Spectrophotometric Determination of Beryllium with 2- Phenoxyq uiniza rin-3,4‘-d isuIfo nic Acid E. GUY OWENS 111 and JOHN H. YOE Pratt Trace Analysis laborafory, Departmenf o f Chemistry, University o f Virginia, Charloffesville, Va.
b Beryllium reacts with 2-phenoxyquinizarin-3,4’-disulfonic acid (dipotassium salt) to form a stable violet = 550 mp). The reaccomplex (A,, tion has been applied to the spectrophotometric determination of beryllium in beryl, copper alloys, aluminum metal, and spiked samples of bronze and steel. The practical sensitivity of the reaction is 1 part of beryllium in 125,000,000 parts of solution. Large quantities of interferences are separated by ion exchange or by electrolysis a t the mercury cathode. Many interferences, including aluminum, present in moderate amounts are masked by adding the calcium salt of (ethylenedinitri1o)tetraacetic acid as a sequestering agent. Out of 64 ions investigated only Cr+3, Mg+*, Zr+4, Th+4, F - I , and interfere seriously when CaEDTA is used as a sequestering agent. Development of the method included a study of the effects of temperature and pH, the stabiliiy of the complex, and its rate of formation.
A
m u m of 2-phenosyquinizarin-3.4’-disulfonic acid as a reagent for the spectrophotometric determination of beryllium is described. A similar reagent, 1,4-dihydroxyantliraquinone-2sulfonic acid, used for the colorimetric dt4ermination of beryllium ( 2 ) has several disadvantages. It is temperature-dependent, and subject to interference by a large numher of ions, and color intensity varies with the ionic strength of the solution and pH. These objections have been eliminated to a large degree in the ncw method. The ionic strength is niaintained nearly constant by using a large amount of pH buffer and a sequestering agent. No hc>ating is required. The use of the calcium salt of (ethylenedinitri1o)tetraacetic acid (CaEDTA) masks many interferences which R ould otherwise require separation. pII adjustment is a critical factor in both methods, but regulation with a buffer largely eliminates this error. EXPERIMENTAL
Apparatus.
Beckman spectropho-
Present address, E. I. du Pont de Nemours & Co., Inc., Kinston. 9.C.
tometer, Model DU, with matched 1cm. Corex cells. Reagents. 2-PHLNOsrQuI~IZARIN~,~’-DISULFOXIC ACIDSOLCTIOK. Prepare a O.lyosolution of the dipotassium salt in distilled nater. The reagent used in this investigation was a mixture of the mono- and disulfonates and is satisfactory. STANDARDBERYLLIUMSOLUTIOX;. TT’eigh accurately about 0.5 gram of pure beryllium metal (Johnson hfatthey and Co.). Dissolve the metal in 250 ml. of water with the addition of enough concentrated hydrochloric acid to make the resulting solution 0.lN in acid when the solution is diluted to 500 ml. The pH of a 5-p.p.m. solution prepared from this standard solution should be about 2. Use a hood and avoid fumes. CAEDTA SEQUESTERTNG AGENT. Dissolve 18.6 grams of disodium EDTA and 12 grams of calcium nitrate, Ca( ~ 0 3 ) 2 . 4 H 2 0in, about 350 ml. of water. Adjust t o pH 4.3 by adding about 6 ml. of concentrated ammonium hydrovide and dilute to 500 ml. AMMOI~IUM ACETATEBUFFER,p H 6. Prepare a 4M solution of ammonium acetate and adjust to the desired pH with acetic acid. CATION EXCHANGE RESIS. Amberlite IR-120, Rohm and Haas Co. General Procedure. Obtain the beryllium in aqueous solution free of interfering ions a t p H 2 to 3.5. Pipet a 5- to 20-ml. aliquot containing 5 t o 35 pg. of beryllium into a 50-ml. volumetric flask and add 10 ml. of CaEDTA solution, 10 ml. of reagent solution, and 5 ml. of 4M ammonium acetate. Dilute to volume, check the pH with the aid of a p H meter, and adjust if necessary to pH 6.0 =t0.1, adding concentrated ammonium hydroxide or acetic acid from a dropper. Allow the solution to stand 1 hour or longer and read the absorbance a t 550 mp against a reagent blank adjusted to the same pH. Determine the amount of beryllium from a calibration curve. Absorbance Curves. I n Figure 1 typical curves of wave length us. absorbance for the unreacted dye and its beryllium complex are compared. hIeasurements were in 1-cm. cells against water as a blank a t p H 6.0 with 0.4M ammonium acetate. Total dye concentration was about lO-5M Effect of pH. The p H is critical because the reagent functions as an arid-base indicator. The effect of p H on the reagent blank and beryllium
