A ZSM-5 zeolite with a \ce{SiO2}/\ce{AlO3} ratio of 23 was used since this maximised the number of sites which were available for ion-exchange due to the higher aluminium content. In addition this increased the efficiency of the ion-exchange process since zeolites with a high Si/Al ratio are hydrophobic\autocite{chen76,han09,olson99} hence the cation solution does not spontaneously enter the zeolite nanopores so ion-exchange happens only at sites close to the pore entrance.\autocite{han09,olson99} This will thus reduce the percentage uncertainties in the values recorded.
\section{Experimental}\label{sec:experimental}
-Standard solutions of \ce{Cu^{2+}} and \ce{Zn^{2+}} (\SI{50.00}{\centi\metre\cubed}) were made using \ce{CuSO4.5H2O} and \ce{ZnSO4.7H2O} with concentration \SI{2.008e-3}{\mole\per\deci\metre\cubed} and \SI{2.02e-3}{\mole\per\deci\metre\cubed} respectively. The absorbance of the standard copper sulphate solution was taken at \SI{806}{\nano\metre} (\num{0.484}) then \SI{20.00}{\centi\metre\cubed} of the standard solutions were added to \SI{0.4810}{\gram} (for the copper solution) and \SI{0.5274}{\gram} (for the zinc solution) of HZSM-5 zeolite with an \ce{AlO3}:\ce{SiO2} ratio of 23 -- forming an opaque white suspension -- before heating both solutions (with stirring) at \SI{70}{\celsius} for one hour. Centrifugation was completed on part of the resultant copper mixture, however time constraints prevented the completion of this process. The two mixtures were thus stored in a fridge for one week until the following laboratory session.
+Standard solutions of \ce{Cu^{2+}} and \ce{Zn^{2+}} (\SI{50.00}{\centi\metre\cubed}) were made using \ce{CuSO4.5H2O} and \ce{ZnSO4.7H2O} with concentration \SI{2.008e-3}{\mole\per\deci\metre\cubed} and \SI{2.02e-3}{\mole\per\deci\metre\cubed} respectively. The absorbance of the standard copper sulphate solution was taken at \SI{806}{\nano\metre} (\num{0.484}) then \SI{20.00}{\centi\metre\cubed} of the standard solutions were added to \SI{0.4810}{\gram} (for the copper solution) and \SI{0.5274}{\gram} (for the zinc solution) of HZSM-5 zeolite with an \ce{SiO2}/\ce{AlO3} ratio of 23 -- forming an opaque white suspension -- before heating both solutions (with stirring) at \SI{70}{\celsius} for one hour. Centrifugation was completed on part of the resultant copper mixture, however time constraints prevented the completion of this process. The two mixtures were thus stored in a fridge for one week until the following laboratory session.
-After one week the zeolite had settled in the bottom of the solutions. The clear solution was decanted and the remainder was centrifuged for 30 minutes before the supernatant was reintroduced to the initially decanted solution producing a slightly cloudy copper solution and a moderately cloudy zinc solution. The solutions were made up to \SI{100.00}{\centi\metre\cubed} before the absorbance of the copper solution at \SI{806}{\nano\metre} was determined (\num{0.110}) and \SI{20.00}{\centi\metre\cubed} aliquots of the zinc solution was titrated against a standard EDTA solution (batch A: \SI{0.4993}{\mole\per\deci\metre\cubed}) with \SI{2}{\centi\metre\cubed} of a pH 10 buffer solution and eriochrome black T as the indicator (colour change from red to light blue).
+After one week the zeolite had settled in the bottom of the solutions. The clear solution was decanted and the remainder was centrifuged for 30 minutes before the supernatant was reintroduced to the initially decanted solution producing a slightly cloudy copper solution and a moderately cloudy zinc solution. The solutions were made up to \SI{100.00}{\centi\metre\cubed} before the absorbance of the copper solution at \SI{806}{\nano\metre} was determined (\num{0.110}) and \SI{20.00}{\centi\metre\cubed} aliquots of the zinc solution was titrated against a standard ethylenediaminetetraacetate (EDTA) solution (batch A: \SI{0.4993}{\mole\per\deci\metre\cubed}) with \SI{2}{\centi\metre\cubed} of a pH 10 buffer solution and eriochrome black T as the indicator (colour change from red to light blue).
