B.  Classification (types) of titrations based on their underlying chemical reactions:

1. Acid-base (neutralization) titrations:

These titrations typically rely on pH changes or electron transfer to find the endpoint. Involves the reaction between an acid and a base to produce salt and water. Common indicators include phenolphthalein and methyl orange.

Example of acid-base titration in water quality analysis:

1. Alkalinity: Determined by titrating a water sample with a standard acid such as sulfuric acid to a specific pH endpoint (e.g., pH 8.3 or 4.5) to neutralize hydroxides, carbonates and bicarbonates.
2. Acidity: Determined by titration of water sample with a standard base such as sodium hydroxide to neutralize mineral acids, weak acids, and hydrolyzing salts.

Acid-base titrations require a single-step, wherein the titrant reacts with the analyte. Hence, these are also called direct titrations.

2. Complexometric Titration:

For direct complexometric titration a metallochromic indicator (dye) is added to a buffered solution with unknown concentration of metal ions (analyte), producing a weak colored metal-dye complex. The analyte solution is then titrated with standard EDTA that selectively chelates the metal ions out of the metal-dye complex. In other words, addition of EDTA breaks the weak metal-dye complex to form a stronger metal-EDTA complex. The endpoint is reached when all metal ions are chelated by EDTA and the weak metal-dye complex is exhausted.

Example of direct complexometric titration in water quality analysis:

Hardness by EDTA titration: A wine-red metal-dye complex is formed when eriochrome black T (EBT) is added to a pH≈10 solution, containing calcium and magnesium (alkaline earth metal) ions. This solution is titrated with standard solution of disodium salt of EDTA, which extracts calcium and magnesium from the dye complex and the dye is changed back to its original blue colour. EBT is used to indicate the endpoint for the titration of calcium and magnesium together (IS 3025 Pt21:2009 RA 2023).

Titrant in this case is standard ethylene diamine tetra-acetic acid (EDTA). Analytes in this case are the water hardness cations, which are primarily Ca²⁺, Mg²⁺ and traces of Fe²⁺, Sr²⁺ & Mn²⁺. Eriochrome black T (EBT) is the indicator. An ammonia-ammonium chloride buffer is added to maintain a pH of ≈10, ensuring EBT works correctly. At least some magnesium is required to ensure a sharp color change, as magnesium-EDTA complexes more effectively with the EBTA indicator than calcium. That is why a small quantity (1.2g in 250 ml) of magnesium salt of EDTA is added to the ammonia-ammonium chloride buffer solution.

3. Redox (Oxidation-Reduction) Titration:

Redox (oxidation-reduction) reactions are chemical processes involving the simultaneous transfer of electrons between species, where one substance loses electrons (oxidation) and the other gains electrons (reduction).

Redox titration determines an unknown analyte's concentration by measuring its reaction with a standard solution (titrant) via electron transfer. Based on simultaneous oxidation (loss of e-) and reduction (gain of e-). Equivalence point is reached when stoichiometric amounts react.

A subclass of redox titration is iodometry where an oxidizing agent reacts with excess iodide (I^-) to release iodine (I₂), which is subsequently titrated with a standard reducing agent, usually sodium thiosulfate (Na₂S2O₃), typically using starch as indicator.

Example of direct redox titration in water quality analysis:

Titration for dissolved oxygen by Winkler method is also an example of redox titration as well as back titration.

The first step in this test is to add manganese sulfate (MnSO₄) to the sample followed by alkali-azide-iodide reagent which contains sodium hydroxide (NaOH), sodium iodide (NaI) and sodium azide (NaN₃). MnSO₄ reacts with (NaOH + NaI) to form Mn(OH)₂. As Mn(OH)₂ is unstable it is immediately oxidized by the dissolved oxygen in the sample to manganic hydroxide [MnO(OH)₂ / manganese (III)] and manganese dioxide [MnO₂ / manganese (IV)]. This is the oxidation step of Winkler method, where in manganese is gaining electrons and dissolved oxygen is losing electrons. The reaction effectively fixes the DO in a higher oxidation state (Mn^3+/4+) for later titration. Hence, this step is referred to as ‘fixing of oxygen’ as the dissolved oxygen is bound down in higher oxidation states of manganese (Mn^3+/4+). The manganic hydroxide [MnO(OH)₂ / manganese (III)] and manganese dioxide [MnO₂ / manganese (IV)] precipitate formed in this step are brown in colour.

The iodide ions (I^-) in the alkali-iodide solution remain largely unaffected during the initial oxidation of to higher-valence manganese oxides. The iodide acts as a reducing agent in the next step where the brown precipitate is dissolved by sulfuric acid, liberating iodine (I₂) in direct proportion to the initial DO. The unused portion of the iodide ions (I^-) from the excess alkali-iodide solution remains in the solution as unreacted, soluble ions. They do not participate in the redox reaction that converts manganese oxides to iodine upon acidification, nor are they consumed during the thiosulfate titration, remaining harmlessly dissolved.

Role of the azide (NaN₃) component in alkali-azide-iodide reagent is to remove interference from nitrite ions especially in sewage or highly polluted samples, and is not a part of the main redox reaction.

Reduction step: The brown precipitate, typically MnO(OH)₂ or MnO₂, dissolves in H₂SO₄, acting as an oxidizing agent converts the previously added iodide ions (I^-) to elemental iodine (I₂). This redox reaction reduces manganese from +3/+4 state to the Mn⁺² state, releasing iodine, which is titrated with thiosulfate. The amount of liberated iodine is equivalent to the original dissolved oxygen. The brown precipitate vanishes as MnSO₄ (Mn⁺² state) is colourless. Instead, a yellowish solution would develop due to iodine. This solution is titrated with sodium thiosulfate (Na₂S₂O₃) using starch as indicator. The end-point is pale-blue to colourless.

