Standardization in analytical chemistry is the process of determining the exact concentration of a solution. This procedure is fundamental for any experiment or analysis requiring highly accurate concentration values. Without standardization, results from subsequent chemical reactions, such as titrations, would be based on an estimated concentration, leading to significant error.
Why Sodium Hydroxide Concentration is Unreliable
Sodium hydroxide (\(\text{NaOH}\)) cannot be used to prepare a solution of accurately known concentration simply by weighing the solid. Solid \(\text{NaOH}\) is inherently unstable and highly hygroscopic, meaning it readily absorbs moisture from the surrounding air. This absorbed water increases the mass of the solid, making the calculated concentration based on the initial mass inaccurate.
Furthermore, \(\text{NaOH}\) reacts easily with carbon dioxide (\(\text{CO}_2\)) present in the atmosphere, forming sodium carbonate (\(\text{Na}_2\text{CO}_3\)). This reaction effectively removes hydroxide ions (\(\text{OH}^-\)) responsible for the solution’s basicity, decreasing the true \(\text{NaOH}\) concentration. Because of these chemical properties, a freshly prepared \(\text{NaOH}\) solution only has an approximate concentration, necessitating standardization.
The Role of the Primary Standard (KHP)
To accurately determine the \(\text{NaOH}\) concentration, a primary standard is used, which is a substance of known purity and stability. A substance must meet several criteria to be considered a good primary standard, including high purity, stability over time, and a high molecular weight to minimize weighing errors. It must also be non-hygroscopic, meaning it does not readily absorb moisture from the air, allowing for accurate mass measurement.
Potassium hydrogen phthalate (\(\text{KHC}_8\text{H}_4\text{O}_4\)), commonly abbreviated as KHP, is the standard substance used for \(\text{NaOH}\) standardization because it meets all these requirements. KHP is a weak, monoprotic acid, meaning it donates a single proton (\(\text{H}^+\)) and reacts with \(\text{NaOH}\) in a straightforward one-to-one molar ratio. This simple stoichiometry greatly simplifies the concentration calculations following the titration.
Performing the Titration Procedure
The first step in the procedure involves accurately preparing the primary standard by weighing a specific mass of dried \(\text{KHP}\) solid, typically between 0.7 and 0.9 grams, to the highest possible precision. This precisely measured mass of \(\text{KHP}\) is then transferred into an Erlenmeyer flask and dissolved in distilled water. A few drops of an acid-base indicator, such as phenolphthalein, are added to the \(\text{KHP}\) solution in the flask.
The \(\text{NaOH}\) solution of unknown concentration is placed in a clean burette, which is a piece of glassware designed to deliver variable, precisely measured volumes of liquid. The burette must first be rinsed with the \(\text{NaOH}\) solution itself to condition the walls and prevent dilution by residual water. The initial volume of the \(\text{NaOH}\) solution in the burette is recorded before beginning the titration.
Titration involves slowly adding the \(\text{NaOH}\) from the burette into the \(\text{KHP}\) solution while continuously swirling the flask to ensure thorough mixing. As the equivalence point of the reaction approaches, the base should be added dropwise. The endpoint is reached when the entire solution in the flask turns a faint, pale pink color that persists for at least 30 seconds, signaling that all the \(\text{KHP}\) has been neutralized.
The final volume reading on the burette is recorded with high accuracy. The difference between the initial and final readings represents the exact volume of \(\text{NaOH}\) solution required to neutralize the known mass of KHP. This volume is the key data collected during the procedure.
Determining the Final Molarity
The true molarity of the \(\text{NaOH}\) solution is determined through a sequence of calculations beginning with the mass of \(\text{KHP}\) used. The number of moles of \(\text{KHP}\) is calculated by dividing the measured mass (in grams) by its molar mass, which is 204.22 grams per mole. Because the reaction between \(\text{KHP}\) and \(\text{NaOH}\) is one-to-one, the moles of \(\text{KHP}\) directly correspond to the moles of \(\text{NaOH}\) that reacted.
The molarity of the \(\text{NaOH}\) solution is then found by dividing the calculated moles of \(\text{NaOH}\) by the volume consumed during the titration, which must be converted from milliliters to liters. To ensure accuracy, the entire titration procedure should be performed multiple times, typically three to five replicates. The final standardized molarity is determined by calculating the average of the molarities found from these trials.