\section{Results}
\subsection{Copper-Exchanged Zeolite}
\begin{table}[h]
- \caption{Masses used in CuZSM-5 preparation.}\label{tbl:cu-masses}
+ \caption{Masses used for the preparation of CuZSM-5.}\label{tbl:cu-masses}
\centering
\begin{tabular}{|c|c|}
\hline
\end{tabular}
\end{table}
+The uncertainty in the absorbance values recorded by the spectrophotometer can be modelled using the following equation:~\autocite{galban07}
+
+\begin{equation}\label{eq:spectrophotometer-uncertainty}
+ \delta Abs = Abs \sqrt{ \left( \frac{0.434}{Abs} k_2 \sqrt{1 + 10^{Abs}} \right)^2
+ + \left( \frac{0.434}{Abs} k_3 \right)^2 }
+\end{equation}
+
+Where $k_2$ is a measure of the expected precision of the instrument itself for a specific solution and $k_3$ is a measure of the uncertainty introduced by replacing the cuvette.
+
+Using the values of $k_2=\num{4.5e-4}$ and $k_3=\num{27e-4}$ recorded by Galb\'{a}n et al.~\autocite{galban07} for the PerkinElmer Lambda 5 spectrophotometer with the ferroin solution since these values produce the largest overall uncertainty, hence giving the most generous reasonable estimate in the uncertainty of the absorbances recorded. Using these values with the absorbances in table \ref{tbl:cu-absorbance} and letting $A_{\ce{Cu}_\text{std.}}$ be the absorbance of the standard \ce{CuSO4} solution and $A_{\ce{Cu}_\text{prod.}}$ be the absorbance of the post-reaction solution:
+
+\begin{align}
+ \delta A_{\ce{Cu}_\text{std.}} &= 0.484 \sqrt{ \left( \frac{0.434}{0.484} \times \num{4.5e-4} \sqrt{1 + 10^{0.484}} \right)^2
+ + \left( \frac{0.434}{0.484} \times \num{27e-4} \right)^2 } \nonumber \\
+ \label{eq:cu-std-abs-uncertainty}
+ &= 0.001 \\
+ \delta A_{\ce{Cu}_\text{prod.}} &= 0.110 \sqrt{ \left( \frac{0.434}{0.110} \times \num{4.5e-4} \sqrt{1 + 10^{0.110}} \right)^2
+ + \left( \frac{0.434}{0.110} \times \num{27e-4} \right)^2 } \nonumber \\
+ \label{eq:cu-prod-abs-uncertainty}
+ &= 0.001
+\end{align}
+
\subsection{Zinc-Exchanged Zeolite}
\begin{table}[h]
- \caption{Masses used in preparation of \ce{ZnSO4} standard solution.}\label{tbl:zn-std-masses}
+ \caption{Masses used in preparation of the \ce{ZnSO4} standard solution utilised in the standardisation of the \ce{EDTA} solution.}\label{tbl:zn-std-masses}
\centering
\begin{tabular}{|c|c|}
\hline
\end{table}
\begin{table}[h]
- \caption{Masses used in ZnZSM-5 preparation.}\label{tbl:zn-zsm-5-masses}
+ \caption{Masses used for the preparation of ZnZSM-5.}\label{tbl:zn-zsm-5-masses}
\centering
\begin{tabular}{|c|c|}
\hline
\end{tabular}
\end{table}
-The reaction which occurred during the titrations between the \ce{EDTA} and \ce{Zn^2+} ions in given in equation \ref{eq:edta-zn-reaction}.
+The reaction which occurred during the titrations between the \ce{EDTA^4-} and \ce{Zn^2+} ions in given in equation \ref{eq:edta-zn-reaction}.
\begin{equation}\label{eq:edta-zn-reaction}
- \ce{Zn^2+ + EDTA^4- -> ZnEDTA^2-}
+ \ce{Zn^2+ (aq) + EDTA^4- (aq) -> ZnEDTA^2-(aq)}
\end{equation}
+%TODO: Reference.
+%The pH 10 buffer was used to ensure the EDTA existed in the deprotonated form which is able to complex to metal cations and also to ensure the eriochrome black T indicator exhibits the desired colour change.
