Hydrocarbon autoxidation proceeds via free radical chain reaction mechanism, as shown in Scheme 1. The radical chain reactions of ROOHs lead to oxygenated hydrocarbon derivatives such as alcohols, ketones and carboxylic acid derivatives via radical terminations pathways.
cite{20}Since the reaction of iodine and TPP is stoichiometric and proceeds with the color change, it used for organic hydroperoxide quantification in non-aqueous media as endpoint detection in the iodometric titration.%%%%%%%%%%%%%%%%subsection{kinetics evaluations}During the rate determining experiments, three mixtures were prepared; one containing: 25 mL Abs.EtOH, 0.5 g of solid potassium iodide, and 0.5 mL of acetic acid-sodium acetate buffer. The second was the stock mixture from EB oxidation reaction containing 9.75 wt.
\% of PEHP, its density, and molarity were 0.87 ce{gcm^-^3} and 0.614 M, respectively. The third mixture was 0.08 molar TPP in freshly distilled EB solution containing 0.01 g of antioxidant (2,6-di-t-butyl-p-cresol).Three mixtures were placed in a water bath at a specified temperature.
As soon as the temperature of all three mixtures reached the bath temperature, 1.0 g of the mixture containing PEHP (1.15 ml, $7.06 imes10^{-4}$moles), it was quickly poured on the first mixture while stirring and the stopwatch was turned on precisely at the moment of adding hydroperoxide. After turning on the stopwatch, one mL of TPP solution ($0.
8 imes10^{-4}$ moles) was added to the mixture immediately. Watch the mixture for the first appearance of a yellow color and stopped the stopwatch and the elapsed time was recorded. Then, again,immediately one mL of TPP was added and the time of the appearance of the yellow was written. It continued until the change of color was no longer observed. The only data obtained in these experiments are the time required to observe changes in color after each mL is added TPP solutions. The results are shown in Table 1 for rates, reaction order, and rate constants at various temperatures.In all such experiments, pseudo-first order condition must be maintained to keep KI in excess and working in the acidic environment. The activation energy of the reaction was also determined by repeating the above reaction at different temperatures, extit{e.
g.} $25,^{circ}mathrm{C}$, $45,^{circ}mathrm{C}$, and $55,^{circ}mathrm{C}$, maintaining other reaction parameters constant.he kinetics of the reaction between PEHP and KI in the acidic environment was studied using a known curve fitting method as described hereinafter. The rate law for this reaction should include the concentrations of iodide, hydrogen ion, and hydroperoxide, and can be determined by performing the experiments at the constant temperature in which the concentrations of iodide and peroxide are varied. However, if the concentration of ce{H^+} and ce{I^-} are held constant throughout the experiment, then the reaction becomes pseudo-first order with respect to PEHP: This results in a relatively simple rate law: From data in Table 1, the average reaction rate (moles of hydroperoxide consumed per liter per second) during color change(Since the addition of TPP until the appearance of yellow color), can be calculated by the following equation:Where the change in concentration is $Delta{PEHP}$, and $Delta{t}$ is the time when the solution changes color.We made eight experiments since the PEHP was added until all of the PEHP was consumed.
Accordingly, in each experiment, one mL of TPP was added and after the eighth experiment, no change in color was observed. However, in order to get better results, we used only six middle experiments for rate calculations. The progress of the reaction can readily be followed by noting the time of appearance of iodine, indicated by the appearance yellow color after the addition of a small known volume of TPP solution. The amounts of PEHP reacted, during the time elapsed since the appearance of yellow color, corresponds to the TPP added. Since the reaction rate depends on the concentration of PEHP present in the reaction mixture, time for the reappearance of yellow color will increase with the progress of the reaction.
The sddition of TPP allowed us to accurately measure the reaction rate between peroxide and iodide ion. Suppose a small amount of TPP solution is added to the mixture of peroxide and iodide ions. Iodine is slowly produced from the reaction between peroxide and iodide ions and immediately and as soon as it is produced, iodine reacts with TPP. As long as excess TPP is present in the solution, no free iodine can accumulate in the solution because it instantly turns into a colorless iodine-TPP complexe. After addition of one mL portions of TPP solution, only some amount of hydroperoxide is consumed, so we cannot directly determine how many moles were consumed. But all of the TPP is consumed and iodine begins to form in solution.
