Eh-pH User Manual
Contents
1. AG n F E 1 where n is the charge transferred in the cell reaction F is the Faraday constant 23045 cal V mol and E is the standard electrode potential in volts Note Give AG data for the selected temperatures usually these values are only available at 25 C Sometimes all the necessary species are not available in the HSC database These missing species can be added to the chemical system specification if the user has the AG values or chemical potential data for these species The new species may be added by inserting an empty row with Insert Row and typing formula and AG values in this row The new species can be inserted into any location but it is recommended to add the same type of species seguentially because it makes the list easier to read and update Note A Do not insert any new elements new elements must be added in the EpH element selection window Fig 1 B Please do not create empty rows In some cases it is necessary to remove certain species from the system e g if some kinetic barriers are found to slow down the reaction rate in the experiments Such species can be removed using Delete Row The Labels and Lines sheets are for the program s internal use and it is not necessary to make any modifications to these sheets The Labels sheet contains format data for the labels such as text area number coordinates font name and properties and label visibility and orientation The Lines sheet contains format data2. The EpH module can also be used as an independent application in these cases the data for the diagram may be read from eph8 files by pressing File Open Normally the system specifications are automatically transferred from the system specification sheet to the diagram menu Fig 2 The calculation results may be saved in a eph8 file by selecting Save from the file menu and loaded back by selecting Open from the file menu which causes the diagram to be automatically recalculated 2 Species and AG data workbook The selected species and calculated AG data based on the enthalpy entropy and heat capacity values of the HSC database is shown in the Species sheet of the diagram workbook Fig 2 The species are arranged according to elements and species type The user can modify the AG data as well as adding or removing species in this sheet The Copyright O Outotec Oyj 2014 HSC 8 EpH O Utotec November 25 2014 Research Center Pori Petri Kobylin Lauri 14012 ORC J 7 16 M enp Antti Roine Kai Anttila Species sheet makes it possible to use data and species which cannot be found in the HSC database for example The published Pourbaix diagrams are often based on AG data which is given in the original papers This AG data can be used in the Species sheet to replace the default values based on the HSC database if reguired Note that these AG values can also be calculated from the standard potential values using Eguation 1
3. 1966 Haung H Construction of Eh pH and other stability diagrams of uranium in a multicomponent system with a microcomputer I Domains of predominance diagrams Canadian Metallurgical Ouarterly 28 1989 July September pp 225 234 Haung H Construction of Eh pH and other stability diagrams of uranium in a multicomponent system with a microcomputer II Distribution diagrams Canadian Metallurgical Quarterly 28 1989 July September pp 235 239 Criss C M and Cobble J W The Thermodynamic Properties of High Temperature Agueous Solutions V The Calculation of lonic Heat Capacities up to 200 Entropies and Heat Capacities above 200 C J Am Chem Soc 86 1964 pp 5385 91 90 93 Archer D G Wang P The Dielectric Constant of Water and Debye H ckel Limiting Law Slopes Journal of Physical Chemistry Reference Data 2 1990 Vol 19 pp 371 411 CRC Handbook of Chemistry and Physics 75th Ed CRC Press 1995 pp 6 15 6 16 Tayer L L Development and Interpretation of Computer Generated Potential pH Diagrams UMI Dissertation Services Arizona State University USA 1995 pp 1 25 Barner H E and Scheuerman R V Handbook of Thermochemical Data for Compounds and Agueous Species John Wiley amp Sons Inc New York 1978 Copyright O Outotec Oyj 2014