+
\begin{table}
\caption{Titration results from standardisation of \ce{EDTA} solution with standard zinc sulphate solution.}\label{tbl:zn-standardisation}
\centering
\begin{align*}
Mr_{\text{unit cell}} = \frac{7.68}{q} Mr_{\text{cation}} &+ (11.5(26.982)+ (96-7.68)(28.085) + 192(15.999) \\
&+ x(2(1.008) + 15.999)) \si{\gram\per\mole} \\
- = \frac{7.68}{q} Mr_{\text{cation}} &+ \SI{5759.4692}{\gram\per\mole} + x(\SI{450.375}{\gram\per\mole})
+ = \frac{7.68}{q} Mr_{\text{cation}} &+ \SI{5759.469}{\gram\per\mole} + x(\SI{450.375}{\gram\per\mole})
\end{align*}
Thus for HZSM-5 where the cation is \ce{H+} and $x \approx 25$~\autocite{donder06}.
\begin{equation}\label{eq:hzsm-5-mr}
\begin{split}
- Mr_{\text{HZSM-5 unit cell}} &= \frac{7.68}{1} \times \SI{1.008}{\gram\per\mole} + (5759.49692 + 25(450.375)) \text{ \si{\gram\per\mole}} \\
- &= \SI{6217.6134}{\gram\per\mole}
+ Mr_{\text{HZSM-5 unit cell}} &= \frac{7.68}{1} \times \SI{1.008}{\gram\per\mole} + (5759.469 + 25(450.375)) \text{ \si{\gram\per\mole}} \\
+ &= \SI{6217.613}{\gram\per\mole}
\end{split}
\end{equation}
\epsilon = \frac{A}{c l}
\end{align}
-Hence using equations \ref{eq:[cuso4]-std} and \ref{eq:beer-lambert-epsilon} with $A_{\ce{Cu}_\text{std.}}$ being the absorbance of the standard \ce{CuSO4} solution:
+Hence using equations \ref{eq:[cuso4]-std} and \ref{eq:beer-lambert-epsilon}:
\begin{equation}
\begin{split} \label{eq:molar-extinction}
\end{split}
\end{equation}
-This hence gives:
-\begin{align}
- \epsilon_{\text{\ce{CuSO4}}} &= \frac{0.484 \times \SI{50.00e-3}{\deci\metre\cubed} \times (63.546 + 32.066 + 4(15.999) + 5(2(1.008) + 15.999)) \text{ \si{\gram\per\mole}}}{\SI{1.0}{\centi\metre} \times \SI{0.5014}{\gram}} \nonumber\\
- \label{eq:molar-extinction-calc}
- &= \frac{0.484 \times \SI{50.00e-3}{\deci\metre\cubed} \times \SI{249.677}{\gram\per\mole}}{\SI{1.0}{\centi\metre} \times \SI{0.5014}{\gram}} = \SI{12.05}{\deci\metre\cubed\per\mole\per\centi\metre}
-\end{align}
+%This hence gives:
+%\begin{align}
+% \epsilon_{\text{\ce{CuSO4}}} &= \frac{0.484 \times \SI{50.00e-3}{\deci\metre\cubed} \times (63.546 + 32.066 + 4(15.999) + 5(2(1.008) + 15.999)) \text{ \si{\gram\per\mole}}}{\SI{1.0}{\centi\metre} \times \SI{0.5014}{\gram}} \nonumber\\
+% \label{eq:molar-extinction-calc}
+% &= \frac{0.484 \times \SI{50.00e-3}{\deci\metre\cubed} \times \SI{249.677}{\gram\per\mole}}{\SI{1.0}{\centi\metre} \times \SI{0.5014}{\gram}} = \SI{12.05}{\deci\metre\cubed\per\mole\per\centi\metre}
+%\end{align}
-\subsubsection{Determination of Percentage of \ce{Cu^2+} Exchanged Compared to the Theoretical Maximum}
+\subsubsection{Determination of \ce{Cu^2+} Ion-Exchange Level}
By rearranging the Beer-Lambert Law (equation \ref{eq:beer-lambert}) for concentration:
\begin{equation} \label{eq:beer-lambert-c}
c = \frac{A}{\epsilon l}
\end{equation}
-Letting $[\ce{CuSO4}]_\text{prod.}$ be the concentration, $A_{\ce{Cu}_\text{prod.}}$ be the absorbance, $n_{\ce{Cu}_\text{prod.}}$ be the amount of \ce{Cu^{2+}} ions and $V_{\ce{Cu}_\text{prod.}}$ be the volume of the solution after the ion-exchange reaction while using equation \ref{eq:beer-lambert-c}:
+Letting $[\ce{CuSO4}]_\text{prod.}$ be the concentration, $n_{\ce{Cu}_\text{prod.}}$ be the amount of \ce{Cu^{2+}} ions and $V_{\ce{Cu}_\text{prod.}}$ be the volume of the solution after the ion-exchange reaction while using equation \ref{eq:beer-lambert-c}:
\begin{align}
[\ce{CuSO4}]_\text{prod.} &= \frac{A_{\ce{Cu}_\text{prod.}}}{\epsilon_{\text{\ce{CuSO4}}} l} \nonumber \\
Using \ref{eq:cu-percent-exchanged} with:
-%TODO: Look up uncertainty in absorbance value for spectrophotometer and that for relative atomic masses.