Therefore, we just need to calculate the number of TPP moles that we have initially poured into the solution.From the equations of (10) and (12), it was seen that one-mole of ce{I2} is liberated for every mole of PEHP and one-mole TPP reacts with one mole ce{I2}. Since the moles of TPP that were in the mixture and the moles of peroxide consumed is equal, the number of moles remains in the reaction mixture after one mL addition of 0.
08 molar TPP solution, can be calculated by subtracting the initial moles of PEHP from moles of reacting PEHP.We make multiple measurements of the rate in which we vary the initial concentration of PEHP but keep the initial concentration of ce{I^-} constant. The results of initial rates at various temperatures for the rate law of $Rate= kPEHP^n$ are tabulated in Table1.
We made a graph of log(Rate) versus $logPEHP$, then we got the linear relationship of; $log(Rate)=nlogPEHP + log(k)$, with the order of hydroperoxide (n) as slope and the y-intercept was equal to $log (k)$. This graph was appropriate for the case where the iodide concentration is constant. However, in this work, there was no need to find the order of the iodide.
By using the known values of n, the value of the rate constant k can be calculated: We used the given initial concentrations and rate for each experiment and calculate the value of k for each experiment.From data obtained from Table 1, it could be concluded that the order of the reaction of PEHP and KI in acidic environments with an excess of ce{I^-} is: n~=1, then the reaction becomes pseudo-first order with respect to PEHP and the rate law is:Ln( extit{k}) was plotted against the reciprocal of the temperature (1/T) and a straight line was obtained. The slope of the line is the ce{-E_a/R}, and the y-intercept is the natural logarithm of the pre-exponential factor. Experimentally, we chose the same procedures described at the beginning of this section, but in different temperatures. The activation energy values were calculated by measuring the rate constant values at several temperatures. Using data from Table 1 and from the above-mentioned plot in excel software we got the following linear equation (with $R^2=0.9928$):This relatively high activation energy means that it is needed to warm the reaction mixture to have a workable rate through PEHP quantification with any one of the methods described in this work.%%%%%%%%%%%%%%%%subsection{Methods Comparison}A comparison of results for PEHP quantifications with both modified TS and TPP iodometric methods using the various solvents and iodine sources at different reaction time is given in Table 2.
The selected samples as hydroperoxide source were from the reaction mixture of EB air oxidation. This mixture also contains acetophenone and $alpha$-phenyl ethanol as side products.In each case, 0.
10 g sample with a concentration around 10\% by weight was added to the titration flask holding at near boiling temperature ($68,^{circ}mathrm{C}$) for 5, 10, and 15 minutes and the hot solutions were titrated with 0.1 N aqueous TS solution or 0.08 molar TPP in EB solution. At least three-run replications were done for all samples. As reference method, we used the method described by Alcantara extit{et al.},cite{17} using NaI-IPA system (run 7, Table 2).As shown in Table 2, the peroxide value is lower by TS titration rather than by TPP, due to one or more of these factors: PEHP is not an easily reduced hydroperoxide; its incomplete reaction with iodide; possible decomposition of PEHP by side reactions, water retarding effect, and loss of part of liberated iodine owing to absorption.By using the TPP solution as the titrant, the fast reaction of remaining PEHP with TPP would be done at first, and then the reaction of TPP with iodine liberated from the relatively slow reaction of iodide with PEHP will be done.
The susceptibility of TPP titration method to interference by atmospheric oxygen can result in higher values than TS method. Adding tiny amounts of the known antioxidant such as 2,6-di-t-butyl-p-cresol to TPP titrant solution results in preventing the TPP oxidation by air.Iodometric titration methods for the determination of hydroperoxides are simple, rapid, and require only unsophisticated equipment.