complex is shown in Figure 2. Thc pivot point between decrease and increase in color intensity with timcl is pH 6.0. For this reason pH 6.0 was chosen as the optimum. Choice of Buffer. T n o buff em werv found satisfactory: ammonium acetate and pyridine-hydrochloric acid The latter has greater buffering capacity than the former (9) but is objectionable because of the unpleasant odor of pyridine. Absorbances of solutions containing 0.3 p.p.m. of berylliuni mere nearly the same regardless of the buffer used. A high concentration of buffer is desirable, so that the ionic strength of the solution will be maintained more nearly constant. This reduces error caused by the indicator salt effect. A concentration of 0.4M ammonium acetate in the final solution was used throughout the investigation. Stability of Complex and Reagent Solution. Stability of the complex is affected by concentration of excess reagent, temperature, and pH. Thc procedure adopted allons for a reaction period of 1 hour or more beforc absorbance measurement. After 1 hour, the decrease in intensity amounts to only about 3y0 over 22 hours. Reagent solutions kept for several weeks gave the same results as those freshly prepared. The reagent is not stable in basic solution. Upon standing several weeks a t room temperature a mold growth sometimes appears. Order of Addition of Reagents. If pure beryllium solutions are used, the order of addition of reagents is immaterial. However, because of the use of the CaEDTA exchange sequestering agent, the following order should be followed: 1. 2. 3. 4.
Sample aliquot CaEDTA Reagent pHbuffer
This order permits exchange between CaEDTA and the interfering ions before the reagent is added. When beryllium a t 0.5 p.p.ni., CaEDTA, and buffer were mixed and allowed to stand for 3 days before addition of the reagent, the 3-day-old solution showed only -2.3% difference from a VOL. 32, NO. 10, SEPTEMBER 1960
1345
freshly prepared solution. Hence, the age of the solution is not critical. Rate of Color Formation. The rate of color formation varies with pH. Below 6.0 the complex is formed slowly and increases in intensity on standing. Above p H 6.0 the color is initially formed rapidly but decreases in intensity on standing. Allowing 1 hour for color development, the intensity is nearly constant within the limits of p H 6.0 f 0.1. Conformity to Beer's Law. The absorbance-concentration curve is nearly linear t o 0.7 p.p.m., but deviates slightly at higher concentrations. Effect of Temperature. The error does not exceed 2% under the conditions specified in the general procedure for a change in temperature of i5' at 22.5' C. The intensity of the complex increases as the temperature is lowered and, conversely, decreases with increase in temperature, but, in practice, fluctuations in temperature may easily be held within the range of i 5 " C. Sequestering Agent. The calcium salt of E D T A permits a large excess of E D T A , thus increasing the sequestering capacity. Calcium ions in moderate concentration d o not interfere in the new method. The CaEDTA complex is colorless and soluble. Most other calcium salts are colorless and water soluble. The CaEDTA complex was chosen as the sequestering agent for this reason. Disodium EDTA has been used as a masking agent in several methods for beryllium (1, 4, 6, 8 ) . Effects of Diverse Ions. Sixtyfour ions have been investigated. The
Table I.
Interference of Diverse Ions
Ion Added
2+; c o +2 Cr +3
c u +2 Fe +3 Hg + l Mg +* Ni +2 Pd + 2
Rh + 3 Ru + 3
s c +3 Sn + 4 Th + 4 Ti +4
UO2+2
Yb +3 Zr +4 F -1 S04-2
1346
(ortho)
Error at 0.5 P.P.M.
Concn., P.P.11.
Be Level, %
100 40 80 40 2 5 60 200 60 10 20,40 GO 100 60 60 24 60 60 5 20 40 60 5 3 20 200
+8.2 +3.1 +3.3 +9.5 +3.1