\begin{align*}
- Mr_{\text{HZSM-5 unit cell}} &= \SI{6217.6134}{\gram\per\mole} \text{ from equation \ref{eq:hzsm-5-mr}} \\
+ Mr_{\text{HZSM-5 unit cell}} &= \SI{6217.613}{\gram\per\mole} \text{ from equation \ref{eq:hzsm-5-mr}} \\
m_{\ce{CuSO4.5H2O}} &= \SI{0.5014 \pm 0.00005}{\gram} \text{ from table \ref{tbl:cu-masses}} \\
- A_{\ce{Cu}_\text{std.}} &= \num{0.484} \text{ from table \ref{tbl:cu-absorbance}} \\
+ A_{\ce{Cu}_\text{std.}} &= \num{0.484 \pm 0.001} \text{ from table \ref{tbl:cu-absorbance} and equation \ref{eq:cu-std-abs-uncertainty}} \\
V_{\ce{Cu}_\text{react.}} &= \SI{20.00 \pm 0.06 e-3}{\deci\metre\cubed} \text{ from method (section \ref{sec:experimental})} \\
- A_{\ce{Cu}_\text{prod.}} &= \num{0.110} \text{ from table \ref{tbl:cu-absorbance}} \\
+ A_{\ce{Cu}_\text{prod.}} &= \num{0.110 \pm 0.001} \text{ from table \ref{tbl:cu-absorbance} and equation \ref{eq:cu-prod-abs-uncertainty}} \\
V_{\ce{Cu}_\text{prod.}} &= \SI{100.00 \pm 0.20 e-3}{\deci\metre\cubed} \text{ from method (section \ref{sec:experimental})}\\
m_{\text{HZSM-5}} &= \SI{0.4810 \pm 0.00005}{\gram} \text{ from table \ref{tbl:cu-masses}} \\
V_{\ce{Cu}_\text{std.}} &= \SI{50.00 \pm 0.06 e-3}{\deci\metre\cubed} \text{ from method (section \ref{sec:experimental})} \\
- Mr_{\ce{CuSO4.5H2O}} &= \SI{249.577}{\gram\per\mole} \text{ from equation \ref{eq:molar-extinction-calc}}
+ Mr_{\ce{CuSO4.5H2O}} &= \SI{249.685}{\gram\per\mole} \text{ \autocite[Physical Constants of Inorganic Compounds, 4-60]{crc-handbook}}
\end{align*}
\begin{align*}
\begin{split}
- \si{\percent} \text{ \ce{Cu^{2+}} Exchanged} &= \frac{2 \times \SI{6217.6134}{\gram\per\mole} \times \SI{0.50140}{\gram}\left(0.484 \times 20.00 - 0.110 \times 100.00\right)\num{e-3} \text{ \si{\deci\metre\cubed}}}{7.68 \times \SI{0.4810}{\gram} \times 0.484 \times \SI{50.00e-3}{\deci\metre\cubed} \times \SI{249.577}{\gram\per\mole}} \\
+ \si{\percent} \text{ \ce{Cu^{2+}} Exchanged} &= \frac{2 \times \SI{6217.613}{\gram\per\mole} \times \SI{0.50140}{\gram}\left(0.484 \times 20.00 - 0.110 \times 100.00\right)\num{e-3} \text{ \si{\deci\metre\cubed}}}{7.68 \times \SI{0.4810}{\gram} \times 0.484 \times \SI{50.00e-3}{\deci\metre\cubed} \times \SI{249.577}{\gram\per\mole}} \\
&\phantom{=} \times \SI{100}{\percent}
\end{split} \\
- &= \SI{-18}{\percent}
+ &= \SI{-37}{\percent}
\end{align*}
\subsubsection{Error Propagation} \label{sec:cu-error-propagation}
\end{split}
\end{equation}
-%TODO: Substitute uncertainties into equation.
