Sara Sotolongo

The oxidation kinetics of Fe(II) in seawater

Abstract The oxidation of Fe(II) has been studied as a function of pH (5 to 9), temperature (5 to 45°C), and salinity (0 to 35). The pseudo-first-order rate constant, k1, −d[Fe(II)] dt = k 1 [Fe(II)] in water and seawater was found to be a second degree function of pH over the pH range of 7.5 to 8.5 at 5°C and 6.0 to 8.0 at 25°C. The overall rate constant (k) −d[Fe(II)] dt = k[Fe(II)][O 2 ][OH − ] 2 was determined from 5 to 45°C and S = 0 to 35. The results have been fit to an equation of the form (T = 273.15 + t°C) log k = log k 0 − 3.29I 1 2 + 1.52I where logk0 = 21.56–1545/T with a standard error = 0.09. The energy of activation for the overall rate constant in water and seawater was 29 ± 2 kJmol−1. The values of the rate constant for pure water (k0) are in good agreement with literature data. The half times for seawater from some previous studies at a pH = 8.0 were slower than our results for Gulf Stream waters. Measurements on Biscayne Bay waters also yield slower half times apparently due to the presence of organic ligands that can complex Fe(II).

The oxidation of Fe(II) with H2O2 in seawater

Abstract The oxidation of Fe(II) with H2O2 has been measured in seawater as a function of pH (2 to 8.5), temperature (5 to 45°C) and salinity (0 to 35). The effect of pH on the rate constant, k, d [Fe(II)] dt = −K[Fe(II)][H 2 O 2 ] was found to be a linear function of [H+] or [OH−] from pH = 6 to 8. The effect of temperature and ionic strength on k at pH = 6.0 was given by log k= 13.73 −2,948/T− 1.707 1 2 + 1.20I with a σ = 0.12 in log k. If the rates are expressed as d( Fe(II)] dt = −k 2 [Fe(II)][H 2 O 2 ][OH − ] the values of k2 are independent of temperature. This is due to the fact that the energy of activation of k is the same order of magnitude of ΔH w ∗ , the heat of ionization of water. The ionic strength dependence of k2 was given by log k 2 = 11.72–2.14I 1 2 + 1.38I with a σ = 0.11 in log k2. The results of k over the entire pH, temperature and ionic strength were given by k = k0αFe + k1αFeOH where αFe and αFeOH, k0 and k1 are the rate constants, respectively, for the oxidation of Fe2+ and FeOH+. The values of k0 and k1 are given by logk0 = 8.37− 1,866/T log k 1 = 17.26 − 2,948/T− 1.70I 1 2 + 1.20I . The addition of HCO3− at a constant pH was found to linearily increase the rate independent of the salinity and temperature. This may be related to FeCO30 reacting faster than FeOH+ with H2O2. At a given pH and ionic strength, the rates in seawater are nearly the same as in NaCl.

The use of buffers to measure the pH of seawater

The pH of seawater can be measured in the field using potentiometric and spectrophotometric methods. The use of pH standards or buffers is an important aspect of the calibration of both methods in a laboratory on a common concentration scale. The buffers can also be used to monitor the performance of pH meter and spectrophotometer during a cruise. A procedure is described for the determination of the pH of seawater, where the proton concentration is expressed as moles kg-H2O−1 using seawater buffers. The buffers are prepared in synthetic seawater in the laboratory by the methods outlined by Bates and coworkers. We have prepared four buffers (Bis, Tris, Morpholine and 2-Aminopyridine) that cover a pH range from 6.8 to 8.8. The emf values of the buffers were measured with a H2, Pt/AgCl, Ag electrode system after their preparation and bottling for use at sea. The measured emf values were found to be in good agreement (±0.05 mV) with the original measurements of Bates and coworkers from 0 to 45°C. The measured pH of these buffers are in good agreement (±0.001 pH units) with the values calculated from the equations of Dickson on the total pH scale based on Bates et al. Studies are underway to access the long term stability of these buffers. We have also used these buffers to calibrate systems used to make potentiometric and spectrophotometric measurements of pH on seawater relative to the H2, Pt/Ag, AgCl electrode from 5 to 45°C.

The PVT properties of concentrated aqueous electrolytes IX. The volume properties of KCl and K2SO4 and their mixtures with NaCl and Na2SO4 as a function of temperature

AbstractThe densities of KCl and K2SO4 were measured from dilute solutions to saturation from 5 to 95°C. The data were combined with literature data to produce density and apparent molal volume, Vφ, equations from 0 to 100°C and to saturation. The standard deviations of the density equations were 30×10−6 g-cm−3 and 32×10−6 g-cm−3, respectively, for KCl and K2SO4. Pitzer equations were used to fit the Vφ data. The resulting infinite dilute partial molal volumes, Vo, were in reasonable agreement with literature data. The densities of the mixtures of the six combinations of the salts KCL, K2SO4 NaCl and Na2SO4 were measured at I=2.0 and t=5, 25, 55 and 95°C. The resulting volumes of mixing were fitted to equations of the form

Densities and compressibilities of aqueous sodium carbonate and bicarbonate from 0 to 45°C

\Delta V_m = y(1 - y)I^2 [\nu _0 + \nu _1 (1 - 2y)]

Uptake of Fe(II) and Mn(II) on chitin as a model organic phase

wherev0 andv1 are interaction parameters. The cross square rule is valid over the entire temperature range although the deviations are larger at higher temperatures. Pitzer θNaKv and

PVT properties of concentrated aqueous electrolytes. 7. The volumes of mixing of the reciprocal salt pairs potassium chloride, potassium sulfate, sodium chloride, and sodium sulfate at 25.degree.C and I = 1.5 M

\theta _{ClSO_4 }^v