In the most established method,cite{11} iodide ion is allowed to be oxidized by ROOHs to iodine; ce{2I^- + 2e^- ->I_2}, iodine is subsequently determined by titration with a standardized TS solution, using starch as an indicator: ce{I2 + 2Na2S2O3 -> 2NaI + Na2S4O6}.In spite of the above-mentioned advantages for volumetric hydroperoxide quantification, there are a large number of inherent disadvantages regarding current iodometric titration, especially for organic hydroperoxides due to one or more of the following factors:-Inadequate lower detection limit, and limited dynamic range,-Requires the use of 99\% isopropyl alcohol (IPA) as the reaction medium,-Need to use a reflux to prepare saturated metal iodide solution in organic solvents,-Need to use a standard solution of TS, which is not so stable against heat, light, and atmospheric oxygen,-Involves a subjective titration endpoint,-The reaction is highly sensitive to oxygen, so accurate and reproducible results require scrupulous oxygen removal,-The titration is highly empirical, so in order to provide reproducible results, many handling issues must be controlled.-Even small changes, such a new technician or different grade of solvent, can cause variations in results, so the method has relatively poor reproducibility.Consequently, it is recommended that this method is primarily used to evaluate relevant samples or determining the evolution of ROOHs in the same samples over time.Ferrous oxidation-xylenol orange (FOX) method,cite{12} that is based on the oxidation of ferrous ce{(Fe^2^+)} to ferric ce{(Fe^3^+)} ions by ROOHs with the subsequent binding of the ce{ Fe^3^+} ion to the ferric-sensitive dye xylenol orange is sensitive (nanomole to micromole levels of ROOHs), inexpensive, and not affected by ambient oxygen concentrations, but samples with more than micromole levels of hydroperoxides must be diluted before analysis.
Hence, it is not appropriate for High-level ROOHs detection also because of its need for colorimetric by ultraviolet spectrophotometry.Spectrophotometric and HPLC hydroperoxide determination using TPP has been done by other authors,cite{6,7,16} in which after hydroperoxide reduction with TPP, the amount of TPPO produced is determined by reverse or normal phase HPLC in combination with an ultraviolet detector by measuring absorption at 220 or 260 nm. However, these longtime spectroscopic methods are not appropriate for industrial process control, which needs rapid methods.In addition, the reaction of ROOHs with TPP at room temperature is sufficiently fast as studied by Hiatt extit{et al.
},cite{22} on reductions of n-butyl, sec-butyl, and tert-butyl hydroperoxides by TPP in ethanol. This rapid, stoichiometric, and irreversible reaction (reaction (1)) has been used extensively in the analysis of autoxidation products.cite{14,15} Measurement of the product alcohol or TPPO allows identification of the original hydroperoxide, and therefore some other workers have chosen to originally quantify the TPPO product by gravimetric or titration,cite{19} but later by the use of gas chromatography,cite{9} HPLC,cite{6} or GC-MS.
cite{7} By using an excess of TPP, the amount of TPP consumed can be measured by the difference, or alternatively, the product TPPO or alcohol can be quantified.Measurement of the TPP consumed by difference requires two analysis and tends to be prone to poor reproducibility at low hydroperoxide levels. However, in the present study, we wish to quantify the PEHP as a class of compounds, not the product alcohol or TPPO.
The use of Abs.EtOH instead of 99\% isopropyl alcohol as the reaction media offers several advantages and gives more accurate results. It is non-toxic and inexpensive solvent for organic substances, including EB oxidation reaction mixture.
The reaction between iodine and TPP is instantaneous in this solvent system and if using excess iodine a blank is unnecessary because there is no oxygen error.Iodine released from the reaction of iodide and hydroperoxide is completely soluble in organic media and form charge transfer complex with TPP in this common organic phase. The titration with TS for organic hydroperoxides was done in double phase (aqueous and organic) and the water retarding effect exists with TS titration.By using a TPP iodometric procedure (explained in this paper), all ingredients are soluble and the reaction media was clear in the same phase; there is not oxygen error and no blank titration is required; the end point is easily visible to within a drop or two of TPP solution and the reaction of iodine with TPP is fast during the titration and the danger of overstepping the end-point is slight. extbf{Reaction Time.