Substituting values into equation \ref{eq:cu-error-propagation} thus yields:
\begin{displaymath}
- \delta v_{\ce{Cu}} = \pm \SI{0.00}{\percent}
+ \delta v_{\ce{Cu}} = \pm \SI{3}{\percent}
\end{displaymath}
-So the percentage of \ce{Cu^2+} exchanged is \SI{-18 \pm 0.00}{\percent}.
+So the percentage of \ce{Cu^2+} exchanged is \SI{-37 \pm 3}{\percent}.
-\subsection{Calculation of Ion-Exchange Efficiency for Zinc Solution}
-\subsubsection{Standardisation of EDTA Solution (Batch A)}
+\subsection{Calculations for Zinc Solution}
+\subsubsection{Determination of EDTA Solution (Batch A) Concentration}
Letting $V_{\ce{Zn}_\text{std.}}$ be the volume, $[\ce{ZnSO4}]_\text{std.}$ be the concentration and $m_{\ce{ZnSO4.7H2O}_\text{std}.}$ be the mass of \ce{ZnSO4.7H2O} used for the preparation of the \ce{ZnSO4} standard solution used to standardise the \ce{EDTA} solution.
\begin{equation}\label{eq:[znso4]-std}
[\ce{EDTA^4-}] = \frac{m_{\ce{ZnSO4.7H2O}_\text{std}.} V_{\ce{Zn}_\text{std. aliquot}}}{Mr_{\ce{ZnSO4.7H2O}} V_{\ce{Zn}_\text{std.}} V_{\ce{EDTA}_\text{std.}}}
\end{equation}
-\subsubsection{Determination of Percentage of \ce{Zn^2+} Exchanged Compared to the Theoretical Maximum}\label{sec:zn-percent-exchanged}
+\subsubsection{Determination of \ce{Zn^2+} Ion Exchange Level}\label{sec:zn-percent-exchanged}
Let $[\ce{ZnSO4}]_\text{std. orig.}$ be the concentration of, $m_{\ce{ZnSO4.7H2O}_\text{orig.}}$ be the mass of zinc sulphate used and $V_{\ce{Zn}_\text{std. orig.}}$ be the volume of the standard zinc sulphate solution created for the ion exchange process.
\begin{equation}
Using equation \ref{eq:zn-percent-exchanged} with:
-%TODO: Uncertainties in Mr.
