Increasing Selectivity of Analytical Reactions by Masking K. 1. CHENG' Metals Division, Kelsey-Hayes Co., New Hartford,
b From the competing equilibrium point of view, masking, demasking, and principal reactions are reviewed and their theoretical aspects are treated. A new term, selectivity ratio, is suggested as an index to evaluate or predict the possibility of unknown masking and principal reactions. The use of new masking agents, especially EDTA, and the combination of several masking agents for a successful masking action are emphasized. A number of new masking actions are presented, covering both inorganic and organic principal agents. By the use of suitable masking agents at optimum conditions, many nonselective reagents can offer highly selective or specific reactions.
T
H E INCREASING USE of new complexing agcnts for masking actions to improve older analytical methods or t o develop new ones is of considerable and practical interest t o the ana!yst. The following conditions should be considered for the use of masking phenomena: selection of the masking and principal agents, pH, pM, use of more than one masking agent, amount of masking and principal agents used, change in oxidation state, rate of reactions, solvent, and temperature. Optimum conditions coupled with suitable masking agents can offer highly selective reactions. For example, bismuthiol 11, 2-thenoyltrifluoroacetone, and dithizone can give specific reactions for thallium, manganese, and silver, respectively. Hydrogen peroxide with Xylenol Orange can mask completely the reaction of zirconium, but not hafnium. Hafnium may be dctcrmined in t h e presence of limited amounts of zirconium. As another example of improving older analytical methods, tellurium may be determined gravimetrically as tellurium dioxide instead of elemental tellurium. Although t h e tellurium dioxide method was described in 1908, i t is not commonly used, probably because hydrolysis of polyvalent metals in a weakly acidic medium would cause interference. When EDTA is used as masking agent, the dioxide method offers a
N. Y.
favorable gravimetric factor and is highly selectivc for tellurium. DEFINITIONS
The terms of masking and demasking have long been used in chemical literature, but a clear exact definition of them is needed. I n addition t o the definitions, two new terms, principal reaction and selectivity ratio, are introduced here. Reactions of the principal reagent with substances in the system are called principal reactions, and are more or less selective reactions. Of special interest are principal reactions t h a t proceed even in the presence of one or more masking agents; such principal reactions are resistant t o masking action. For example, silver in the form of the silver ammine complex will not yield a precipitate with hydroxide or chloride ion, but it will with iodide. The principal reaction of silver with iodide is resistant to the masking action of ammonia. Masking is a process in which a substance, without physical separation of i t or its reaction products, is so transformed t h a t certain of its reactions are prevented. For example, addition of cyanide t o a n ammonical solution containing zinc leads to the formation of the very stable zinc cyanide complex, thus preventing the reactions of zinc ion with Eriochrome Black T or (ethylenedinitri1o)tetraaeetic acid (EDTA). Demasking is the process in which a masked substance is released from its masked form and regains its ability to enter certain reactions. For example, addition of formaldehyde t o t h e solution containing the zinc cyanide complex destroys this complex and frees zinc ion, which thereby regains its ability to react with Eriochrome Black T or EDTA. Selectivity Ratio a n d Masking Ratio. These two terms a r e introduced for evaluating a n d predicting t h e equilibria of t h e masking and principal reactions involved i n a system. These terms can serve as a n index of the extent of masking or principal reactions for a particulur system. They may be defined mathematically as follows:
Masking ratio (M.R.) 1 Present address, RCA Laboratories, Princeton, N. J.
=
(PMm)' PMP
where pM, and pM, are t h e negative
logarithms of the metal ion concentration dissociated, respectively, from the metal-principal reagent complex and the metal-masking agent complex. The pM, and pM, shown in Tables I and IT are calculated on the basis of 1M concentrations of the metal-principal agent and metal-masking agent complexes. The apparent or conditional stability constant should be used instead of the thermodynamic stability constant in the calculation, if they are d 8 e r e n t under the experimental conditions. Although the stability constant is a n expression of the stability of a reaction equilibrium, i t can only be used directly as a n index for comparing complex reactions having the same metal to ligand ratio. For instance, t h e solubility product constants of BiBa and HgS are From direct comand 3 X parison of these values, it is not possible to state which is the most insoluble substance. Actually, the concentrations of bismuth and mercury ions dissociated in water from the sulfides are 10-14.4 and 1.7 X or pBi = 14.4 and pHg = 26.8. Thus, the value of p M is more useful in predicting solubility or stability of a complex. When the metal ions are present with a masking agent and a principal agent, the principal reaction is favored if pM, is larger than pM,. When pM,/pM,>1, the position of the equilibrium has a tendency to shift toward the principal reaction. Furthermore, the appearance of the principal reaction depends on the magnitude of pM,- that is, regardless of the prc$ence of a competing masking agent, the higher the value of pM, the easier iu the formation of a complex or precipitate. Therefore, in considering the two factors, pM,/pM, X pM, or (pM,)*/pM, would be an index of whether a principal reaction takes place or not in a certain system under ordinary conditions when a masking agent is present. Both pM,/pM, and (pM,)2/pM, would serve as such a n index; however, the latter expression gives larger numerical values which are easy to manipulate (Tables I and 11). When the former expression has a value of >1.1, or the latter has a value of >7, there will be a tendency for the principal reaction to predominate; at lower values there will be a tendency for the masking reaction to succeed. I n case a metal-principal agent complex has B VOL 33, NO. 6, MAY 1961
783
Table 1.
