Titration is a fundamental technique in quantitative chemical analysis, used to determine the unknown concentration of a substance, known as the analyte, within a solution. This process involves the slow, controlled addition of a reagent, the titrant, whose concentration must be known with extreme accuracy. Before the main experiment can yield reliable results, a preliminary procedure called standardization must be performed on the titrant solution. This initial step ensures that the concentration of the solution used for measurement is precisely established, eliminating a major source of error in the final analysis.
Defining Standardization and Its Purpose
Standardization is the laboratory process of determining the exact concentration, typically expressed in molarity, of a solution that will serve as the titrant. Solutions with approximate concentrations, known as secondary standards, must undergo this process before they can be used for accurate analysis. Many common laboratory reagents, such as sodium hydroxide (NaOH) or hydrochloric acid (HCl), are considered secondary standards because their initial concentration is unreliable due to factors like absorbing moisture or reacting with atmospheric gases.
This instability means that simply weighing the chemical and dissolving it into a known volume of water does not produce a solution of precisely known concentration. Standardization corrects for these inaccuracies by reacting the secondary standard with a compound of known, stable properties. Without this calibration step, any concentration data collected from the subsequent main titration experiment would be unreliable. The standardization reaction ultimately provides a correction factor, which is multiplied by the solution’s nominal concentration to yield its true molarity.
Understanding Primary Standards
The compound used to standardize the secondary solution is called a primary standard, which acts as the ultimate reference material. A primary standard must meet rigorous chemical requirements to ensure its mass and purity accurately represent the moles it contains.
Primary standards must possess several key characteristics:
- High purity, often exceeding 99.9%, ensuring the weighed mass is not contaminated by impurities.
- High chemical stability, meaning it does not decompose or react with components in the air during weighing and storage.
- A high equivalent weight, which minimizes the relative error associated with the balance measurement.
- An exactly known chemical formula.
- Readily solubility in the solvent being used.
Common examples of reliable primary standards in acid-base chemistry include potassium hydrogen phthalate (KHP) for standardizing bases and anhydrous sodium carbonate for standardizing acids.
The Practical Steps of Standardizing a Solution
Standardization begins with accurately weighing the primary standard, which is typically dried and cooled to ensure complete removal of absorbed moisture. A precise mass of the dry standard (e.g., 0.7 to 0.8 grams of KHP) is measured and dissolved in a precisely measured volume of solvent, usually distilled water. This solution is placed in a flask, and an appropriate chemical indicator, such as phenolphthalein, is added to monitor the reaction.
The titrant solution requiring standardization is loaded into a burette, a specialized glass tube that allows for the precise, dropwise delivery of liquid. The titrant is slowly added to the flask containing the primary standard until the reaction reaches the endpoint, signaled by a sudden color change from the indicator. The volume of titrant used is recorded, and the entire process is repeated multiple times to ensure a highly accurate average volume is obtained. Care must be taken in reading the burette volumes, often to the nearest 0.02 mL, to maintain the necessary precision.
Determining the Exact Concentration
Once the titration is complete, a stoichiometric calculation determines the exact molarity of the secondary standard solution. The known mass of the primary standard is converted into moles using its molecular weight. This precise mole quantity is the foundation of the calculation, providing the exact amount of reactant present in the flask.
The balanced chemical equation is used to establish the mole ratio between the primary standard and the titrant. This ratio allows the moles of the primary standard to be converted into the moles of the titrant that reacted. Finally, the standardized molarity is calculated by dividing the determined moles of the titrant by the average volume consumed during the titration trials. This final calculated molarity is the true, accurate concentration that is then used for all subsequent analytical work with the now-standardized solution.