4. Cu has been selected in this example but S may be selected as the main element by pressing S in Fig 3 Up to 7 elements can be selected but it is recommended to use fewer because a large amount of elements and species increases calculation time and could cause some other problems S has been selected in this example Note It is not necessary to select H and O because these are always automatically included Search Mode This selection specifies the type of species which will be collected from the database It is recommended to use the default selections Note Condensed solid substances Aqueous Neutrals dissolved species without charge Aqueous lons dissolved ions Gases gaseous species without charge Gas lons gaseous ions and Liquids liquid species Select Species Usually you can select all species for the diagram by pressing All In some cases however it is useful to remove unnecessary species from the system This will decrease calculation time and simplify the diagrams Note A By pressing simultaneously the Ctrl key and clicking with the mouse it is easy to make any kind of selection B By double clicking the species it is possible to see more information on the species Load CUS25 eph8 file to see the selected species of this example The selection of species is the most critical step in the EpH calculation specifications For kinetic reasons the formation especially of large molecules may take quite a long time in aque
5. Petri Kobylin Lauri 14012 ORC J 6 16 M enp Antti Roine Kai Anttila 17 3 E pH Diagram Menu E pH Diagram Basic Settings o x File Edit Format Help A B D E F G 2 _1 Cu S System at 25 00 C 2 Molality Pressure Temperature 25 000 C 3 Cu 1 00E 00 1 00E 00 Show Colors FALSE 4 S 1 00E 00 1 00E 00 Dielectric Coi 78 382 5 DeltaG of H2 56 678 6 Davies Methi FALSE 7 lon Strength 0 000 8 Connection F 1 000 9 Criss Cobble FALSE 10 Max Eh 2 000 11 Max Eh 2 000 12 Min pH 0 000 13 Potential Scale 1 000 Max pH 14 000 14 Search modes Condensed Agueous lons Show Predon FALSE 15 Species DeltaG Area X1 Y1 X2 Y2 16 kcal mol no pH L Vv pH Vv 17 Cu 0 000 18 Cu FCC 0 003 19 CuO 30 974 20 CuO T 30 563 21 Cu20 35 335 22 Cu OH 2 89 079 k K 4 HI Chart 1 AKK 4 w gt Outotec Fig 2 Default values for the E pH diagram Table button The EpH diagram menu shows a summary of the chemical system specifications as well as the selected default values for the diagram The fastest way to go forward is to accept all the default values and press Draw for simple E pH diagram Usually at the beginning there is no need for modifications to the default values However it is important to understand the meaning of these settings because they may have a strong effect on the diagram The details of the diagram menu options are explained in the following paragraphs 1 File Open
6. HSC Chemistry EpH module is quite a simple task However several aspects must be taken into account when specifying the chemical system and analyzing the calculation results for example 1 A basic knowledge of chemistry aqueous systems and electrochemistry or hydrometallurgy is always needed in order to draw the correct conclusions 2 The EpH module carries out the calculations using pure stoichiometric substances In practice minerals may contain impurity elements and the composition may deviate slightly from the stoichiometric one 3 There are always some errors in the basic thermochemical data of the species This may have a significant effect on the results especially if the chemical driving force of the reaction is small Usually small differences between Pourbaix diagrams from different sources can be explained by the slightly different basic data used 4 Sometimes data for all existing species is not available from the HSC database or from other sources This will distort the results if the missing species are stable in the given conditions The missing unstable species will have no effect on the results 5 The EpH module does not take into account the non ideal behavior of aqueous solutions However in many cases these ideal diagrams give a quite good idea of the possible reactions in aqueous solutions especially if the driving force of the reactions is high 6 Thermochemical calculations do not take into account th
7. of the eguilibrium lines of the diagram Species names line area numbers line endpoint coordinates and line properties 3 Temperature Pourbaix diagrams are drawn at a constant temperature The user must select one temperature for the diagram from the list of temperatures Fig 2 The actual temperature values can only be changed from the system specification form Fig 1 No temperature selection is needed here for the Combined diagrams The default AG values in the Species sheet are calculated at the given temperatures 4 Other parameters The default values for the Dielectric Constant and AG of H20 are automatically calculated on the basis of the selected temperature and pressure The calculation of Dielectric Constant is based on experimental values and water vapor pressure which are valid from 0 to 373 C and from 1 to 5000 bar Outside this range the Dielectric Constant will be extrapolated The lon Strength and Correction Factor constants are automatically calculated by the program and usually they do not need to be modified by the user Copyright O Outotec Oyj 2014 HSC 8 EpH O utotec November 25 2014 Research Center Pori Petri Kobylin Lauri 14012 ORC J 8 16 M enp Antti Roine Kai Anttila 5 Potential and pH scale ranges The user may change the default range Max Eh and Min Eh for the potential scale as well as the range of the pH scale Max pH and Min pH The scale settings can also be changed by
