of excess. Additional tests on the ring oven showed that a maximum sensitivity was also obtained at this p H and that the p H was critical to within about 1 p H unit. The buffer, ethylenediamine-HCl, was chosen for its somewhat volatile nature to avoid the collection of too large a bulk of solids in the ring zone. Paper. A study was made to determine the most suitable filter paper
Table 1.
Paper
on which to carry out the reaction on the ring oven. The following papers were tested: Whatman Nos. 40, 41, 44, 54, 541, 50, 52; Munktells OK, 3, 0, OOH; Reeve-Angel 711. The problem was to find a rapid paper with a low blank which had the u e t strength to permit handling when the paper containing relatively large amounts of basic buffer was washed in water. The results of these studies are summarized in Table I. All the tests on the various filter papers were carried out n ithout masking, and the rings containing beryllium were made with 0.04 pg. of the metal. There was a definite correlation betwee11 the speed of the paper and the sharpness of the ring formed. The slower the paper, the more irregular a a s the ring produced, apparently because too much evaporation occurred before the ring zone was reached. The only papers which gave a good color for 0.04 pg. of Be Rere also the ones which formed irregular rings. Therefore, a weaker color had to be accepted. Of the four papers Rhich gave no blank, Whatman No. 41 was the best, although its wet strength left much to be desired. The final decision was to use Whatman No. 41 and exercise care in handling the paper to minimize tearing. Transfer of Sample to Ring. T o ensure that all the beryllium in the sample introduced into the center of the filter paper reached the ring zone, a fen nashings nere made with O.1N HC1 before the system was washed w i t h buffer.
Characteristics of Selected Filter Paper
Speed
Blanli
Strength
Remarks
Whatman 54 1
Good
Sone
hl unk tells OX
Slow
Retained reagent Good
Munktells
Rapid
Slight
Munktells 0 Whatman
Slow Very rapid
Some retention of reagent None
Poor
Weak color for Be ring
\Thatman
Very slow
Slight
Good
Ring forms very irregularly
Khatman
Very slow
Retained reagent Good
Ring forms very irregularly
Reeve-Angel Rapid
None
Fair
Whatman
Slow
Slight
Fair
Whatman
Very slow
Slight
Good
Whatman
Rapid
Kone
Excellent
Ring with Be exceedingly faint About the same as Whatman 41 Ring formed in irregular manner Ring formed in irregular manner
Munktells OOH
Slow
Slight
Good
3
41
52 50
711
40
44 54
Excellent
Tendency for rings to form irregularly
Satisfactory, Color of Be ring rather weak Satisfactory \Teak color for Be ring
VOL. 34, NO. 4, APRIL 1962
559
Washing. As previously described, washing of the final ring was required to remove excess reagent. I n addition, long standing of the reagent in contact with the beryllium led to further color development which continued for as much as 24 hours. Also, several ions formed colored reaction products which were water soluble. For these reasons, the ring, as soon as formed, was washed in distilled water to remove all soluble matter, including excess reagent. I t was necessary to wash the ring with continuous agitation to prevent the uneven destruction of the ring. Washing was continued until no trace remained of any yellow, brown, orange, or red color. While agitation was required to maintain a uniform ring, too vigorous a disturbance was to be avoided. Kashing the ring with a jet from a wash bottle, for instance, dislodged some of the beryllium reaction product. The method adopted consisted of placing the ring in a Petri dish of distilled water and gently agitating the paper. Several organic solvents were tried for the washing step but presented no particular advantage over water. Order of Reagents. The order of addition of reagents to form the ring is critical. Masking agent must be added both before and after the sample so that regardless of rates of diffusion the sample will come into contact with the masking agent before any other substance added. The addition of three portions of acid before buffering ensures that all the beryllium present is washed to the
Table
II.
Determination of Beryllium
Ratio of Test t o Standard Solution Taken Found4 0.90 0.904 zk 0.092* 1.20 1.198 =k 0.081* a Based on averaging 5 values calculated from 3 rings each according to the method of Weisz (3). b Calculated at 90% confidence level.
