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• — }-- tLlgjunction polonitsl included) ___�_ _ _ Tempera Hr,0 l M Ha,30 M MHt.r,35 M HE,40 M Hr,Sold O • Hr0 0 1 M O -HrQ,Sold lure •C Calomel Calomel Calomel Calomel Celome! Calomel CerOmel ' 10 03362 02602 02556 - 02543 — —04564 12 03362 —03731 15 03362 -- — — 02511 —03710 —04560 ' 20 03359 02569 02520 — 02479 —03676 —04555 25 03356 02549 02501 02459 02444 —03642 —04554 30 03351 02530 02481 02438 02411 —03607 —04550 35 03344 --- — — 02376 —03573 —04548 38 033311 02496 024411 02402 02115 —03512 —04547 ' 40 03336 02487 02439 02393 02340 — —04547 Table 5.4 standard Potentials Ehca1 in Volts)of Sorne cells Used for pH Measurements ' An acid solution would release deadly cyanide gas 3 Bleaching Pulp Therefore, the system generally Incorporates pH The pulp and paper Industry uses a number of oxl- controt The first stage reactions are dizing agents, usually based on chlorine, for bleaching ' Ct2 + 2e— = 2CI— E= = 1 35V pulp Many of these applications also incorporate pH Minus control Since a variety of organic compounds are OCN—+H2O+2e` =CN—+20H` E$= — (-097V) oxidized, 11 Is not possible to write exact equations for 232 the pulp-bleaching process ' Net Reaction However, excess reagent is often used to ensure Ci + CN` + 20H — = 2Cf— + OCN — + H Q completion of the reaction Therefore, the ORP will 2 2 tend toward the standard potential of the added The ORP at the equivalence point, using a silver reactant Again, the solution pH aflecls the absolute -silver chloride reference electrode,would be ORP value represented by Et} + Eq 1 35 — 097 pH of Hydrogen Fluoride Solutions '*Eeq = 2 --E = 2 {0 199) Fluoride concentrations greater than 10—6M in — —0 009 V vs AglAgCl acid solutions will attack the glass electrode The greater the concentration, the greater the severity of The second step in cyanide oxidation takes place at ' a controlled pH In the 7-t3 range The reactions are the attack One method of determining pH In hydrogen fluoride acidic solutions is by Indirect measurement Cl2+2e` =2CI` with a metallic electrode ' 2CO2+N2+21-12O+6e— =20CN — +40H — The oxidation-reduction potential (ORP) of a solution The net reaction Is containing quinhydrone is pH-dependent Therefore, if 3Cl2+20CN— +40H— —6CI— +2CO2+N2+21-12O no Interfering substances, such as strong oxidants or reductants,are present, the pH of the test solution can ' 2 Reduction of Chromate Waste be measured by Ilrst saturating It with qufnbydrone Metal-plating and treating industries are also one of then measuring the pH with a metallic electrode and a the largest producers of chromate bearing wastes reference electrode The gold thimble and the HF ' Chromate reduction, like cyanide oxidation, requires pH resistant calomel electrodes. In phenolic resin bodies, control In the first step the pH Is lowered and controlled are specifically designed for this application and are at about 2 to 2 5 Sulfur dioxide Is commonly used as not attacked by HF ( See Photo following) ' the reductant The reactions are These HF-reslstant electrodes cannot be used Cr 0- +14H+ +fie` =2Cr3+ +7H O directly to measure pH, but rather, quinhydrone must 2 7 2 first be added to the solution to establish a reversible SO T+4H+ +2e— =H2SO3+H2O and measurable redox couple The overall reaction is Cr20 7+2H + +3H2SO3=2Cr3+ +3SO4= +41-12O In the second part of the process the waste liquors are neutralized to a pH level of 7 to 8 At this pH the Wchromic Ion precfpltates as a sludge,and is sent Io tarifiers for ultimate disposal or recovery