8. 1 00E 000 1 00E 000 Fig 3 E pH diagram of Cu S H O system at 25 C using Cu as the main element The molalities of Cu and S are 1 mol kg H20 Copyright O Outotec Oyj 2014 Outotec an Research Center Pori Petri Kobylin Lauri 14012 ORC J M enp Antti Roine Kai Anttila 11 16 Chart1 x S Cu System at 25 00 C Eh Volts Element Molality Pressure S 1 00E 000 1 00E 000 Fig 4 E pH diagram of the S Cu H O system at 25 C using S as the main element The molalities of Cu and S are 1 mol kg H20 Selecting Format H20 Stability Lines opens the format dialog which makes it possible to modify the water stability lines formats and properties NB This dialog cannot be opened by double clicking the water stability lines Copyright O Outotec Oyj 2014 Outotec Research Center Pori Petri Kobylin Lauri M enp Antti Roine Kai Anttila 3 Scales Parameters and Axes Dielectric Constant 78 382 AG of H20 56 678 7 Use Davies Method for Activity lon Strength Correction Factor 7 Criss Cobble Diagram Axes Min Eh 2 000 Max Eh 2 000 Min pH 0 000 Max pH 14 000 Potential Scale Hydrogen gt Calomel Fig 5 Menu for formatting y axis note the different potential scale options HSC 8 EpH November 25 2014 14012 ORC J 12 16 The scale format dialog can be opened by clicking the Parameters and Axis see Fig 5 A special feature of the E pH diagr
9. HSC 8 EpH Outotec November 25 2014 Research Center Pori Petri Kobylin Lauri 14012 ORC J 1 16 M enp Antti Roine Kai Anttila 17 E pH Pourbaix Diagrams Module E pH diagrams show the thermodynamic stability areas of different species in an aqueous solution Stability areas are presented as a function of pH and electrochemical potential scales Usually the upper and lower stability limits of water are also shown in the diagrams by dotted lines Traditionally these diagrams have been taken from different handbooks However in most handbooks these diagrams are available only for a limited number of temperatures concentrations and element combinations The Eh pH module EpH module is used hereafter in HSC Chemistry allows the construction of diagrams in a highly flexible and fast way because the user can draw the diagrams exactly at the selected temperature and concentration The EpH module is based on STABCAL Stability Calculations for Aqueous Systems developed by H H Haung at Montana Tech USA Copyright Outotec Oyj 2014 HSC 8 EpH O Utotec November 25 2014 Research Center Pori Petri Kobylin Lauri 14012 ORC J 2 16 M enp Antti Roine Kai Anttila 17 1 Introduction E pH diagrams are also known as Pourbaix Diagrams after the author of the famous Pourbaix diagram handbook The simplest type of these diagrams is based on a chemical system consisting of one element and a w
10. N Re U Cu2 OH 2 2a Br Ga Na Rh V Cu3 OH 4 2a G Gd Nb Rn W HCuO2 a Ca Ge Nd Ru Xe HS a cd H Ne 5 Y HS2 a Ce He Ni Sb Yb HS3 a Cf H Np Sc Zn HS4 a cl Hg O Se Zr HS5 a HS6 a Search Mode HSO3 a Gases HSO4 a Gas Ions HSO5 a Liquids HS203 a Condensed HS204 a 3 Aqueous Neutrals HS205 a Aqueous Ions Select All Parameters and Axes v Temperature 25 00 in Molality Pressure a Element E mol kg H20 bar Cu 1 00E 00 1 00E 00 S 1 00E 00 1 00E 00 Show predominance areas of ions Create new page Show colors Table Combiner Draw Cu v Outotec Fig 1 Selecting elements and species for the E pH diagram 4 16 The EpH Diagram selection in the HSC main menu will show the EpH form Fig 1 The user must specify the chemical system which will be used to calculate the diagram in this form Let us assume that the system contains Cu S and H2O The following steps should be specified in order to create the diagram Copyright O Outotec Oyj 2014 HSC 8 EpH O utotec November 25 2014 Research Center Pori Petri Kobylin Lauri 14012 ORC J 5 16 M enp Antti Roine Kai Anttila 1 Select Elements Select one or more elements from the list The element chosen first will be used as the main element in the first diagram i e all species which are shown in the diagram will contain this element The user can easily change the main element selection later on in the diagram form see Fig 3