\begin{align*}
- Mr_\text{HZSM-5 unit cell} &= \SI{6217.6134}{\gram\per\mole} \text{ from equation \ref{eq:hzsm-5-mr}} \\
+ Mr_\text{HZSM-5 unit cell} &= \SI{6217.613}{\gram\per\mole} \text{ from equation \ref{eq:hzsm-5-mr}} \\
V_{\ce{Zn}_\text{prod. aliquot}} &= \SI{20.00 \pm 0.06 e-3}{\deci\metre\cubed} \text{ from method (section \ref{sec:experimental})} \\
V_{\ce{Zn}_\text{std.}} &= \SI{100.00 \pm 0.20 e-3}{\deci\metre\cubed} \text{ from method (section \ref{sec:experimental})} \\
V_{\ce{EDTA}_\text{std.}} &= \SI{31.95 \pm 0.2 e-3}{\deci\metre\cubed} \text{ from equation \ref{eq:edta-v-standardisation}} \\
V_{\ce{Zn}_\text{std. aliquot}} &= \SI{10.00 \pm 0.04 e-3}{\deci\metre\cubed} \text{ from method (section \ref{sec:experimental})} \\
V_{\ce{Zn}_\text{prod.}} &= \SI{100.00 \pm 0.20 e-3}{\deci\metre\cubed} \text{ from method (section \ref{sec:experimental})} \\
m_{\text{HZSM-5}} &= \SI{0.5274 \pm 0.00005}{\gram} \text{ from table \ref{tbl:zn-zsm-5-masses}} \\
- Mr_{\ce{ZnSO4.7H2O}} &= (65.38 + 32.066 + 4(15.999) + 7(2(1.008) + 15.999)) \text{ \si{\gram\per\mole}} \\
- &= \SI{287.547}{\gram\per\mole}
+ Mr_{\ce{ZnSO4.7H2O}} &= \SI{287.578}{\gram\per\mole} \text{ \autocite[Phyical Constants of Inorganic Compounds, 4-96]{crc-handbook}}
\end{align*}
+Gives:
+
\begin{displaymath}
\text{\si{\percent} \ce{Zn} Exchanged} = \SI{66}{\percent}
\end{displaymath}
+ \left( \frac{\delta V_{\ce{Zn}_\text{std.}}}{V_{\ce{Zn}_\text{std.}}} \right)^2
}
{
- V_{\ce{Zn}_\text{prod. aliquot}} V_{\ce{Zn}_\text{std.}} V_{\ce{EDTA}_\text{std.}} m_{\ce{ZnSO4.7H2O}_\text{orig.}} V_{\ce{Zn}_\text{orig.}} - V_{\ce{Zn}_\text{std. orig.}} V_{\ce{EDTA}_\text{prod.}} m_{\ce{ZnSO4.7H2O}_\text{std.}}
+ \big( V_{\ce{Zn}_\text{prod. aliquot}} V_{\ce{Zn}_\text{std.}} V_{\ce{EDTA}_\text{std.}} m_{\ce{ZnSO4.7H2O}_\text{orig.}} V_{\ce{Zn}_\text{orig.}} - V_{\ce{Zn}_\text{std. orig.}} V_{\ce{EDTA}_\text{prod.}} m_{\ce{ZnSO4.7H2O}_\text{std.}}
} \\
%New Line.
&\frac{
+ V_{\ce{Zn}_\text{std. orig.}}^2 V_{\ce{EDTA}_\text{prod.}}^2 m_{\ce{ZnSO4.7H2O}_\text{std.}}^2
}
{
- V_{\ce{Zn}_\text{std. aliquot}} V_{\ce{Zn}_\text{prod.}} \hfill
+ V_{\ce{Zn}_\text{std. aliquot}} V_{\ce{Zn}_\text{prod.}} \big)^2 \hfill
} \\
%New Line.
&\frac{
\end{split}
\end{equation}
-%TODO: Substitute uncertainties into equation.
Substituting values into equation \ref{eq:zn-error-propagation} gives:
\begin{displaymath}
- \delta v_{\ce{Zn}} = \pm \SI{0.00}{\percent}
+ \delta v_{\ce{Zn}} = \pm \SI{3}{\percent}
\end{displaymath}
-Hence the percentage of \ce{Zn^2+} exchanged is \SI{66 \pm 0.00}{\percent}.
+Hence the percentage of \ce{Zn^2+} exchanged is \SI{66 \pm 3}{\percent}.
\section{Analysis}
%Over 100% exchange is possible e.g. due to formation of oxide species phyllosilicate outside zeolite e.t.c. influence on cobalt salt precursers on cobalt speciation and catalytic properties of H-ZSM-5 modified ... mhamdi
%TODO: Compared molar extinction coefficient value to literature value.
%TODO: Note the acidity of some metal ions formed other substances in solution.
-%TODO: Add note about centrifugation used for zeolite separation due to recommendation by Russell: nano-sized zeolite particles clog filter paper and prevent efficient filtration of solution.
-
-%TODO: Titre value too small due to ZSM-5 suspended in solution: displaces liquid so actual aliquot size is smaller than appears.