} Most methods to date recommended only 1-5 minute reaction time for the complete reaction between PEHP and iodide.cite{17} However, three separate studies on the PEHP quantification all concluded that 5 minute reaction time was not enough. While organic ROOHs is reduced relatively slow with iodide to produce iodine, the ROOHs react rapidly with the TPP.
As shown in Table 2, in the case of iodometry with TS solution, the reaction completion time was up to 15 min. except for run no. 2 and 10 for which the maximum was at 10 and 5 min., respectively.
This is due to the solubility of NaI / KI and hydroperoxide mixture in Abs.EtOH than in the IPA. So, the time needed to get a complete reaction is less in Abs.EtOH and if using NaI. For titration with TPP solution, the best reaction time for IPA-KI, Abs.EtOH-KI, IPA-NaI, and Abs.
EtOH-NaI systems was 15, 15, 5, and 5 min. respectively. Comparison between runs using KI and runs using NaI shows that the reaction rate in solvent-NaI is more than in solvent-KI systems, due to the more solubility of NaI than KI in these solvents. This result is validated by comparing run 2 and 7 or 5 and 11. Anyway, using any selected iodine source at a solvent incipient boiling temperature could produce sufficient iodide ion in solution for ROOHs reduction.On the other hand, PEHP samples have impurities like acetophenone and $alpha$-phenyl ethanol, which can improve solvent polarity; results in better dissolving iodine sources. Longer reaction time at $65-70,^{circ}mathrm{C}$ results in the removal of liberated iodine by evaporation or absorption.
This was verified experimentally. However, when analyzing materials containing water, where the liberation of iodine is found to be slow, it is better to heat the analytical solution longer to ensure complete reaction before titration.From the experimental results, we concluded that the reaction time of 10 min. in relatively dry condition with Abs.
EtOH-NaI / KI system is the best for PEHP quantification. extbf{Reaction temperature.} The selected reaction temperature was near Abs.
EtOH boiling point of $65-70,^{circ}mathrm{C}$. At this temperature, both the solubility of NaI / KI in the reaction medium and also the reaction rate of PEHP with iodide to liberate iodine are high. By using excess iodine source, it is possible to prevent liberated iodine at this relatively high temperature.cite{12}As verified experimentally, thermal decomposition of PEHP at $65-70,^{circ}mathrm{C}$ during up to 15 min. is almost zero, thus we can do titration at this temperature. extbf{Solvent compositions.
} The choice of solvent for titration depends on the following factors: It should be non-toxic, EB, PEHP, and KI/NaI have good solubility in it, it should be inexpensive and has no reaction with the materials in the titration solution. Acetone, EtOH, and IPA, each react with iodine in the presence of water and acid, so are unsuitable for organic ROOHs assay in aqueous solution. Among other solvents, Abs.EtOH was more appropriate than the 99\% IPA as a reaction medium because it is inexpensive and more environmentally friendly than the IPA. The solubility of reaction ingredients in Abs.EtOH at near boiling temperature was more than IPA.
The IPA can be easily oxidized in the presence of atmospheric oxygen in the air to produce hydrogen peroxide, therefore, the reaction in the IPA solvent requires blank correction at the beginning of the titration. But Abs.EtOH does not easily oxidize in the air. Therefore, Abs.EtOH was our selected solvent. extbf{Iodine source.
} KI and NaI at the selected reaction temperature of $65-70,^{circ}mathrm{C}$ both have sufficient solubility in Abs-EtOH to produce saturated iodine solutions. We used, excess iodide as in the known traditional methods,cite{11} a complex of ce{I3^-}that reacts similarly to free iodine is formed. This strategy keeps the equilibrium; ce{I2 + I^- <->I3^-} far to the right. As a result, since the tri-iodide ion is not volatile, lost of liberated iodine either from boiling the reaction mixture or from purging the reaction flask with a stream of inert gas was prevented and the reaction was not affected by atmospheric oxygen. By using excess iodide source, even though the peroxide-iodide and iodine-TPP reactions tend to consume iodide ions, the excess iodide source immediately provides the iodide to the reaction and the concentration of iodide ion in the solution remains constant at its original value. Powdered solids KI / NaI are convenient to use, as it is sufficiently soluble in hot Abs.EtOH and both prevent bumping and maintain the reaction solution saturated.
extbf{Sample sizes and concentrations.} The sample sizes are based on the approximate PEHP concentration in the samples and were in the range of 0.05 to 0.3 g for 25\% to 0.1\% by weight PEHP in the samples, respectively.
