Application of Neutralization Titrations

Reagents for Neutralization Titrations• Neutralization Titrations – acid/base titrations• Strong acids and bases are used – Reason: sharper endpoints (more pronounced)

• Preparation of standard acid solutions– Prepared by diluting an approximate volume of

conc reagent then standardized – Less frequently: density measurements (Available

data: conc. vs density)

• HCl– widely used for titration of bases– Stable indefinitely– Do not cause troublesome pptn rxns with most cations– Reported that 0.1 M HCl soln can be boiled for 1 hr with

loss of acid; 0.5 M for 10 minutes– Stock solution of HCl

• Prepared by dilution of a quantity of conc acid with a known volume of water then distilled

• Final quarter of distillate is clled the constant boiling HCl• Has fixed and known composition,, dependent only on

pressure• For P of 670-780 torr:

• Stds solns are then prepared by diluting weighed this acid to known volumes

• HClO4 and H2SO4 are stable and used for titration when Cl– interferes by forming pptes

• HNO3 is seldom used because of its oxidizing properties

Standardization of acids• Sodium Carbonate– Frequently used – PS grade is available commercially– Can be obtained by heating to 270–300oC for 1 hr– Has two endpoints• 1st: at pH 8.3 - to • 2nd: at pH 3.8 - to • Boiled to eliminate carbonic acid to destroy buffering effect• Titration is resumed after cooling yielding to sharper endpoints

2𝐻+¿

1𝑁𝑎2𝐶𝑂3¿

– Tris-(hydroxymethyl)aminomethane (TRIS or THAM)• Rxn:

• Greater mass/mole of proton

– Na2B4O7▪H2O • Rxn

1𝐻+¿

1𝑇𝑅𝐼𝑆 /𝑇𝐻𝐴𝑀 ¿

2 𝐻+¿

1𝐵4𝑂72− ¿

• Prepation of Std Solns of Base– NaOH is the most common base– KOH or Ba(OH)2 is also used– None has PS purity, so standardization is reqd

• Effect of CO2 on Basic Solutions– Bases rapidly reacts with atmospheric solutions to

produce carbonates:

– When using a acid-range indicator (eg bcg or mo), no error occurs during analyses since

– But indicators used are usually are base-range, resulting to carbonate error:

– To eliminate: • use same indicator in analysis and standardization, though

less sharp endpoints• Steps are also taken to remove carbonate ions b4

standardization• For conc base solutions, Na2CO3 has low solubility; solids

are removed by decantation or vacuum filtration• Water must be free of CO2, should be boiled then cooled

(hot alkali soln absorb CO2 rapidly)• Deionized water is safer to use• Use of tightly capped PE bottle (short term only)• Do not use glass bottles (loss of 0.1 to 0.3%/wk, freezing)

• Standardization of Bases– Potassium Hydrogen Phthalate (KHP, KHC8H4O4, [204.2])• An ideal PS, can be used without purification

– Benzoic Acid (C6H5COOH)• Can be obtained in high purity• Has limited solubility on water• Dissolved in ethanol before dilution in water

– Potassium hydrogen iodate• High MW/mol H+

• A strong acid• Can be titrated with any indicator with a transition range

b/n a pH of 4-10

1𝑂𝐻−

1𝐾𝐻𝑃

1𝑂𝐻−

1𝐵𝑒𝑛𝑧𝑜𝑖𝑐 𝐴𝑐𝑖𝑑

1𝑂𝐻−

1𝐾𝐻 ( 𝐼𝑂 3 )

Application • Elemental analysis – Kjeldahl Method – determination of organic

nitrogen• Sample is decomposed in hot sulfuric acid (1 hr), and

ammonia produced is condensed and titrated with acid• Converts amino and amide groups to NH3, but not nito,

azo or azoxy groups and certain aromatic heterocyclic compounds eg pyridine• %protein = f*%N; f=6.25 for meats, 6.38 for dairy, and

5.70 for cereal

– Sulfur• Found in organic and biological materials• Sample is burned in a stream of oxygen (SO2, SO3)• Collected in dilute H2O2:

• H2SO4 is then ttd with std base

– Other non-metallic elements determination is listed in Table 16-1

Determination of Inorganic Substances• Ammonium Salts– Kjeldahl method can be used

• Nitrates and Nitrites– Reduced first to NH4

+ by Devarda’s (50%Cu, 45%Al, 5%Zn) or Arnd’s (60%Cu, 40%Mg) alloy

– Then proceed Kjeldahl method • Carbonate and Carbonate Mixture– Compatible combinations of NaOH, NaHCO3, Na2CO3

– No more than two of these three constituents can exist in appreciable amount in solution because rxn eliminates the third

– NaOH + NaHCO3 → Na2CO3 +H2O• If NaOH is used up, NaHCO3 and Na2CO3 are present

• If NaHCO3 is used up, NaOH and Na2CO3 are present

• if equimolar are used, only and Na2CO3 is present

– Using different indicators (acid- or base-range)

Titration Curves of Compatible Mixtures

• 1st endpoint:

• 2nd endpoint: (2 sources)

• Organic Functional Groups– Carboxylic and sulfonic acids – readily titratable

with base using base-range indicator – Amines – titrated with strong bases for aliphatic;

for aromatic and cyclic amines are tirated in non-aqueous ie anhydrous acetic acid to enhance basisity

– Esters - treated with excess base (saponification), then backtitrated with std acid

– Alcohols– can be determined by esterification (rxn of acetic or phthalic anhydrides with ROH in pyridine); excess anhydride is hydrolize to give acetic acid; acetic acid is titrated with alcoholic NaOH/KOH, then compared to blank

• Carbonyl compounds– Aldehydes and ketones– Reacted with hydroxylamine hydrochloride:

– Liberated HCl is titrated with a base

Problems