Table 111. Errors Found in Ten Determinations of Beryllium Using the Knodel and Weisz Method of Calculation
Absolute Value of Error in a Single Determination” Average Maximum Minimum
0.075
0.167
0.008
The unite of these measurements are ratio of the test solution concentration to that of the standard solution. a
ring zone before any precipitation of beryllium by the basic buffer can occur. One portion of buffer is added before the reagent to ensure that the reagent will be in its basic form before encountering any sample. Buffer is used for the final formation of the ring zone to prevent any side reactions with the reagent, which undergoes several color changes with pH. Stability. I n the procedure as finally developed, the color of the rings was stable for over 1 month. Interferences. I n the first survey of interferences, two tests were made for each ion investigated. For the first test, 10 pl. of a solution containing 1 pg. per p1. of the potential interferer was used as the sample. In the second test, 20 pl. was taken of a solution containing equal volumes of 1 pg. per p1. of ion and 0.01 pg. per pl. of Be. The rings were formed according to the procedure given above. In all cases where the first test was identical to a blank using all the reagents or was less intensely colored than 0.05 pg. of Be, and the second test gave the same intensity of coloration, judged visually, as 0.1 pg. of beryllium, the ion was said to be noninterfering. The following ions were established as noninterfering: Li, Na, K, Cu, Rb, Cs, Ca, Zn, Sr, Cd, Ba, borate, Ga, Ce(III), Tl(I), carbonate, silicate, Ti(IV), germanate, Zr(IV), Pb, ammonium, nitrate, vanadyl, vanadate, As(III), As(V), Bi(III), sulfide, thiosulfate, sulfite, sulfate, molybdate, tungstate, chloride, chlorate, perchlorate, N n ( I I ) , permanganate, bromide, bromate, iodide, iodate, perrhenate, Co, Xi, Ru, Rh, Pd(II), Os(VIII), Ir(IV), Pt(IV), Sc, Ta, La, Y, Nb, and In. The other ions studied behaved as fOllO\~S. Ten pg. of Cr(V1) reduce the sensitivity of the Be reaction, but 5 pg. give no interference. Ten pg. of Se(IV), Ag, Te(VI), or Fe(II1) give a coloration which would not be mistaken for beryllium, nor did such amounts cover up the test for 0.1 pg. of Be. Ten pg. of Sb(II1) or Se(V1) give a coloration which would not be mistaken for beryllium. They discolor the test for 0.1 pg. of Be, but do not render it unusable. Au(II1) gives a strong test identical to that for Be; 0.1 pg. of Au(II1) is acceptable, but 1 pg. is not. Mg reduces the sensitivity of the Be reaction a t a ratio of 100 to 1, but is satisfactory a t a ratio of 50 to 1. Five pg. of Al+3, Th+4, F-, or Crf3 reduce the sensitivity of the Be reaction, but 1 pg. does not affect the test for 0.1 pg. of Be. Ten pg. of U(V1) give a reaction which would be mistaken for 0.05 pg. of Be, but 5 pg. do not interfere. Of these interferences, only those due
560
ANALYTICAL CHEMISTRY
to Mg, Al, Th, and Cr are likely to be of significance in studies of air pollution, and the worst of these can still be tolerated in an amount 10 times that of the beryllium being determined. Because of the significant reduction in sensitivity of the reaction in the presence of fluoride, this method should not be applied in cases where it is necessary to use H F to ensure solution of the sample. Quantitative Estimation. The application of the reaction between beryllium and Eriochrome Cyanine R for estimation of beryllium by the Weisz technique was investigated by the preparation of several standard scales and the study of two solutions of known beryllium content. Standard scales prepared from 0.05, 0.1, 0.2, 0.4, 0.6, 0.8, and 1.Opg. of beryllium were compared visually after a month with similar freshly prepared scales, and no differences were noted. For the study of the reproducibility of analyses, standard scales were prepared using 5-pI. drops of 0.01 pg. per pl. of Be solution. Fifteen rings were prepared for each of the test solutions-the rings containing 1, 2, 2, 3, 4,4,5, 6, 6 , 7, 8, 8, 9, 10, and 10 of the 5pl. drops of the test solution. The rings were divided into sets of three, according to a pattern arbitrarily chosen before the rings were made, and each group of three rings was treated as a separate sample with the rings compared to the standard scale and the average value for the ratio of test solution to standard computed by the method of Weisz (3) as described above. The results of these determinations are presented in Tables I1 and 111. While the work of Weisz and others indicates that the general technique of the ring oven yields determinations correct within 5 to 10% or better, the theoretical basis for this statement assumes that in all cases the rings will be matched to the standard scale correctly ( 2 ) . In practice, of the 30 rings used in this study, 13 were not matched correctly with the standard scale. In spite of this high proportion of errors in visual matching, an average error of 7% resulted, which further demonstrates the power of the averaging technique developed by Weisz. LITERATURE CITED
(1) Hill, U. T., ANAL. CIIEM. 30, 521-4
(1959).
(2) Knodel, W., Weisz, H., Mikrochim. Acta ( Wien) 1957, 417. (3) Weisz, H., “Microanalysis by the Ring Oven Technique,” pp. 70-6, Pergamon, New York, 1961.
RECEIVED for review October 30, 1961. Accepted January 30, 1962. Work waa supported by PHS research $rant No. 7481 from the National Institutes of Health, Puhlic Health Service.
