Electroanalytical Chemistry CEM 837
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This 9 page Class Notes was uploaded by Ladarius Rohan on Saturday September 19, 2015. The Class Notes belongs to CEM 837 at Michigan State University taught by Greg Swain in Fall. Since its upload, it has received 58 views. For similar materials see /class/207678/cem-837-michigan-state-university in Chemistry at Michigan State University.
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Date Created: 09/19/15
Potentiometry BF pp 7482 Measurement of a potential E that reflects the concentration of an analyte species in solution according to a Nernst like equa on E E0 OO59nlogOXRed lon internal lon external solution solution B or Selective Membrane Em RTziF log aiBai pX logX If activity of species i is held constant in one phase the potential difference between the two phases often called the membrane potential Em responds in a Nernst like fashion to the ions activity in the other phase Potentiometry gtThis is the essence of ionselective electrodes Measurements with these devices are essentially determinations of membrane potentials gtJunction potentials develop at the membrane gt The performance of any single system is determined to a large extent by the degree to which the species of interest can be made to dominate the charge transport in part of the membrane gt A key is the membranes selectivity for the ion or analyte of interest The ion or analyte should have a tranference number of 1 and all other species should be 0 ti lzi uiCiZ zj uJCj Potentiometry There are several types of ionselective membrane electrodes minimal solubility ion conductivity and selectivity for species of interest gt Crystalline Membranes Single crystal LaF3 for F39 Polycrystalline AgZS for 8239 and Ag gt Noncrystalline Membranes Glass materials Liquid ion exchangers eg crown ethers Immobilized liquids in rigid polymers Potentiometry Gass Membrane Electrode lon selective properties have been know since the early 20th century Membrane regularly used for measurement of pH and alkali metal ion activity HgHgZCIZIKCI sat dTest solnGlass membraneHCI 01 MAgCAg a z Internal Ref Electrode SCE J V Glass Electrode The properties of the test solution influence Em constant RTziF n aisoln the overall cell potential difference at two points One is the liquid junction between the test solution and the SCE reference The other is at the membrane Potentiometry Test Dry Glass Internal Filling Solution SOIUtlon Silicate network 5100 nm 50 um 39 5100 nm gt The silicate membrane has an affinity for certain cations which are adsorbed at anionic sites gt Action creates charge separation internally that leads to a potential difference gt Even though H conduction through the membrane does not occur what happens at the interfaces controls the membrane potential Potentiometry Equil Adsorption Equil Adsorption lt gt SiO4 sites 9 Test Dry Glass Internal Filling Solution Solution Silicate network L A r r 5100 nm 50 pm 5100 nm Hquotsoln Gl39glass e HGl39 external HGl39egt H Gl39 internal soln glass Charge neutrality must be maintained Complicated mechanism Potentiometry There are several factors that can affect a pH measurement Alkaline error pH gt 12 Na Acidic error pH lt 05 Dehydration Variations in junction potential Potentiometry Potential measured develops due to a difference in activity of an ion or charged molecule across a membrane which is selective for the species of interest Electrical contact 3 Module housing E K 00592 log x Reference element A901 Ion selective Figure 244 A typical plastic membrane ISE Courtesy of memb ane Orion Research Inc An example of a permselective membrane made onlastic polymer Remember that the membrane must be chemically inert insoluble and exhibit a selective interaction with the target analyte An active substance is placed in the polymer membrane The active substance is either a cation exchanger Ca2 an anion exchanger Br or a neutral macrocycle Potentiometry TABLE 241 Typical Commercially Available IonSelective Electrodes Concentration pH Species Type RangeM Range Interferences Ammonium t L 10 1 to 10 6 5 8 KNaM 2 BariumBa29IH4 L 10391m10 5 5 9 K m c LDR 4396 orders 0f mag s 3121 Br 2 S 13030 5 7 2 12 I l32 2Cl1q 2 2 F 3 Ca 39umCd s 1010 3 7 A 11 Cu Pb 0 Calcium Ca2 L 103107 49 3 Mg Napb2 LOD 10395 to 10396 M S N 3 Chloride or s 1 to 5 x 10 5 2 11 1 32 CN Br CopperCu2 s 10 1t010 7 0 7 AgHg2sz C1 Br Cyanide CNt s 10392m 10396 10 14 82quot SenSItIVIty 00592 V Fluoride F39 s 1 to 10 7 5 8 011 Iodide 1 s 1 to 10 7 3 12 52 r LeadPb2 s 10quotm10 5 0 9 Agngsz Cd2Cu2Fe3 Precision lt 5 n10 Nitrate N03 L 1 ms x 106 3 10 C1 Br N02 FSO 39 Nimwmog L 1010 6 39 31o c139Br NoF so Potassium Kt L 110 10 6 4 9 NaCa2Mg1 Stability good Silver Ag s 1 to 10397 2 9 52 Hg Sodium Nat G Sat d to 10 6 9 12 Li 1U NIH Sul de 3239 s 1 to 10397 12 14 Ag Hg2 G glass L liquid membrane S solid state Typical temperamre ranges are 0 50quotC for liquid membnnc and 040 C for solidstate electrodes Detection limit largely governed by leaching of the ions from the internal to the external solution through the membrane
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