The study was designed to provide duplication for approximately 50% of the experiments. The precision of these duplicate experiments and the precision obtained from the standard method of addition experimentis were of the same order of magnitude. It was therefore concluded that in the future, repetition will be used only for spot checks and regression analysis will be the main statistical tc 101 for investigating precision and accuracy.
matrix for subsequent analysis by atomic absorption spectrophotometry. RECEIVED for review May 5, 1972. Accepted October 24, 1972. Reference to specific makes and models of equipment is made for identification only and does not imply endorsement by the Bureau of Mines.
SUMMARY
The procedure detailed here permits t h e rapid and reliable quantitative determination of beryllium, cadmium, calcium, cobalt, copper, lithium, potassium, magxiesium, manganese, and nickel in coal at sample weights as low as 10 mg. This method may be amenable to the analysis i3f other elements in coal because all metals present are in s(i3lution. The combination of fuming nitric acid, hydroflur )ric acid, and boric acid has been found to be an excellent mi2dium for oxidation and solvation of coal, and in addition provides a suitable
CORRECTION Resolution by Gas-liquid Chromatography of Diastereomers of Five Nonprotein Amino Acids Known to Occur in the Murchison Meteorite In this article by G. E. Pollock [ANAL.CHEM.,44, 2368 (1972)], the second last line of the abstract should read “6amino-isobutyric’’ instead of “6-amino-n-butyric.”
I CORRESPONDENCE Influence of pH on the Response of a Cyanide Ion Selective Membrane E,lectrode SIR: Recently, an operational mode:l for cyanide ion selective electrode was reported ( I , 2 ) . At the silver iodide membrane surfacze, in contact with cyanide ions, the following reaction occurs :I
Ag I
+ 2 CN- e Ag(CN)z- + I-
K
‘v
lo4
(1)
The equilibrium constant is large enough and it is postulated that the iodide ion concentration fixes the membrane potential at 25 “C through the equation:
E
=
Eo - 0.059 log [I-]
=
Eo - 0.059 log
C * KHCN [H+]
AgI
+ 2 HCN $ Ag(CN)2- + 2 H+ + I-
(4)
The equilibrium constant of this reaction is
(2)
This model can explain the electrode response in cyanide solutions in the pH range 7-11, that is, %#hereassociation of CN- with H+ions occurs. The potential response of the cyanide CI) selective electrode as a function of pH has been reported ( I , 3, 4 ) : E
This Equation 3’ does not fit our results or the results of others (5). Experimental values are reported in Figure 1 with the line describing Equation 3’. We suggest that the following reaction must also be considered to obtain the relation between pH and the electrode response :
1
+ KH,,G 2
(3)
because, from Equation 1 and from the di ssociation of HCN,
where Kpsis the solubility product of AgI, Kps = 8.3 X KHCNis the dissociation constant of HCN, KHCS= 5 X 10-lo, and K A ~ ( c N )is~ -the dissociation constant of Ag(CN)z-, K A ~ ; c N= ) ~10-2l. From relationship 5, we can calculate the concentration of I- as a function of pH. Since it can be postulated that: [Ag(CNh-I
=
[I-]
(6)
Equation 5 becomes (3 ’) where C is the analytical concentration of cyanide and K H Cis~the dissociation constant of HCN.
Mass balance is given by 2[Ag(CN)2-]
(1) (2) (3) (4)
B. Fleet and H. von Storp, ANAL.CHEM.,43, 1575 (1971). D. H. Evans, ibid.,44, 875 (1972). E. Pungor and K. Toth, Airalyst (Loi?d0/7),95, 625 (1970). K. Toth and E. Pungor, Aiial. Chinz. Acta, 51, 221 (1970).
614
0
ANALYTICAL CHEMISTRY, VOL. ~ 1 5 ,NO. 3, MARCH 1973
+ [HCN] + [CN-] = C
(8)
( 5 ) G. A. Rechnitz and R. Llenado ANAL.CHEM., 44, 468 (1972). See Figure 6.
6
8
7
9
10
where C is the analytical concentration of cyanide. Substitution of Equation 8 in 6 gives: [HCN]
=
c - 2[I-] 1
+ KHCS
(9)
And Equations 9 and 7 give: [I-]
=
1.4 2.8
x
x
+
10-7 c [H+l KHCN
+
Equation 10, rather than Equation 3‘ is the relationship that holds at pH 5 9. In Figure 1, Equations 3’ and 10 are compared with experimental points in the pH range 7-11 for C = 10-2M. The agreement with Equation 10 is evident. Equations 3’ and 10 will coincide at pH 2 9 where [H+] AHCN