The sample should be diluted when more than 5 to 10 mL of 0.1 N TS or 0.08 molar TPP solutions is consumed for more accurate reading; if less than one mL titrant (TS or TPP solution) is consumed, 0.
01 N TS or 0.008 molar TPP should be used or increase the sample. As shown in Table 3, the sample size has not an important effect in PEHP quantification. Also, as experiments proved, for the EtOH-KI system and titration with TPP solution, dilution has not important effect on PEHP quantification. The samples were diluted with freshly distilled EB and concentrated by vacuum distillation at a temperature below $100,^{circ}mathrm{C}$.textbf{Oxygen and water effect.} Longer reaction time increases the diffusion of oxygen into solutions and results in higher peroxide value because of the slow hydroperoxide reaction with iodide changes to a fast reaction of oxygen with iodide.
In order to avoid possible interference by oxygen, usually, the reaction mixture including samples was de-aerated prior to analysis and was kept under a blanket of dry nitrogen. However, such precaution was unnecessary as it was proved through testing for the methods described in this work. A blank solution after standing for up to two days in an open flask resulted in the liberation of a minute amount of iodine. Therefore, no blank or prior treatment is required in modified and TPP methods described in this work. In the absence of PEHP, the solution remains colorless. After 24 hours, the color becomes light yellow by the reaction with diffused atmospheric oxygen. Thus, the “oxygen error” is eliminated.
As previously reported by Mair extit{et al.},cite{11} the reaction between iodide and peroxides was retarded in the presence of water in acidic solutions. This result is valid for titration with TS solution, which appreciable amounts of water must be present at the titration endpoint in order to avoid over titration due to the slowness of reaction between iodine and TS, especially when the titer is small. However, as shown in Table 5, in the case of PEHP and titration with TPP solution, water has no important effect on the hydroperoxide reduction by iodide. Because the iodine released from the reaction of iodide with peroxide has better solubility in organic than in aqueous solutions. This is the reason that we prefer to use the TPP dissolved in EB as titrant.
Anyway, PEHP determination was done in relatively dry condition. The starch indicator cannot be used in dry conditions. However, by adding water, it is possible to use starch indicator, but this produces a heterogeneous system and was not necessary.%%%%%%%%%%section{Conclusions}A new protocol was developed for quantitative measurement of organic hydroperoxides in organic mixtures.As a case, the analysis of PEHP of EB oxidation reaction mixtures studied in detail. It involves the use of the organic TPP solution instead of the aqueous TS solution in the iodometric titration. The proposed method was effective and simple with good precision.
There was generally good agreement between the iodometric hydroperoxide quantification by TPP and TS solutions as titrant. However, the iodometric assay by TS appeared to have some difficulty in consistently quantifying high and low peroxide levels, whereas the TPP assay measured peroxide concentrations in nearly all concentrations. Compared with the current iodometric assay, the TPP method consistently generated less variable peroxide values, is inexpensive, not sensitive to ambient oxygen levels, and can rapidly generate peroxide measurements. The method is widely adaptable, many types of organic/inorganic peroxides and hydroperoxides, both liquids, and solids provided that they are soluble in Abs.EtOH. No blank is required for this selected solvent and method. The determination is independent of the quantities of alcohol and reagents used; is precise; accurate; and convenient.The rate of the reaction between PEHP and KI in an acidic environment with excess KI was pseudo-first order with respect to PEHP.We have concluded that; the peroxide iodometric determination using the TPP solution as titrant offers a more accurate measurement for non-easily reduced hydroperoxide content than does the traditional iodometric method using TS as titrant.
