Added function ConvertRefPot().

Function takes a potential and its reference electrode, and converts to a specified reference electrode scale, either another or the same.
I made it to simplify converting between different reference electrode scales in running text.

common.R

@@ -3,6 +3,7 @@
 # Taha Ahmed, Jan 2011
 
 # CONTENTS
+# >>>> ConvertRefPot
 # >>>> Celsius2Kelvin
 # >>>> Kelvin2Celsius
 # >>>> as.radians
@@ -11,6 +12,55 @@
 
 
 
+##################################################
+################# ConvertRefPot ##################
+##################################################
+ConvertRefPot <- function(argpotential, argrefscale, valuerefscale) {
+   # Converts from some reference potential scale into another
+   # SHE:     standard hydrogen electrode scale
+   # Ag/AgCl: silver silver-chloride electrode scale
+   # SCE:     standard calomel scale
+   #
+   ##### Add more reference electrodes here >>
+   refpotatSHEzero <- c(     0,     -0.21,  -0.24,        3)
+   refrownames     <- c( "SHE", "Ag/AgCl",  "SCE", "Li/Li+")
+   refcolnames     <- c("SHE0",   "AgCl0", "SCE0",    "Li0")
+   ##### Add more reference electrodes here <<
+   #
+   SHE0 <- data.frame(matrix(refpotatSHEzero, ncol=length(refpotatSHEzero), byrow=T))
+   refpotmtx <- matrix(NA, length(SHE0), length(SHE0))
+   refpotmtx[,1] <- matrix(as.matrix(SHE0), ncol=1, byrow=T)
+   for (c in 2:length(SHE0)) {
+      # loop over columns (except the first)
+      for (r in 1:length(SHE0)) {
+         # loop over rows
+         refpotmtx[r, c] <- refpotmtx[r, 1] - refpotmtx[c, 1]
+      }
+   }
+   refpotdf <- as.data.frame(refpotmtx)
+   names(refpotdf) <- refcolnames
+   row.names(refpotdf) <- refrownames
+   ## So far we have made a matrix of all the possible combinations,
+   ## given the vector refpotatSHEzero. The matrix is not strictly necessary,
+   ## but it may prove useful later. It does.
+   #
+   # Match argrefscale to the refrownames
+   argmatch <- match(argrefscale, refrownames, nomatch = 0)
+   # Match valuerefscale to the refrownames
+   valuematch <- match(valuerefscale, refrownames, nomatch = 0)
+   # We simply assume that the match was well-behaved
+   valuepotential <- argpotential + refpotdf[valuematch, argmatch]
+   # Check that arg and value electrodes are within bounds for a match
+   if (argmatch == 0 || valuematch == 0) {
+      # No match
+      # Perform suitable action
+      message("Arg out of bounds in call to ConvertRefPot")
+      valuepotential <- NA
+   }
+   return(valuepotential)
+}
+
+
 
 ##################################################
 ############### Celsius2Kelvin ###################