Releasing Effects in Flame Photometry Determination of Calcium JOSEPH I. DlNNlN
U. S.
Geological Survey, Washington 25, D. C.
b Strontium, lanthanum, neodymium, samarium, and yttrium completely release the flame emission o f calcium from the depressive effects o f sulfate, phosphate, and aluminate. Magnesium, beryllium, barium, and scandium release most o f the calcium emission. These cations, when present in high concentration, preferentially form compounds with the depressing anions when the solution i s evaporated rapidly in the flame. The mechanism o f the interference and releasing effects is explained on the basis of the chemical equilibria in the evaporating droplets o f solution and i s shown to depend upon the nature o f the compounds present in the aqueous phase of the solution. The need for background correction techniques i s stressed. The releasing effect i s used in the determination o f calcium in silicate rocks without the need for separations. HE DEPRESSIVE IXTEWFEEEKCE EFFECTS in t,lie flame photoinetric
determination of calcium have been well documented. They have been explained as being due to the formation of compounds with high vaporization temperatures. Until recently it was tacitly assumed t h s t the formation of the refractory compountls takes place in tlie flame. If, lion-ever, it is assumed that the formation of the compounds takes place in the evaporating droplets of solution, a valuable insight can he gained into a mechanium for the eliinination of the interferelice. One of the techniques that has been developed to cope with the deprwrive anionic interferences is that known in spectroscopy as addition in excess ( 5 ) . It works ell v\-ith moderate concentrations of calciim if the ralciuin to deprwsant ratio is less than l to l . However, in very lon. concent,rzt,ions of calcium arid mole ratios of calcium to t l e r m s a n t leqs than 1 to 1, as in many miiieral? and rock;.. the technique virtually eliniinatea the cnlciuni emission. .\nothcr technique is that based upon an ohservation b>. Mitchell and Robertsoil (11) that a high concr,ntr:ition of d st,rontiiiiii \ \ ~ o ~ i Iroli i i o i i of ~~:iIciiiiii j ' r i , i i ! t!lc. iiilii!)itorj. 1.E-
fects of aluminum, and that conversely, a high concentration of calcium would release the flame eniission of strontium (I, s>f 5 ) . Abresch and Dobner (I) have studied the effects of 25 compounds on the flame emission of calcium. They were concerned primarily with the action of the compounds as radiation buflers, that is as compounds that ITould nullify or make very small the variations in the calcium emission caused by varied concentrations of silica and other elements in blazt furnace slag or steel-making products. They found most of the compounds to be ineffective as buffering agents, although strontium chloridr was found to be t8hebest. Most of the reports have not adequately taken into account the background emissions contributed by t'he releasing agents or radiation buffers. S o t only has this obscured the underlying mechanism of the releasing action, h i t i t has, on occasion, given rise to n t releasing action where none Combining a releasing agent, n-ith one or more anionic depressants in t'he form of radiation buffers has most of the disadvant,ages of the addition in excess technique. Much of the action of the releasing cation is consumed in nullifying the effect of the added depresing anion. Although the combination of ions may level out or buffer the radiation, if escesive amounts of depressant are present the calcium emission may he depressed excessively. Although a general description of the use of t'he alkaline earths as releasing agents in the determination of other alkaline earths has been presented by Burriel-Marti and Ramirez-1Iuiioz ( b ) , few quantitative data on the technique are available. The purpose of this v o r k is to present some of the comparative information on releasing agents needed for the design of methods for the determination of calcium. YofB and Finkelstein's (15) succesP with the use of lanthanum and iron, led to the study of these cations also. Because of the similarity in chemical properties, the effects of several ot,her rare earth c:lcrnents :ind tlie rlements bcqdlium, yttriurn. arid scantlium
were studied also. An attempt is also made to show the mechanism of the action of the releasing agents. The methods described do not eliminate the need for standard or known materials. They do, however, make much less critical the exact matching of the concentrations of the interferences in standard and sample. The effects of many interferences can IJC overcome without the need for separations, with only consideration for the order of magnitude of the concentrations of the interferences. EXPERIMENTAL
Instrument a n d Emission Measurement Technique. A Beckman Model
DU spectrophotometer with flame photometer attachment S o . 9200 and photomult'iplier attachment S o . 4300 was used for most of the work described here. A Beckman hIode1 B spectrophotometer with photomultiplier attachment S o . 4600 and flame photometer attachment S o . 9125 JTas used also. T h e flame source was a Beckman KO.4030 burner using hydrogen and oxygen gases. Emission measurements were macle on the most concentrated solutions with the selector switch set. a t 1: photomultiplier voltage full, and the slit widths designated in Tables I and 11. I n most cases, the slit width selected was that' required t'o give a net emission of calcium approaching 300 units (% T X 10). Emission measurements on blank solutions and solutions of low concentrations were macle with the selcrtor set a t 0.1. All emission measurements reported in this study were made by the method of background correction (5, 6) and represent net emissions unless othern-ise noted. They were obtained by making measurements a t two wave lengths, one a t the wave length of the emission peak of t'he element under study, thc second a t the closest neighboring wave length that gives, while the element being det'ermined is being aspirated, an emission approaching that given by distilled water. IIeasurements a t the neighboring wave length represent the emissions of the flame, water, and other elements contributing to the background emission and are subtracted from the gross cmissioils o b t a i n d a t thc sensitive 'ivare lengtli. VOL. 32, NO. 1 1 , OCTOBER 1960
1475
Table 1.
Cation Sr Mg
Maximum Release of Calcium Emission from Depression Caused by Aluminum and Phosphorus (5 p.p.m. Ca, 2% HC10,) ALUMINUM PHOSPHORUS Mole Ratio P/Ca = 5.6/1 Mole Ratio Al/Ca = 30/1
Wave Length, Mp Ca Background
Ba
554 554 422.7 622 422.7
556 566 420.0 635 420.0
Background emission ratio, Slit Width, background Mm. Net Ca 0.07 0.07 0.10 0.10 0.04b
Mole ratio, cation A1
Release,
10 30 30 30 306
100 90" 90" 80 a
3.2 0.25 0.50 0.60 3.6
%
80
Background emission ratio, background Ket Ca
Mole
ratio, cation P
Release,
%
1.4 0.25 0.35 ... 0.75
10 50 50
100 90=
17
90
...
...
...
005
. .
Calcium emission depressed a t higher ratios.
* Slit reduced because of high background emission. Maximum mole ratio tested.
Table II.
Cation6 Ca Sr
Maximum Release of Calcium Emission from Depression Caused by Sulfur (5 p.p.m. Ca, 2% HClO,) Mole Ratio S/Ca = 5/1
Wave Length, Mp Ca Background
...
...
Slit Width, Mm.
Background emission ratio background Net Ca
Mole ratio cation S
...
...
...
- -
0.07 0.75 ,566 420.0 0.10 0.28 566 0.07 0.23 0.10 635 0.28 La 420.0 0.10 0.75 0.10 Xd 420.0 0.38 0.48 0.10 Sm 420.0 420.0 0.40 0.10 Y (566) 0.10 sc 420.0 0.33 (566) 0.10 420.0 Be 0.20 566 (635) 0.10 1.4 Fe(II1) 420.0 6 Ba could not be tested because of precipitation of BaSOd.
-
554 422. i' 554 622 422.7 422.7 422.7 422.7 (544) 422.7 (554) 422.7 554 (622) 422.7
Release,
% , . .
5 10 10 10 25
