Use of an osmotic pump to create a flowing reference junction for ionic-activity sensors
In an ionic-activity sensor, an osmotic pump drives a reference-cell electrolyte to flow through an interface with the solution to be measured. This minimizes contamination of the reference cell by that solution. The driving force results from expansion of an electrolytic-agent reservoir into which solvent from a solvent reservoir diffuses through a semi-permeable membrane. The electrolytic-agent reservoir contains an electrolytic-agent solution in which a quantity of undissolved is disposed to keep the electrolytic-agent solution saturated as solvent diffuses into it.
1 . An ion-activity sensor comprising:
A) reference and measurement half cells arranged for coupling to a solution to be measured, the coupling of the reference half cell being provided by a coupling path extending between the reference half cell and an external junction for exposure to the solution to be measured, wherein the coupling path includes a reference-cell electrolyte;
B) a measurement circuit, electrically coupled to receive the reference and measurement half cells″outputs, for generating a sensor output that indicates an ionic activity of the solution to be measured, and
C) an osmotic pump that drives the reference-cell electrolyte through the reference half cell's external junction.
2 . The ion-activity sensor of claim 1 , wherein the ionic activity indicated by the sensor output is oxidation/reduction potential.
3 . The ion-activity sensor of claim 1 , wherein the ionic activity indicated by the sensor output is ionic concentration.
4 . The ion-activity sensor of claim 3 , wherein the ionic concentration indicated by the sensor output is hydrogen concentration.
5 . The ion-activity sensor of claim 3 , wherein the ionic concentration indicated by the sensor output is fluoride concentration.
6 . A pH sensor comprising:
A) reference and measurement half cells arranged for coupling to a solution to be measured, the coupling of the reference half cell being provided by a coupling path extending between the reference half cell and an external junction for exposure to the solution to be measured, wherein the coupling path includes a reference-cell electrolyte;
B) a measurement circuit, electrically coupled to receive the reference and measurement half cells' outputs, for generating a sensor output that indicates the pH of the solution to be measured, and
C) an osmotic pump that drives the reference-cell electrolyte through the reference half cell's external junction; wherein the osmotic pump includes:
i) a solvent reservoir containing a solvent;
ii) an osmotic-agent reservoir containing an osmotic agent and disposed in fluid communication with the solvent reservoir through a diffusion path;
iii) a semi-permeable membrane interposed in the diffusion path, the semi-permeable osmotic membrane being more permeable to diffusion of the solvent than to the osmotic agent; and
iv) iv)an actuator so operatively coupled to the osmotic-agent reservoir and the flowing electrolyte as to be urged by expansion of the osmotic-agent reservoir to drive the reference-cell electrolyte through the reference half cell's external junction.
7 . A pH sensor as defined in claim 6 wherein the osmotic reservoir further comprises dissolved osmotic agent and a quantity of undissolved osmotic agent so disposed therein as to keep the osmotic agent solution saturated as solvent diffuses into the osmotic-agent reservoir.
8 . A pH sensor as defined in claim 6 wherein said external junction comprises an alumina ceramic.
9 . A pH sensor as defined in claim 6 wherein said measurement half cell further comprises a pH glass membrane.
10 . A pH sensor as defined in claim 9 wherein said pH glass membrane has a substantially dome shape.
11 . A pH sensor as defined in claim 9 wherein said pH glass membrane comprises a glass composition comprising about 33 to about 36 mole percent Li 2 O, about 0.5 to about 1.5 mole percent of at least one oxide selected from the group consisting of Cs 2 O and Rb 2 O, about 4 to about 6 mole percent of a lanthanoid oxide, about 4 to about 6 mole percent of at least one oxide selected from the group consisting of Ta 2 O 5 and Nb 2 O 5 , and about 54 to about 58 mole percent SiO 2 .
12 . A pH sensor as defined in claim 6 wherein said actuator is a piston.
13 . A pH sensor as defined in claim 6 wherein said osmotic agent is an alkali or alkali earth metal halide.
14 . A pH sensor as defined in claim 6 wherein said osmotic agent is sodium chloride or potassium chloride.
15 . A pH sensor as defined in claim 6 wherein said electrolyte comprises potassium chloride.
16 . A pH sensor as defined in claim 6 wherein said electrolyte comprises potassium chloride and one or more components selected from the group consisting of ethylene glycol, propylene glycol, sodium chloride and silver nitrate.
17 . A pH sensor as defined in claim 6 wherein the reference-cell electrolyte flows through the reference half cell's external junction at a rate of about 24 μL per day.
18 . A pH sensor as defined in claim 6 wherein the reference-cell electrolyte flows through the reference half cell's external junction at a rate of about 1 μL per hour.
19 . A pH sensor as defined in claim 6 wherein said semi-permeable osmotic membrane consists essentially of a material selected from the group consisting of cellulose acetate, aromatic polyamides, aliphatic polyamines, or a mixture thereof.
20 . A pH sensor as defined in claim 6 wherein said semi-permeable osmotic membrane consists essentially of cellulose acetate or an aromatic polyamide.