Ion-Selective Electrode (I.S.E.) Principle of Ion-Selective Electrode (I.S.E.) I.S.E. consists of a thin membrane across which only the intended ion can be transported. The transport of ions from a high conc. to a low one through a selective binding with some sites within the membrane creates a potential difference.

Electrodes and Potentiometry Use of Electrodes to Measure Voltages that Provide Chemical Information Various electrodes have been designed to respond selectively to specific analytes Indicator Electrode: Electrode that responds to analyte and donates/accepts electrons Reference Electrode: Second ½ cell at a constant potential Cell voltage is difference between the indicator and reference electrode

Reference Electrodes Reference electrode, [Cl-] is constant Potential of the cell only depends on [Fe2+] & [Fe3+] Pt wire is indicator electrode whose potential responds to [Fe2+]/[Fe3+] Unknown solution of [Fe2+] & [Fe3+]

Reference Electrode Silver-Silver Chloride Reference Electrode

Reference Electrode Saturated Calomel Reference Electrode

Types of I.S.E. 2. Solid-state electrode 3. Liquid-based electrode 1. Glass membrane 2. Solid-state electrode 3. Liquid-based electrode 4. Compound electrode

Electrolytes Type of ions Cations – Positive charge move toward the cathode Na+ = Extracellular cation K+ = Intracellular cation Anions – Negative charge move toward the anode Cl– Extracellular anion HCO–3 – Extracellular anion 11

Sample Collection Serum Collected in heparin bulb Plain EDTA can not be use ….? Urine Collected in plain Vaccutte 12

Routinely measured electrolytes Sodium (90%)Major cation Extracellular fluid outside cells Normal values Serum = 135-145 mEq/L Urine (24 hr ) = 40-220 mEq/L Functions Influence on regulation of body water Osmotic activity Neuromuscular activity 13

Sodium (Na) Sodium determination Ion-selective (specific) electrode Membrane composition = lithium aluminum silicate glass 14

Hyponatremia <135 mEq/L Increased Na+ loss Aldosterone deficiency = Addision disease Diabetes mellitus Diabete Ketoacidosis Potassium depletion K normally excreted , if none, then Na Loss of gastric contents 15

Routinely measure electrolytes Potassium (K) (2%)major cation Intracellular fluid inside cell Normal value Serum- 3.5-5.3 mEq/L Urine- 25-125 mEq/L Function Neuromusculer cells Heart muscle contraction Increase K Promotes muscular excitability Decrease K =Paralysis and arrhythmiasis 16

Hypernatremia Excess water loss resulting in dehydration (relative increase) Sweating Diarrhea Burns Dehydration from inadequate water intake Diabetes insipidus 17

Hyperkalemia Increased K concentration Causes Renal disease Acidosis (Diabetes mellitus ) H+ competes with K+ to get into cells & to be excreted by kidneys Decreased insulin promotes cellular K loss Hyperosomolar plasma (from ↑ glucose) pulls H2O and potassium into the plasma 18

Chloride ( Cl - ) Chloride Major cation Extracelluler fluid Normal value Chloride usually follows Na (if one is abnormal, so is the other) Function - not completely known body hydration osmotic pressure electrical neutrality & other functions 19

Chloride ( Cl - ) Normal values Serum – 100 -110 mEq/L 24 hour urine – 110-250 mEq/L varies with intake CSF – 120-132 mEq/L 20

Hypochloremia Decreased serum Cl loss of gastric HCl salt loosing renal diseases metabolic alkalosis increased HCO3- & decreased Cl- 21

Hyperchloremia Increased serum Cl dehydration (relative increase) excessive intake (IV) congestive heart failure renal tubular disease metabolic acidosis 22

Advantages of I.S.E. 1 . Linear response: over 4 to 6 orders of magnitude of A. 2. Non-destructive: no consumption of analyte. 3. Non-contaminating. 4. Short response time: in sec. or min. useful in indust. applications. 5. Unaffected by color or turbidity.

Limitations 1. Precision is rarely better than 1%. 2. Electrodes can be fouled by proteins or other organic solutes. 3. Interference by other ions. 4. Electrodes are fragile and have limited shelf life. 5. Electrodes respond to the activity of uncomplexed ion. So ligands must be absent or masked. m must be kept constant

Functions Volume and osmotic regulation Myocardial rhythm and contractility Cofactors in enzyme activation Regulation of ATPase ion pumps Acid-base balance Blood coagulation Neuromuscular excitability Production of ATP from glucose 25

Application of ISE Electrolyte Arterial Blood Gas Analysis Sodium pO2 Potassium pCO2 Calcium pH Lithium HCO3- Iodine Magnesium Chloride Fluoride Glucose Urea 26