Ion selective electrode element

An ion selective potentiometric electrode component consisting essentially of a solid, plate-like interface element prepared by simultaneously co-precipitating Ag.sub.2 S, Ag.sub.2 Se, CdS and CdSe followed by compressing the precipitated material at an elevated temperature to provide a substantially impervious agglomerate having an X-ray diffraction pattern distinct from a composite pattern of the individual four components and characterized by improved response properties and enhanced sensitivity to cadmium ion concentration in solution.

BACKGROUND OF THE INVENTION 
The present invention relates to an improved physical component for an 
ion-selective potentiometric electrode. More particularly, the invention 
is directed to a solid state interface plate or membrane providing greater 
ion definitiveness, improved ion sensitivity, and enhanced operational 
characteristics in the detection and quantitative determination of 
specific ions in solution. 
Potentiometric techniques utilizing ion-selective electrodes for 
qualitative detection and for quantitative measurement of specific ions in 
solution have in recent years replaced other analytical methods including 
colorimetric, spectrographic and reagent-based procedures. Extensive 
developmental research time and funds have been directed toward improving 
the sensitivity, the specificity, and the response characteristics of 
ion-selective electrodes. A significant part of this work has dealt with 
improvements in solid state membrane compositions. Notwithstanding a 
substantial expenditure of investigative resources and the discovery of 
relevant parameters, much fundamental knowledge remains to be gleaned. 
SUMMARY OF THE INVENTION 
The present invention provides an improved interface plate or membrane for 
an ion-selective potentiometric electrode, characterized by enhanced ion 
sensitivity and superior quantitative response characteristics. 
A principal aim of the invention is to provide a membrane element for use 
in an ion-selective electrode to facilitate qualitative detection of and 
quantitative measurement of specific ions in solution. 
In a preferred embodiment of the invention, the membrane constitutes a 
sintered-like mass of particles formed as a disc-like element or plate 
sensitive to and responsive to cadmium ions in solutions. 
An important feature of the invention is an enhanced ion sensitivity and a 
successful realization of an increased incremental voltage change as a 
function of ion concentation, all derived from a utilization of the 
improved solid state membrane of the invention. 
A related feature of the invention is that ion-selective electrodes 
incorporating the membrane of the invention exhibit essentially linearly 
proportional voltage response curves over at least an eight-fold 
logarithmetic cadmium concentration range including over a range of from 
about 0.1 molar to about 10.sup.-8 molar. 
Yet another feature of the invention is the ease with which the improved 
compositions are prepared and processed for use. 
An important facet of the invention is the discovery that a close 
structural relationship between different component salts may contribute 
to the establishment of a crystalline structure responsible for the 
enhanced efficacy of the solid membrane of the invention in analytical 
procedures. 
Related important properties of the solid membrane of the invention are 
believed to be attributable to the formation of solid solutions of salts 
of elements to be detected, for example, cadmium. 
Still another feature of the composite four-component co-precipitate of the 
invention as transformed into a unitary solid membrane through application 
of pressure and heat, is that an X-ray diffraction spectrum gives no lines 
characteristic of any of the four pure components, indicating the 
composite is a substitutional solid solution and not a mixture. 
It is a feature of the four-component solid solution of the ion-selective 
electrode element of the invention that the X-ray diffraction pattern is 
also different from that of a pattern derived from a mixture of a 
co-precipitate of Ag.sub.2 S and Ag.sub.2 Se with a co-precipitate of CdS 
and CdSe.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The preferred embodiment of the present invention is a four-component 
equi-molar co-precipitate of Ag.sub.2 S, Ag.sub.2 Se, CdS and CdSe 
prepared in accordance with the procedure set forth below: 
Preparation of Ag.sub.2 S/Ag.sub.2 Se/CdS/CdSe Coprecipitate 
Silver nitrate (10.85 g.) and cadmium nitrate (10.32 g.) were dissolved in 
1,000 ml. of deoxygenated, deionized water, which was previously acidified 
with 5 ml. of glacial acetic acid. Sodium sulfide nonahydrate (7.36 g.) 
and sodium selenide (4.00 g.) were dissolved in 1,000 ml. of deoxygenated, 
deionized water. Nitrogen was bubbled through this solution while the two 
ingredients were being dissolved. The mixed nitrate solution was slowly 
poured into the sulfide/selenide solution with vigorous stirring. The 
resulting dense black co-precipitate was allowed to settle and the mother 
liquor was decanted. The co-precipitate was washed ten consecutive times 
at room temperature with one liter portions of deionized water and twice 
with one liter portions of isopropanol at 60.degree. C. Each wash 
consisted of stirring for 15-20 minutes followed by settling and 
decantation of the supernatant. The product was collected by suction 
filtration of a sintered glass filter funnel and dried at 100.degree. C. 
for eight hours to yield 13.3 g. (99%) of co-precipitate. (Co-precipitate 
A) 
In fabricating the solid state pellet or membrane of the invention the 
above-described co-precipitate was compressed at an elevated temperature. 
Typical conditions imposed were pressure of 100,000 psi for one hour at 
about 350.degree. F. The product, in the form of an ion-selective 
electrode pellet was subjected to x-ray diffraction examination using a 
General Electric X-ray Diffraction Spectrometer, employing K radiation 
(.lambda.=1,50405 A). A typical spectrum is shown in FIG. 3. 