11. am scale dialog is the scale unit option You can select between the Hydrogen Saturated Calomel and Ag AgC scales The default scale is Hydrogen Eh which is used in the calculations The difference between the Min and Max values must be at least 0 2 units 4 Printing Print Settings Preview Printer porsr006 PORPRO17 x Setup Print Header Cu S System at 25 00 C Fonts Footer Font Orientation Portrait 5 Landscape Margins 25 25 23 Content Bitmap Keep aspect ratio 5 Vector Use background fill a aa Use graph fill Double resolution Refresh Fig 6 E pH Diagram Print Dialog 3 CICMA BO AA n k Copyright O Outotec Oyj 2014 HSC 8 EpH O Utotec November 25 2014 Research Center Pori Petri Kobylin Lauri 14012 ORC J 13 16 M enp Antti Roine Kai Anttila You can open the Print dialog by clicking Print see Fig 3 This dialog allows the user to select margins and size of the diagram as well as the orientation see Fig 6 5 Other Options The default diagram font dialog can be opened by pressing Font or by selecting Format Default Font from the menu Selecting Copy as well as Edit Copy will copy the diagram into the Windows Clipboard which makes it possible to paste the diagram into other Windows applications in Windows Metafile format Selecting Edit Copy All will also copy the molality and pressure values into th
12. ater solution for example the Mn H O system The system can contain several types of species such as dissolved ions condensed oxides hydroxides oxides etc The E pH diagram shows the stability areas of these species in the redox potential pH coordinates Usually the redox potential axis is based on the Standard Hydrogen Electrode SHE scale designated Eh but other scales can also be used The redox potential of the system represents its ability to change electrons The system tends to remove electrons from the species when the potential is high E gt 0 These conditions may exist near the anode in an electrochemical cell but can also be generated with some oxidizing agents Cu H20 CuO H20 In reducing conditions when the potential is low E lt 0 the system is able to supply electrons to the species for example with a cathode electrode or with some reducing agents The pH of the system describes its ability to supply protons H a to the species In acid conditions pH lt 7 the concentration of protons is high and in caustic conditions pH gt 7 the concentration of protons is low Usually a large amount of different species exist simultaneously in the agueous mixtures in fixed E pH conditions Pourbaix diagrams simplify this situation a lot by showing only the predominant species whose content is highest in each stability area The lines in the diagrams represent the E pH conditions where the content of the ad
13. clicking the x or y axis on the diagram form The minimum difference between Min and Max values is 0 2 and there is no upper limit for the difference 6 Molality and Pressure The diagrams are calculated using constant molalities concentrations for all the elements The default values may be changed in the table in the bottom right corner of the diagram menu see Fig 2 Molality values are given in mol kg H20 units The total pressure of the system is also given in the Molality table The EpH module uses the maximum value given in the Pressure column as the chemical system total pressure It is not possible to select a smaller pressure value than the water vapor pressure at the selected temperature In other words the total pressure must always be bigger than the water vapor pressure at the selected temperature The default value for the pressure is 1 bar 7 Show Predominance Areas of lons Selecting Show Predominance Areas of lons causes the EpH module to calculate two diagrams for the same system The first one is a normal E pH diagram with all species and the second one is the predominance diagram with only aqueous species Both diagrams are drawn into the same figure the first diagram in black and the second one in blue This option is recommended for use only with normal E pH diagrams see section 17 4 8 Draw and Combiner Buttons Draw starts the calculations and automatically shows the normal Pourbaix diagram Combiner will