Silver Ammonin
Comparison of Equilibrium Constants of Complexes and Precipitates of Silver
Equilibrium
+ 2NHa * Ag(NHs)1+ 2Ag+ + 2 0 H - SAgSO + HsO Ag+ + EDTA-' Ft AgEDTA-* 2Ag+ + Cr04-*S AgrCrO, Ag+ + CIAgCl Ag+ + 1 0 8 - 5 AgIOs Ag+ + Br--A Br Ag+ + I Agf 2A$+ + CIO'-' S AgsCiO' Ag + 2S201-*e Ag(&O&-l Ag+ + SCN- S AgSCN Ag' + 2CN- e Ag(CN)*2Ag+ + S-' S AgsS Ag+ + CrrH~~OSsN1* Ag+
1 . 5 x 10'
2.4
x 101 . 9 x 10-1' 1 x 10-10 2 x .lo-' 5 x 10-1' 8 . 5 x 10-11 5 x lo-" 1 x 10-1' 1 x 10-'0 8 . 1 x 10-1' 2
:&$ride 2 x 107 3 . 6 Chromate $ Chloride Iodate Bromide Iodide $ Oxalate Thiosulfate 1.7 x 10" 4 . 6 Thiocynnnte 5 . 6 x lo1' 6 . 4 Cyanide Sulfide pDimethy1nminobenAgCI~HlrO&Ns zalrhodanine 1,2,3-Benzo- Ag+ C ~ H ~ NS -10-14 I - AgCB'Nt triazole Mercnpte 1 Ag+ CaHsNrS,- g AgC~HcNfin phen ylthie thiodiazolone K = Stability constant. S = Solubility product constant. pAg+ = Calculated baaed on 1M complex present. Adjusted to pH 10 with sodium hydroxide.
+ +
( M . R . ) (S.R.) Complex 3.41 4 . 0 No ppt. Complex 3 . 6 1.00 1.00 3 . 6 3 . 6 No ppt. 3 . 8 1.06 0.95
0.72 0.95 0.59 0.45 1.04 0.78 0.60 0.56 0.22 0.38
2.69 3.41 2.12 1.62 3.73 2.82 2.16 2.02 0.79 1.36
7.0 4.0 10.3 17.8 3.4 5.9 10.0 11.4 73.8 25.1
iXtppt. Complex Ppt. Complex Ppt. Ppt.
1.94 0.51
1.85
13.6
Ppt.
1.39 1.06 1.69 2.22 0.97 1.28 6 . 0 1.67 1.78 16.3 4.53 9 . 5 2.64 6.0 3.8 6.1 8.0 3.5
-7 1
>7
9%.
Ppt.
Ppt.
Table II. Reaction of Some Metals with DMG in Presence of EDTA and BHEG
++ EDTA --'eCuBHEG CuEDTA-* CU + 2DMG e Cu(.DMG)r Ni+l + EDTA-' S NiEDTA-* NiBHEG Ni+* + BHEGNi + 2DMG E Ni(DMG)' Co+*+ EDTA-' * CoEDTA-* C o + l + BHEG- S CoBHEG CO+'+ 2DMG Co(DMG)r Zn+* + EDTA-4 S ZnEDTA-1 Zn+' + BHEG- e ZnHBEG Zn+l + 2DMG e Zn(DMG)z Cu +*
Cu+* BHEG+)
B
= brown; R
-
18.8 13.36 23.30 18.6 10.8 21.7 16.2 8.8 18.94 16.26 8.62 13.9
red; S = solution; P
pM, value of 4 and a metal-masking agent complex has a pM, value of 3, the pM,/pM, ratio will be 1.3, and the metal-principal agent complex may not be formed in the presence of a slight excess of masking agent because the (pM,)2/pMm will be 5.3. The p M cipal reaction may succeed if a large excess amount of principal agent is added. In such cases the value of (pM,)*/pM, is more meaningful and is apt to be
