An indicator, Fe(III), is used in this experiment and acts by imparting a red coloration to the solution
with the first slight excess of thiocyanate, as indicated in equation three below:

     The titration must be carried out in an acidic environment to prevent the hydrolysis of the iron (III)
from the indicator.  The titration error in the Volhard Method is small as the indicator is extremely
sensitive to the thiocyanate ions.  A measured excess of standard silver nitrate solution is added to the
sample, and the excess is determined by back titration with a standard thiocyanate solution.
     The error resulting from the Volhard Method is what our group is most concerned with.  By
comparing the solubility products in equations four and five below (Pietrzyk and Frank, 326), one can
easily conclude that silver chloride is more soluble than silver thiocyanate.

As a consequence, equation six below dominates:

This reaction above occurs to a significant extent near the end point in the back-titration of the excess
silver ion.  As a result, low values are determined in the chloride analysis.  Rigorous calculations have
shown that an overconsumption of 1.6 mL or more of excess 0.1 M thiocyanate will occur (Skoog and
West, 227). 


     To avoid this error, three methods may be utilized.  The three most common methods are:  (1)
filtration, followed by the titration of the aliquot of the filtrate, yields excellent results provided that the
precipitated silver chloride is briefly digested; (2) the use of the Caldwell and Moyer modification
which entails coating the silver chloride precipitate with nitrobenzene, substantially removing it from
contact with the solution; (3) the use of the Swift, Arcand, Lutwack, and Meier modification which
requires neither filtration nor the coating agent, but merely increasing the iron (III) ion to a sufficiently
high concentration.  This method development lab will utilize methods two and three above (coating
agent and increasing indicator concentrations). 


     It must be noted that the above modifications are not required for the indirect titrations of Br^- and I^-. Once again, refer to the solubility products of silver bromide and silver iodide in equations seven and
eight below.  Because these silver salts are more insoluble compared to the complex of AgSCN, no error is encountered.

     The most widely used method of the Volhard Method is the use of nitrobenzene as the coating agent around AgCl.  Our interests lie in the use of two other aromatic organic compounds, namely benzene
and aniline.  The structures of the three aromatic compounds follow:

     Now is an appropriate time to talk briefly about each of the aforementioned three molecules.
Nitrobenzene is a stable aromatic compound, due to its derivation from benzene through the reaction of
benzene with the nitronium ion (Fox & Whitesell, 546).  Nitrobenzene possesses the functional group
[-NO₂] (nitro), which tends to be an electron withdrawing group because of the presence of the two
oxygen atoms. 


     Benzene is unusually stable because it contains two electrons more than a multiple of four.  German chemist Erich Huckel provided more insight to the organic chemistry world by formulating his popular
Huckel's Rule, which explains that any planar, cyclic, conjugated system containing (4n + 2) pi (p)
electrons (where n is an integer) experiences unusual aromatic stabilization (Fox & Whitesell, 70).
Benzene and its substituents (e.g. nitrobenzene, aniline) are very water insoluble and are also more
dense than water.  In fact, benzene dissolves in water only to the extent of 0.01%, which makes it form the immiscible layer around the precipitated AgCl.  These factors make it a perfect candidate in coating the precipitate. 


     Aniline is produced by reacting nitrobenzene with strong reducing agents such as tin or zinc in the
presence of acid.  Sodium bicarbonate is then added to neutralize the acid from the first step to produce aniline (Fox & Whitesell, 553).  Aniline possesses the functional group [-NH₂] (amino), which tends to
be an electron donating group onto the benzene ring. 


     The second part of this method development lab deals with the third modification of the Volhard
Method involving increasing the concentration of the indicator (namely Fe³⁺) and performing a back
titration.  This method was formulated by four chemists at the California Institute of Technology at
Pasadena, California.  To determine the amount of iron required in the solution to disregard the
dissolution of the AgCl, rigorous calculations have been proposed (Skoog and West, 228), concluding
that [Fe³⁺] should be approximately 0.72 mole/liter.  Obviously, this is a very concentrated solution
when it consists of the indicator.  The chloride sample that is to be determined is dissolved in 30 mL of
solution, but kept at an acid environment (100 millimoles HNO₃) to avoid, as mentioned above, the
hydrolysis of iron (III).  Excess standard silver nitrate is added to the solution.  To this, 2.2 M ferric
nitrate indicator is added to the solution followed by 1.00 mL of 0.01 M KSCN.  The complex formed
between the indicator and the KSCN (equation 3) is then titrated against standard silver nitrate until the
endpoint is observed (Analytical Chemistry, 308).
 