14. e diagram Edit Copy Special will copy scaled diagrams Selecting Save or File Save will save the E pH diagram in Windows Metafile format WMF These diagrams cannot be read back to HSC Selecting Menu or File Exit will reactivate the Diagram Menu form see Fig 3 Selecting Help will open the HSC Help dialog 6 Example Cu S H20 System The Cu S and S Cu diagrams are shown in Fig 3 and Fig 4 These diagrams provide a lot of valuable information For example the dissolution behavior of copper can easily be estimated from Fig 3 It is easy to see that in neutral and caustic solutions metallic copper is stable near zero potential values It will form oxides in anode conditions E gt 0 and sulfides in cathode conditions E lt 0 However it will dissolve as Cu 2a in acid conditions at the anode E gt 0 and precipitate on the cathode E lt 0 in metallic form Copyright O Outotec Oyj 2014 HSC 8 EpH O Utotec November 25 2014 Research Center Pori Petri Kobylin Lauri 14012 ORC J M enp Antti Roine Kai Anttila 17 5 Combined E pH Diagrams This is not available in HSC 8 Copyright O Outotec Oyj 2014 14 16 HSC 8 EpH O Utotec November 25 2014 Research Center Pori Petri Kobylin Lauri 14012 ORC J 15 16 M enp Antti Roine Kai Anttila 17 6 E pH Diagrams in Practice The HSC EpH module enables fast and easy creation of Pourbaix diagrams for the reguired chemical system in u
15. e speed of the reactions kinetics For example the formation of the SO4 2a ion may be a slow reaction In these cases metastable diagrams created by removing such species from the system may give more consistent results with experimental laboratory results Due to these limitations and assumptions the HSC user must be very careful when drawing conclusions from E pH diagrams However these diagrams can offer extremely valuable information when combining the results with experimental work and with a good knowledge of agueous chemistry There is no universal kinetic or thermochemical theory available that could entirely substitute traditional experimental laboratory work with pure theoretical calculation models More information on E pH diagrams calculation methods and applications can be found in different handbooks for example from the Pourbaix Atlas Copyright Outotec Oyj 2014 Outotec HSC 8 EpH November 25 2014 Research Center Pori Petri Kobylin Lauri 14012 ORC J M enp Antti Roine Kai Anttila Chemical System Specifications Eh pH Diagram File Help Diagram Parameters a Select Elements Select Species Ac Cm Ho Os Si S gt Ag Co I P Sm S M A Cr I Pa Sn 03 Am Cs Ir Pb Sr s03 8 Ar Cu K Pd Ta SO3X G As Dy Kr Pm Tb Cu a At Er La Po Tc Cu 2a Au Es Li Pr Te CuO2 2a B Eu Lu Pt Th CuOH a Ba F Mg Pu Ti Cu OH 3 a Be Fe Mn Ra T Cu OH 4 2a Bi Fm Mo Rb Tm Cu20H 3a Bk Fr
16. ig 2 1 Main Elements The E pH diagrams show only the species that contain the selected main element The default main element Cu must be selected in the system specification form Fig 1 However the active main element can easily be changed in the diagram form by selecting Element inside Draw button see Fig 3 for the Cu S H O diagram and Fig 4 for the S Cu HO diagram Usually it is useful to check all the diagrams with different main elements to get a better idea of the equilibria 2 Labels and Lines The EpH module locates the area labels automatically on the widest point of the stability areas You can easily relocate the labels by dragging them with the mouse cursor if necessary The text of the labels and headings can be modified by inserting the cursor into the correct location within the text row and then starting to type You can start the Label Format dialog by double clicking the label This dialog makes it possible to change text and lines properties such as font type size line width color etc Note that you cannot delete the default labels but you can hide these labels by removing all the text from the label Copyright Outotec Oyj 2014 HSC 8 EpH Outotec November 25 2014 Research Center Pori Petri Kobylin Lauri 14012 ORC J 10 16 M enp Antti Roine Kai Anttila Chart1 x Cu S System at 25 00 C Eh Volts Element Molality Pressure Cu 1 00E 000 1 00E 000 s
17. jacent species is the same in the equilibrium state However these species always exist in small amounts on both sides of these lines and may have an effect on practical applications The lines in the diagrams can also be represented with chemical reaction eguations These reactions may be divided into three groups according to the reaction types 1 Horizontal lines These lines represent reactions that are involved with electrons but are independent of pH Neither H a ions nor OH a ions participate in these reactions 2 Diagonal lines with either a positive or negative slope These lines represent reactions that are involved with both electrons and H a and OH a ions 3 Vertical lines These lines represent reactions that are involved either with H a or OH a ions but are independent of E In other words electrons do not participate in these reactions The chemical stability area of the water is shown in the E pH diagrams by dotted lines The upper stability limit of water is based on the potential when oxygen generation starts on the anode It is specified by the reaction 2 H20 O2 g 4 H a 4 The lower stability limit is based on hydrogen formation on the cathode It is specified by the reaction 2 H a 2 H2 g Copyright O Outotec Oyj 2014 Outotec 11 Research Center Pori Petri Kobylin Lauri 14012 ORC J 3 16 M enp Antti Roine Kai Anttila The construction of the diagrams with the
