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Chapter 7

Let the Titrations Begin

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Terms and Concepts

What is Titration Addition of reagent in increments

until a reaction is complete. A base being added to an acid until the

acid is fully neutralized. NaOH + HCl = H2O + Na+ + Cl-

Na+ + OH- + H+ + Cl- =

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Titration

Titrate

 trans. To ascertain the amount of a constituent in (a mixture, or (less usually) a compound) by volumetric analysis; i.e. by adding to a solution thereof of known proportion, a suitable reagent of known strength, until a point is reached at which reaction occurs or ceases.

OED

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Titration

Citations of the term 1870 G. E. DAVIS in Eng. Mech. 4 Mar. 605/2 In titrating iron

solutions, the ferrocyanide is not used. 1872 WATTS Dict. Chem. VI. 154 It is easy by means of the latter to titrate sulphuric, oxalic, or any other acid with perfect certainty. 1899 J. CAGNEY Jaksch's Clin. Diagn. i. (ed. 4) 4 Tauszk weighs the blood used, and titrates

with tropæolin or litmus.

OED

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Titrations Which spring from a French word.

Titre or Titer

1. The fineness of gold or silver; in Chem. the strength of a solution as determined by titration; in Med., the concentration of an antibody, as measured by the extent to which it can be diluted before ceasing to give a positive reaction with antigen.

1839 URE Dict. Arts 858 The French rule for finding the par of a foreign gold coin,..is to multiply its weight by its standard or titre. 1868 Chem. News 13 Mar. 132/1 Provided the ‘tinctorial’ power, and consequently ‘titre’ of the ammonia standard be correctly ascertained in terms of the iodine solution, the former may be dispensed with. 1903 Amer. Chem. Jrnl. Mar. 188 The solution was kept cooled to 15°. One cc. was removed at intervals (5 cc. in all), and the ‘immediate’ titer was found to gradually decrease from its original value of 6·4 to 1·2 in about one-half hour, the total active oxygen content..remaining the same. 1918 [see PEPTIDASE]. 1947 Ann. Rev. Microbiol. I. 337 If soluble antigen has been released it may be detected in relatively high titer in the supernatant fluid. 1958 Spectator 19 Sept. 379/1 The shot I'm going to give you..ought to moderate the symptoms until you've developed a high anti-body titer of your own. 1979 Jrnl. R. Soc. Arts CXXVII. 421/1 As the anti-body titres waned they would go through a succession of mid-gestation abortions. 

OED

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Titration

Devices

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Titration Wide variety of reactions can be set up

to work with titration Acid Base Precipitation

Ag+ + Cl- = AgCl(s) Complex formtion

EDTA4- + Ca2+ = Ca(EDTA)2-

Redox 2 MnO4- + 5 H2C2O4 + 6 H+ = 2 Mn2+ + 10 CO2 +

8 H2O

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Titration Equivalence Point

The point were all the reactants are used up to make product. No excess titrant.

End Point The point were you sense that the

above has been reached. You need a small excess of titrant to change the color of solution.

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Titration

Indicators Acid Base

Phenolphthalein Methyl Red Many, many more.

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Structure of Phenolphthalein

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Titration

This difference in the equivalence point and the end point is call the - Titration error and can be

determined by doing a blank titration.

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Titrations

Since you are comparing moles of titrant to moles of analyte then you must know the moles of titrant very accurately.

This is where standards come in.

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Titration

A standard is a solution, solid or some material that we know its concentration or purity very accurately.

We can pay someone to provide this for us. For example, in the vinegar lab we used the “cube”. This was made up by Fisher and they gave us the value.

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Titration

This might not work depending what your lab regulatory environment is.

You might need to check to see if Fisher has given you the proper concentration.

They make errors!!! This would be a raw materials quality

control issue.

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Titration

We will need often to go back to a primary standard.

For Acid Base titrations this is KHP KHP is not Potassium Hydrogen

Phosphorus.

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Titration

KHP Potassium Hydrogen Phthalate

Or Potassium Biphthalate or other possible names.

It is the mono-potassium salt of phthalic acid

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Titration

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Titration

What are characteristics of a primary standard. 99.9 pure or better (but known

exactly) Not decompose on standing Not decompose on drying High equivalent weight

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Titration

Once you have prepared your solution you now have your standard. This concentration will be used in your determination.

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Titration

Vinegar Lab We made up a solution of NaOH.

NaOH is not available as a primary standard since the material is sold as solid hemispherical pellets that absorb water and CO2

So we prepare an approximate concentration and standardize this solution with the acid from the “cube”.

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Titration

We depend on the supplier to give us the proper assay. For us it has been 0.0998M, 0.1000M and 0.1003M

We are fairly safe with this since it is hard to introduce contamination to the “cube”.

What was the expiration date???

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Titration

Issues such as expiration date and supplied concentration are issues in GLP.

We then determined the assay of the NaOH and used this value to determine the percent acid in the vinegar.

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Titration We did a direct titration. Added base

until the endpoint was reached. We also could have done a back

titration. This is where you add excess titrant and titrate the excess and determine the number of moles of titrant used by the initial analyte.

This is the basis of a Kjeldahl titration

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Titration The Kjeldahl is used to determine

% nitrogen (protein content). Important in the feed industry. Protein digested to NH3

NH3 then bubbled into excess HCl (aq) The amount of excess HCl is

determined by titration with NaOH. Amt (NH3) = Amt HCl (start) – Amt HCl

(left over)

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Titration

We can, and often do, carry out titration without indicators. We can follow a titration with an instrumental response.

Spectrophotometer. (Spec 20) Potentiometric Titration (pH or

other electrode)

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Titration

Spectrometric Example. HST titrated with Ferric nitrilotriacetate

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Titration

Precipitation Titration It is common to titrate halides with

silver to form the precipitate. Monitor this titration with a silver

electrode.

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Titration

Electrode will respond to the potential of the silver ion in solution. pAg

pAg = -log[Ag+]

pCl = -log[Cl-]

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Titration

We monitor the pAg as we add silver ion. We will see the classical titration presentation.

We can calculate this curve based on our understanding of chemistry.

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Titration

We can calculate the amount of silver ion from three separate regions of the curve.

Before EP (Silver limited by common ion) At EP (Only Silver in solution due to Ksp) After EP (Silver just added to water)

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Titration

Since Ksp of the three halides are significant different then we can use this procedure to determine mixtures of halide ions in solution.

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Titration We can also use indicators for silver

titrations. Also referred to as Argentometric titrations.

Endpoint methods. Volhard – soluble colored complex at end

point Fajans – adsorption of colored indicator on

the precipitate at the end point

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Titration

Volhard Ag+ + Cl- = AgCl(s) Remove precipitate Back titrate excess silver with

thiocyanate Ag+ + SCN- = AgSCN(s) Fe3+ + SCN- = FeSCN2+ (red)

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Titration

Fajans Adsorption indicator is used and when

the end point is reached the indicator attaches to the precipitate and changes color.

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Titration

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Titration

Do the problem on my home page.

It will be an excellent review of Ksp