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Topic: Acid Base Neutralization  (Read 2910 times)

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Offline deezy

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Acid Base Neutralization
« on: May 03, 2018, 06:02:09 PM »
I am attempting to make 2 separate clean liquid solutions of citric acid fully chelating Cu 2+ and Mn 2+ while at the same time neutralizing (reaching a pH ~7.5-9) with Aqua Ammonia.  I have formulas for the molar ratios below which should remain very clean and stable, however in trialing these they continue to precipitate.  Any help would be appreciated as I’m still learning (currently studying Chemistry: The Central Science textbook).  I thought I would post here to learn from your real world experience and insights.  Thanks in Advance.

Products and Amounts: A note - the Cu and Mn amounts are what I need, please keep that in mind so any recommended adjustments should not affect these amounts.

Copper Solution:
•   Water – 180.98 g / 10.05 mol
•   Citric Acid Anhydrous 99% AI – 68.95 g / 0.356 mol
•   Copper Sulfate Pentahydrate – 91.63 g / 0.367 mol
•   Aqua Ammonia 23.5% AI – 112.04 g / 3.197 mol of 23.5% Aqua Ammonia Solution / 0.751 mol of Ammonia (if my math is right)
•   Ending pH is stated to be 7.5 in the formula

Manganese Solution:
•   Water – 78.93 g / 4.385 mol
•   Citric Acid Anhydrous 99% AI – 87.99 g / 0.453 mol
•   Manganese Sulfate Monohydrate – 71.21 g / 0.319 mol
•   Aqua Ammonia 23.5% AI – 215.45 g / 6.149 mol of 23.5% Aqua Ammonia Solution / 0.1445 mol of Ammonia (if my math is right)
•   Ending pH is stated to be 8.9 in the formula

Questions:
1.   Do these molar ratios look correct or realistic to achieve what I’m after?

2.   What is the correct classification for this reaction where you have neutralization occurring as well as chelation?  Would it be an acid base neutralization?  If so, it should produce a soluble
        salt (Copper Citrate & Manganese Citrate) and water correct?

3.   Is it possible to construct a balanced molecular or ionic equation for these?

4.   Citric Questions:

a.   I understand this is a weak acid and can be expressed as H3C6H5O7 to represent that it can depronate one H+ from each of its carboxyl groups.  I also understand each of these have a
        different dissociation constant and can or will happen in a step wise fashion. 

b.   What do the dissociation constant values represent?  Is it an amount of energy that is required?  Is it only under certain conditions where each H+ will depronate such as pH value?

c.   Since Cu and Mn are 2+ I assume the goal would be to get the citric acid to depronate two hydrogens so that it could from the chelate ring around the metal ion.  Given the molar ratios
        above does this seem likely to happen?

d.   Is there a condition where citric acid would fully depronate all 3 H+ and if so is that necessarily required to fully chelate a divalent metal cation such as Cu 2+ or Mn 2+ while having a
        neutral to slightly basic pH?

e.   I am assuming the precipitate is copper hydroxide (light blue in color) and manganese hydroxide (white in color).  Would that mean the citric chelation is not happening to some degree
        and allowing the hydroxide ions to form with the metals?

5.   Aqua Ammonia Questions:

a.   I understand this is commonly referred to as Ammonium Hydroxide NH4OH, my textbook states Aqua Ammonia is a weak base and only about 1% of NH3 will react in water to form NH4+
        and OH-.  Is this correct?  I’m assuming this the case when NH3 is only present in water?

b.   When reacting Aqua Ammonia with an acid in aqueous solution does the % of NH4+ and NH3 shift?  For example below pH of 7 will NH4+ and OH- increase due to the presence of more
        H+ ions?  And vice versa, above a pH of 7 will the equilibrium shift towards increased NH3 and H2O due to the reduction in H+ ions?

c.   What is the role of Aqua Ammonia in a mixture like this?  In part it is neutralizing the acid so does that mean there is sufficient OH- reacting with the H+ from citric acid to form water and
        thus increase pH?  I also understand NH3 can act as a common ligand with the lone pair of electrons on the N.  Is it likely the NH3 would be complexing any portion of the metals or
        affecting the citric molecule?  The reason I ask is because I’ve been told the key to these types of mixtures is to chelate the metal with citric acid first and then raise the pH with the
        Ammonia which helps to strengthen the citric chelate.  Trying to understand why the raising of pH with Ammonia would help strengthen the citric chelate.



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Re: Acid Base Neutralization
« Reply #1 on: May 04, 2018, 03:13:14 AM »
This is long, I will try to address the most obvious things.

Quote
1.   Do these molar ratios look correct or realistic to achieve what I’m after?

Hard to tell without spending few hours calculating the exact equilibrium, but just skimming: look at the amounts of copper and citric acid in the first solution, there is no way to fully chelate copper with citric acid. Doesn't mean remaining copper won't be complexed by ammonia.

Quote
2.   What is the correct classification for this reaction where you have neutralization occurring as well as chelation?  Would it be an acid base neutralization?  If so, it should produce a soluble salt (Copper Citrate & Manganese Citrate) and water correct?

It is not a single reaction, so there is no simple classification.

Quote
3.   Is it possible to construct a balanced molecular or ionic equation for these?

For each reaction separately.

Quote
4.   Citric Questions:

a.   I understand this is a weak acid and can be expressed as H3C6H5O7 to represent that it can depronate one H+ from each of its carboxyl groups.  I also understand each of these have a
        different dissociation constant and can or will happen in a step wise fashion. 

b.   What do the dissociation constant values represent?  Is it an amount of energy that is required?  Is it only under certain conditions where each H+ will depronate such as pH value?

Please check the dissociation constant definition, I have no problems with explaining details once you get confused about something, but this subject is covered in every general chemistry textbook and I don't feel like writing one now.

Please look for equilibrium calculations, these are taught as either part of GenChem, Analytical or Physical chemistry. You have a system with several competing reactions, each of them has its own equilibrium constant, finding what is really happening in the solution can be challenging.
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info

Offline deezy

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Re: Acid Base Neutralization
« Reply #2 on: May 04, 2018, 05:11:15 PM »
Thanks for the feedback, appreciate it.

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