-%TODO: Add notes about titration results: final anomalous reading possibly due human error or water in pipette filler, more solid in aliquot than others since less solution in volumetric flask and solid started to settle on bottom hence aliquot smaller than others.
%TODO: add note explaining when separate ZnSO4 solution prepared for standardisation of EDTA solution - not a primary analytical standard.
%TODO: Analysis Points:
-%Explain intention of storing in fridge.
-%Losses: centrifuge tube: unable to transfer all of zinc solution out of sample bottles into centrifuge tubes. Insufficient time to centrifuge zeolite with distilled water as well to rinse tubes.
-%After centrifuging solutions were still cloudy so still zeolite suspended. Higher absorbance than true for copper - could have effected zinc as well. Use titrimetric method to get more accurate copper reading.
%Also other byproducts (non-useful) formed - see paper.
+\subsection{General}
Between laboratory sessions the solutions were stored in a fridge in an attempt to reduce the rate of ion exchange since some of the ZSM-5 had already been separated out of the copper solution. This is not likely to have been very effective since the temperature of the fridge is still fairly high and the samples were left for a long period of time (one week), hence both samples are likely to have reached new equilibriums during this time thus effecting the results collected. It would have been better if the initial centrifugation of the copper solution was not completed since then both mixtures would have been exposed to the same conditions, hence allowing direct comparison of the ion exchange results.
-Losses in the non-exchanged ions are likely to have occurred for both solutions during the centrifugation process since some metal ions will have remained within the precipitate and in the tube when the supernatant fluid was collected. To reduce this loss distilled water could be added to the centrifuge tube and additional centrifugations performed. This was not completed due to time constraints.
+Losses in the non-exchanged ions are likely to have occurred for both solutions during the centrifugation process since some metal ions will have remained within the precipitate and in the tube when the supernatant fluid was collected. To reduce this loss distilled water could be added to the centrifuge tube and additional centrifugations performed, hence washing the tubes. This was not completed due to time constraints.
+
+Both the copper and zinc solutions were cloudy following the centrifugation indicating that some ZSM-5 remained suspended in the solutions. Further centrifugations wold have reduced the amount of suspended zeolite from the solutions and hence the errors resultant from this (see below). Centrifugation was chosen instead of filtration to separate the zeolite since the nano-size particles of ZSM-5 can block the filter paper during filtrations hence resulting in very long filtration times.~\autocite{russell}
+
+While monomeric species such as \ce{Cu^2+} and \ce{Zn^2+} are likely to be the predominant species present in the ZSM-5 zeolite after the ion exchange process other species such as (\ce{[ZnOH]+} which subsequently form \ce{[ZnOZn]^2+} dimeric bridges upon drying) and \ce{[Cu2(OH)2]^2+} may alternatively be formed.\autocite{almutairi11,schreier05,mhamdi09} The formation of these species allows a 1:1 exchange between the hydrogen and the metal cations thus enabling an ion exchange level greater than that calculated,~\autocite{almutairi11,schreier05} however the exchange of the monomeric species is the preferred thermodynamic product and the other species only form at isolated \ce{Al} centres when using aqueous phase ion exchange as the preparation technique.~\autocite{aleksandrov10,penzien04} It is thus unlikely that a large amount of the dimeric species was present in the products created.
\subsection{Copper Exchanged ZSM-5}
-As seen in section \ref{sec:cu-percent-exchanged} the percentage of copper calculated to have been exchanged with the HZSM-5 was negative. This can be explained by the fact that the solution placed in the spectrophotometer still contained some suspended zeolite (seen by how the solution was slightly cloudy after the centrifugation), hence this increased the absorbance value of the sample over the true value and thus resulting in the negative yield calculated. To reduce the effect of any zeolite remaining suspended in the solution a titrimetric method to calculate the copper ion concentration would have been better such as titrating against an \ce{EDTA} solution using Fast Sulphon F as the indicator.~\autocite{denby-copper-conc} This is also a better method since it allows a more direct comparison between the copper and zinc ion exchange processes since very similar methods are used to determine the cation concentrations which may partially compensate for unforeseen systematic errors.