     To adequately test our hypothesis, the following method is followed to complete part I of this
experiment.  Approximately 9.8 grams of KSCN is dissolved in one liter of water and mixed well.
Three 25 mL samples (to nearest 0.01 mL) of standard silver nitrate are measured into Erlenmeyer flasks and diluted to approximately 100 mL.  To this solution, 5 mL of boiled 6 M HNO₃ is added followed by 5 mL of iron(III) ammonium sulfate indicator.  These solutions are then titrated with the KSCN solution until a red-brown color of the FeSCN²⁺ (equation 3) is stable for one minute.  Standardization of the KSCN yielded a 0.0973 N solution.


     Four unknowns (in the range of 0.25-0.30 grams) are weighed to the nearest 0.1 mg and transferred to 500 mL titration flasks.  One-hundred milliliters of deionized water is added to the titration flask, along with 5 mL of boiled 6 M HNO₃ and an excess of standard silver nitrate (pipet 50 mL).  The solution is then shaken vigorously to coagulate the AgCl precipitate.  Five milliliters of ferric ammonium sulfate indicator is added along with (benzene/aniline) and again shaken vigorously.  These solutions are then titrated with the standard KSCN to an orange/pink colored endpoint which is stable for one minute.
 

     Approximately 250 mL of 3.33 M HNO₃ is prepared and boiled.  Four chloride samples are weighed (in the range of 0.35-0.45 grams) and added to four 30 mL aliquots of the prepared 3.33 M HNO₃. Titrate 0.1 N AgNO₃ until a clear point is reached, and then add 2-3 mL in excess.  Ten milliliters of 2.2 M ferric nitrate is then added to the solution.  (NOTE:  Ferric nitrate plays a much better role as the indicator than ferric ammonium sulfate does because Fe(NO₃)₃ is much more soluble in 6 M HNO₃ than ferric ammonium sulfate.  This solubility results because all nitrates are soluble.  Therefore, a much greater amount (80.80 grams) of Fe(NO₃)₃ can be dissolved in 100 mL of 6 M HNO₃ to yield the required 2.2 M ferric indicator solution).  One milliliter (by pipet) of 0.01 M KSCN is added to the analyte.  The Fe(SCN)²⁺ complex (equation 3) is then notably formed.  Standard silver nitrate is then titrated with the analyte, which is vigorously swirled, until the disappearance of the ferric thiocyanate complex.  This solution is then compared to a prepared blank that has been similarly titrated, but which has an excess of 1-2 drops of AgNO₃ added.  The final volume of this solution is approximately 150 mL.
 

                                      *This sample was spilt before the titration was completed
 
 
 

     These results reinforce our null hypothesis that stated the percentage chloride from using benzene as the coating agent would not result in a significant difference than the same method using nitrobenzene. Implementing the F-test, there is no significant difference between the two methods.

     Experimental difficulty was experienced during the titration of the analyte consisting of the coating
agent aniline.  From our observations taken during lab, it was noted that upon addition of the aniline, the
unknown solutions changed color to that of turquoise.  When titrated with KSCN, the solution
precipitated a light blue powder that was very fine.  The actual solution was a deep green color at the
endpoint; which was extremely difficult and nearly impossible to detect.  If excess KSCN was added,
the solution changed to a brown-green color with the blue precipitate still present. 


     Looking at the statistical summary of this method involving aniline, it is apparent that this method
failed to provide an accurate percentage of chloride.  In fact, two of the values were rejected.  These
values were rejected by using the Q-test and the values 57.85% and 57.42% chloride as the accepted
values in this experiment.  Obviously, only two values in even this range would require all the values to
be outliers (2 values using the Q-test will reject both of them).  Therefore, it can be concluded that the
use of aniline as the coating agent failed and resulted in a significant difference to the same method
using nitrobenzene.
 

                                      ** These samples were treated each with 50 mL AgNO₃ (excess)
                                           when the clear point was reached

     This modification of the Volhard Method involving a concentrated iron (III) indicator yielded
extremely good results.  The group was very surprised that the results turned out the way that they did
because this procedure was very easy to do.  As the iron cyanate complex was formed when adding 1.00 mL of the 0.0104 N KSCN, the silver nitrate was then implemented to perform the back titration.  The endpoint was very obvious.  This method, we conclude, has very good precision.
 

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