18. ous solutions For example the formation of large polysulfide sulfate etc molecules S406 2a HS206 a HS703 a may take quite a while If these are included in the chemical system then they may easily consume all the sulfur and the formation of simple sulfides AgS CuoS will decrease due to the lack of sulfur Therefore in some cases large molecules should be rejected from the chemical system The data on some species may also be unreliable especially if the reliability class of the species in the database is not 1 Such species may be rejected from the chemical system Note also that it is not recommended to use species with the ia suffix see Chapter 28 section 28 4 for details Temperature The user must specify at least one temperature for the diagram Up to four temperatures may be specified in order to draw combined diagrams which show the effect of temperature The temperatures 25 50 75 and 100 C have been selected in this example Fig 1 and Fig 2 Criss Cobble This option enables HSC to extrapolate the heat capacity function of the aqueous species if this data is not given in the database see Chapter 28 28 4 Press Import Data This option selects data for calculation Draw Pressing the Table button will show the Diagram specification form see section 17 3 and Fig 2 By pressing Draw in Fig 2 you will see the default diagram Copyright O Outotec Oyj 2014 Outotec 11 Research Center Pori
19. ser specified conditions These diagrams contain the basic information of the agueous system in a compact and illustrative form These diagrams have found many applications in corrosion engineering geochemistry and hydrometallurgy since the publication of the famous Pourbaix Atlas handbook In hydrometallurgy E pH diagrams may be used for example to specify the conditions for selective leaching or precipitation In corrosion engineering they may be used to analyze the dissolution and passivation behavior of different metals in agueous environments These diagrams may also be used to illustrate the chemical behavior of different ions in agueous solutions Geochemists use Pourbaix diagrams quite frequently to study the weathering process and chemical sedimentation The weathering process is used to predict what will happen to a mineral exposed to acid oxidizing conditions at high temperature and pressure Pourbaix diagrams can also be used to estimate the conditions needed to form certain sediments and other minerals in the geological past Some application examples of the EpH module and Pourbaix diagrams are given in Chapter 18 Copyright Outotec Oyj 2014 HSC 8 EpH O Utotec November 25 2014 Research Center Pori Petri Kobylin Lauri 14012 ORC J 16 16 M enp Antti Roine Kai Anttila 17 7 References 1 2 Pourbaix M Atlas of Electrochemical Eguilibria in Agueous Solutions Oxford University Press U K
20. show one more menu for combined diagram specifications see section 17 5 for further details This is not yet implemented in HSC 8 9 Other options The Chart may be printed clicking Print Data table can be printed by clicking Table and selecting Print from the file menu Edit Copy provides normal copy and paste operations Edit Copy All copies all three sheets to the clipboard The worksheet layout may be changed using the Format menu which contains dialogs for column width row height font alignment and number formats Press Exit or select File Exit when you want to return to the system specifications form Fig 1 The Help menu opens the HSC Help dialog 10 Example of Normal Pourbaix Diagrams Cu S H O system Accept all the default values and press Draw Copyright O Outotec Oyj 2014 HSC 8 EpH O utotec November 25 2014 Research Center Pori Petri Kobylin Lauri 14012 ORC J 9 16 M enp Antti Roine Kai Anttila 17 4 Normal E pH Diagrams The calculated E pH diagrams are shown in Fig 3 In the Diagram window it is possible for example to modify the layout and format of the diagram The solid black lines show the stability areas of the most stable species on the pH and E scales The dotted black lines show the upper and lower stability areas of water see section 17 1 The stability areas of ions are shown by a blue dotted line if the Show Predominance Areas of lons option has been selected see F
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