+As seen in section \ref{sec:cu-percent-exchanged} the calculated exchange level for the \ce{Cu^2+} ions with the HZSM-5 was negative. This can be explained by the presence of the suspended ZSM-5 in solution whichincreased the absorbance value of the sample over the true value thus resulting in the negative yield calculated. To reduce the effect of this suspended zeolite a titrimetric method for calculating the copper ion concentration could have been used for example using \ce{EDTA} solution as the titrant and Fast Sulphon F as the indicator.~\autocite{denby-copper-conc} This would also allow a better comparison between the copper and zinc ion exchange processes since the similar methods could compensate for common systematic errors.
\subsection{Zinc Exchanged ZSM-5}
-From section \ref{sec:zn-percent-exchanged} the percentage of zinc calculated to have been exchanged with the ZSM-5 zeolite was \SI{66 \pm 0.0}{\percent}. This value is much higher than expected since S.A. Yasnik et al. calculated an exchange efficiency of \SI{48}{\percent} for \ce{CuSO4} ZSM-17 following contact for 48 hours at room temperature.\autocite{yashnik05}
+From section \ref{sec:zn-percent-exchanged} the percentage of zinc calculated to have been exchanged with the ZSM-5 zeolite was \SI{66 \pm 3}{\percent}. Tamiyakul et al. completed an ion exchange between HZSM-5 with an \ce{SiO2}/\ce{AlO3} ratio of 30 and \ce{Zn(NO3)2} at \SI{70}{\celsius} for 12 hours and obtained an ion exchange level of $\frac{0.64}{1.5} \times \SI{100}{\percent} = \SI{43}{\percent}$.\autocite{tamiyakul15} The exchange level calculated in this project is expected to be slightly greater than that reported by Tamiyakul et al. due to the lower \ce{SiO2}/\ce{AlO3} ratio of ZSM-5 used, however the $\SI{66}{\percent} - \SI{43}{\percent} = \SI{23}{\percent}$ difference in ion exchange levels is fairly large and at least art of the difference is likely to be due to the systematic errors discussed (see below): all of which result in an ion exchange value which is too great.
+
+The zeolite suspended in the zinc solution resulted in the aliquot volume being too small since the suspended zeolite displaced some of the solution when the aliquot volume was being measured. This thus reduced the titre volume recorded which can be seen to have inflated the ion exchange level calculated (by inspection of equation \ref{eq:zn-percent-exchanged}.
-The zeolite suspended in the zinc solution resulted in the aliquot volume being less than expected since the suspended zeolite displaced some of the solution when the aliquot volume was being measured, hence making it too small. This may explain the anomalous final titre volume (run 4) obtained (see table \ref{tbl:zn-analytical-titration}) since some of the solid zeolite may have settled in the bottom of the volumetric flask, hence for this final titration the pipette contained a greater number of suspended zeolite particles thus reducing the analyte volume and resulting in the anomalously small titre volume.
+This may partially explain the anomalous final titre volume obtained in the titration (see run 4 in table \ref{tbl:zn-analytical-titration}) since some of the solid zeolite may have settled in the bottom of the volumetric flask, hence for this final titration the pipette contained a greater number of suspended zeolite particles thus reducing the analyte volume and resulting in the anomalously small titre volume.
-Further centrifugations could have been completed to reduce the amount of suspended zeolite from the solutions. Centrifugation was used instead of filtration to separate the zeolite since small nano-scale particles ZSM-5 often block the filter paper during filtrations hence resulting in a very slow filtration.~\autocite{russell}
+\subsection{Uncertainties}
+The percentage uncertainty in both of the obtained results is fairly high at \SI{3}{\percent}, although the actual error is greater than this due to the systematic errors discussed. This could be reduced by instead using Diffuse Reflectance Fourier Transform Infrared Spectroscopy (DRIFT-IR)\autocite{pollanen05} on the ZSM-5 samples and obtaining the ion exchange level through comparing the integration of the \SIrange{3570}{3630}{\per\centi\metre} peak between the ion exchanged ZSM-5 samples and the original HZSM-5 sample~\autocite{yu12} instead of using a titrimetric method on the solutions used, thus reducing the number quantities involved in the calculations and hence reducing the number of errors introduced.
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\printbibliography
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howpublished = {personal communication}
}
+
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+}
projects / chemistry / university-chemistry-lab-reports.git / commitdiff