The Winkler method for dissolved oxygen (DO) is technically considered an indirect or back-titration-like procedure because the oxygen is first trapped (fixed) by introducing an excess of reagents (MnSO₄ & NaOH) to fix the oxygen and produce an equivalent amount of iodine, which is then titrated with sodium thiosulfate, rather than directly titrating the oxygen. The stoichiometry relies on the generated iodine, making it a two-stage indirect redox reaction.

4. Precipitation Titration (Argentometric Methods):

In precipitation titration the titrant reacts with the analyte to form an insoluble, solid precipitate. It is primarily used to determine the concentration of halide ions such as chlorine (Cl^-) and iodine (I^-) or metal ions in a sample by measuring the volume of titrant needed to complete the precipitation.

Example of direct precipitate titration in water quality analysis:

Argentometry (Mohr’s method) for chloride (IS3025Pt32:2025/APHA SM4500-Cl^-) is a precipitation titration technique that measures chloride ion (Cl^-) concentration by reacting it with silver nitrate (AgNO₃) to form an insoluble white precipitate of silver chloride (AgCl).

The titrant in this case is a standard solution of AgNO₃. The water sample for titration turns pale yellow after due to addition of potassium chromate indicator. As the titrant is added, chlorides in the sample react with AgNO₃ to form AgCl, which is a white precipitate). When chlorides are exhausted and precipitation is complete, the titrant (AgNO₃) reacts with the indicator (K₂CrO₄) leading to the appearance of reddish-brown/bright-red silver chromate (Ag₂CrO₄), which is the indicator of titration-endpoint.

C. Summary:

Table 1: Overview of Titration Types, Reactions in Water Quality Testing.

Frequently Asked Questions About Titration:
(Titration FAQs)

How does one reagent measure the quantity of another chemical?

One reagent measures another chemical through stoichiometry, which uses balanced chemical equations to determine precise molar ratios between substances. By reacting a known volume and concentration of a standard solution (titrant) with an unknown sample, the quantity is determined based on when the reaction reaches completion, often shown by a color change or indicator.

What is stoichiometry?

Stoichiometry is the quantitative study of reactants and products in a chemical reaction, determining the exact amounts (mass or volume) needed or produced based on balanced equations. Stoichiometry is based on the law of conservation of mass, which dictates that in a closed system, the total mass of reactants equals the total mass of products—no atoms are created or destroyed, resulting in no, or immeasurably small, loss of mass.

Titration is based on stoichiometry. Stoichiometry is based on the law of conservation of mass in a closed system. But titrations happen in open vessels. How can we be sure that the system is closed?

Titrations work despite being in open vessels because they are liquid-phase, aqueous reactions where the reactants and products remain dissolved in the flask, keeping the reacting species confined. While technically open to air, the negligible volatility of the titrant and analyte means no atoms are lost to the atmosphere, fulfilling the law of conservation of mass.

While the burette and the flask are open to the air, it is a closed system for the reactants, as they do not escape into the environment. The accuracy of a titration depends on the moles of the titrant added and moles of analyte in the solution. Hence, stoichiometry holds even if tiny amounts of water evaporate.

Negligible Loss: Volatile products are rarely formed in standard titration reactions. In acid-base titration for alkalinity, carbonate & bicarbonate species are converted into carbonic acid, which breaks down into acid and CO2. The produced CO2 remains dissolved in the solution for a period of time, though it may eventually escape, particularly if the sample is agitated. Accuracy of alkalinity titration is not affected if the sample is not left standing and endpoint is reached promptly.

Why titrant mass is expressed in moles?

Titrant amount is expressed in moles (or millimoles) because titrations are based on stoichiometric (mole-to-mole) ratios from balanced chemical equations, not mass-to-mass relationships. Moles provide a direct count of reacting particles, allowing precise calculation of analyte concentration regardless of the volume changes that occur during the titration.

When performing titration; Why add sodium hydroxide slowly?

When NaOH is added slowly, the reaction proceeds at a more controlled rate. This allows for better observation of the reaction's progress and a more accurate determination of the endpoint (the point where the indicator changes color, which is an approximation of the equivalence point).

Both Winkler method for DO and Argentometry for Chloride have a precipitate. Why one is Redox and the other Precipitate Titration?

The Winkler method is a redox titration because the endpoint depends on the oxidation-reduction reaction between iodine and thiosulfate, with the precipitate [MnO(OH)₂ / MnO₂] serving as a pre-step. Argentometry is a precipitate titration because the titration itself relies on forming an insoluble salt (AgCl) to determine the concentration.

How to improve granularity and accuracy of titration?

Following are some of the strategies to improve granularity of titration

What is rough titration?

A rough titration is the initial, fast-paced trial run in a titration experiment used to estimate the approximate endpoint. By quickly determine the approximate volume of titrant needed for the reaction, one can save time by quickly adding titrant a little short of the estimated volume of titrant needed and then add titrant drop-by-drop to reach the endpoint. Purpose of rough titration is to figure out approximate volume of titrant required to reach endpoint. This information is used to optimize titration operation. Rough titration results are not to be reported as test result. Instead, rough titrations should be followed by accurate or fine titrations where titrant is added slowly near the expected endpoint.

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