In order further to explore the nature of the novel four-component 
precipitate of the invention, a product was made by co-precipitating 
Ag.sub.2 S with Ag.sub.2 Se, mixing the product with a co-precipitate of 
CdS with CdSe and transforming the mixture into a solid state pellet. The 
detailed laboratory procedures for preparing each co-precipitate are set 
forth below. 
Preparation of Ag.sub.2 S/Ag.sub.2 Se Co-Precipitate 
Silver nitrate (20.69 g.) was dissolved in 1,000 ml. of deoxygenated, 
deionized water, which was previously acidified with 5 ml. of glacial 
acetic acid. Sodium sulfide nonahydrate (7.35 g.) and sodium selenide 
(4.00 g.) were dissolved in 1,000 ml. of deoxygenated, deionized water. 
Nitrogen was bubbled through this solution while the two ingredients were 
being dissolved. The sulfide/selenide solution was slowly added to the 
silver nitrate solution with vigorous stirring. The co-precipitate was 
allowed to settle and the mother liquor was decanted. The co-precipitate 
was washed ten consecutive times at room temperature with one liter 
portions of deionized water and twice with one liter portions of 
isopropanol at 60.degree. C. Each wash consisted of stirring for 15-20 
minutes followed by settling and decantation of the supernatant. The 
product was collected by suction filtration on a sintered glass filter 
funnel and dried at 100.degree. C. for eight hours to yield 16.5 g. 
(99.8%) of co-precipitate. 
Preparation of CdS/CdSe Co-Precipitate 
Cadmium nitrate (23.52 g.) was dissolved in 1,000 ml. of deoxygenated, 
deionized water which were previously acidified with 5 ml. of glacial 
acetic acid. Sodium sulfide nonahydrate (9.23 g.) and sodium selenide 
(5.00 g.) was dissolved in 1,000 ml. of deoxygenated, deionized water. 
Nitrogen was bubbled through this solution while the two ingredients were 
being dissolved. The cadmium nitrate solution was slowly poured into the 
sulfide/selenide solution with vigorous stirring. The resulting 
reddish-orange curdy precipitate settled very slowly. After the mother 
liquor was decanted, the co-precipitate was washed ten consecutive times 
at room temperature with one liter portions of deionized water and twice 
with one liter portions of isopropanol at 60.degree. C. Each wash 
consisted of stirring for 5-10 minutes followed by settling and 
decantation of the supernatant. The product was collected by suction 
filtration on a sintered glass filter funnel and dried at 100.degree. C. 
for eight hours to yield 11.81 g. (92%) of co-precipitate. 
Equimolar Ag.sub.2 S/Ag.sub.2 Se and CdS/CdSe co-precipitates were 
physically mixed together with a mortar and pestle in quantities which 
yielded a composition (Composition B) possessing equimolar concentrations 
of Ag.sub.2 S, Ag.sub.2 Se, CdS, and CdSe as found in Coppt. A. 
Composition B was also pressed into an electrode pellet under the same 
conditions as those used in the case of Coppt. A, and the pellet was 
assembled into a conventional ion selective electrode. 
Evaluation of the two electrodes as cadmium ion selective electrodes 
yielded the following results (also see FIG. 5). 
______________________________________ 
Coppt. A Composition B 
______________________________________ 
Limit of Detection (Cd.sup.++) 
2 .times. 10.sup.-7 
5 .times. 10.sup.-5 
Slope (mV per decade) 
29.2 18.7 
______________________________________ 
The results and the attached graphical data clearly indicate the 
superiority of Coppt. A over Composition B. The electrode prepared from 
Coppt. A possesses a limit of detection which is 250 times lower than that 
of the electrode prepared from Composition B. Furthermore, the slope of 
the response line for the electrode of Coppt. A is essentially equal to 
the theoretical slope expected (29.6 mV) whereas that determined for the 
electrode of Composition B is significantly reduced, indicating a poorer 
response to cadmium ion. 
The co-precipitate of the four-component combination silver and cadmium, 
sulfide and selenide of the invention shows structural differences from 
both the pure individual substances or from dual co-precipitates that are 
physically mixed. These data demonstrate a re-ordering of the crystalline 
matrix, of the four component co-precipitate showing a structural pattern 
incorporating each component into a new structure not akin to physical 
mixtures or combinations of co-precipitates and physical mixtures. 
That is, the four-component co-precipitates produce crystalline patterns 
unlike those of physical mixtures even if parts of the physical mixture 
are individual co-precipitates. That the structures produced by the 
co-precipitates are different is shown by different X-ray diffraction 
patterns (See FIGS. 2 and 3). A new chemical/physical material is clearly 
indicated. 
The X-ray diffraction pattern of the four component co-precipitate of the 
present invention is markedly different from that of the physical mix of 
the Ag.sub.2 S/Ag.sub.2 Se and CdS/CdSe co-precipitates, indicating that 
the four component co-precipitate possesses a crystalline structure 
different from either of the two individual solid solutions. Since the 
X-ray diffraction spectrum of the four component co-precipitate also 
differs from either the sum of the individual ingredients or the sum of 
the two Ag and Cd solid solutions, one can conclude that a new 
four-component solid solution is formed in the co-precipitate process. 
Comparision of the electrodes prepared from the mixture of the Ag and Cd 
solid solutions shows that this four-component solid solution possesses a 
new and unique crystalline structure which imparts cadmium sensing 
potentiometric properties that are superior.