STANDARDIZATION AND TITRATION CURVES

volumetric standards (4 criterias)

the ideal standard solution for a titrimetric method will:

• be sufficiently STABLE so that it is necessary to determine its concentration only once

• react RAPIDLY with the analyte so that the time required between additions of reagent is minimized

• react more or less COMPLETELY with the analyte so that satisfactory endpoints are realized

• undergo a SELECTIVE reaction with the analyte that can be described by a balanced equation

Should concentrations of volumetric standard solutions be ascertained?

YES

Standardization

• The concentration of a volumetric solution is determined by titrating it against a carefully measured quantity of

  • a primary standard

  • secondary standard

  • an exactly known volume of another standard solution

Primary Standards

highly purified compound that serves as a reference material in titrations and in other analytical methods

6 Characteristics of Primary Standards

1. high purity. established methods for confirming purity should be available

2. atmospheric stability (does not react easily when kept in its pure form; low reactivity; does not react easily with atmospheric oxygen)

3. absence of hydrate water so that the composition of the solid does not change with variations in humidity

4. modest cost

5. reasonable solubility in the titration medium

6. reasonably large molar mass so that the relative error associated with weighing the standard is minimized

In older editions of the USP, sodium carbonate is used as the primary standard instead of tromethamine. What advantages does tromethamine have over sodium carbonate?

Tromethamine as Primary Standard (THAM or TRIS): 121.4 g/mol

Sodium Carbonate: 105.99 g/mol

• higher molecular weight (sodium carbonate has a lower equivalent weight)

• batter atmospheric stability (sodium carbonate is hygroscopic)

Secondary Standards

compound whose purity has been determined by chemical analysis. the secondary standard serves as the working standard material for titrations and for many other analyses

how are weaker or stronger volumetric solutions prepared

by using the official procedure with proportionate amounts

what glassware is used for volumetric solutions

• are always prepared with volumetric glassware for volume measurement and a suitable analytical balance for weighing

  • They are precisely calibrated glassware used to measure and transfer specific volumes

Volumetric solutions are to be prepared at the standard temperature of?

25 degrees Celcius, if carried out at a markedly different temperature, standardize the volumetric solution used as a the titrant at that different temperature or make a suitable temperature correction

Empirical Concentration

• refers to the actual, experimentally determined concentration of a solution, which may differ from its theoretical or nominal concentration due to factors like impurities, reaction side effects, or environmental influences

Stnadardization of Volumetric Solution

the volumetric solution’s concetration is STILL unknown

Hydrate compounds

contain variable water content which changes over time due to absorption or loss of moisture

anhydrous substances

ensure that the measured mass corresponds exactly to the number of moles of the substance

drying the standard is often necessary for some primary standards True or False

True

A higher molecular weight means less substance is needed per mole, making errors less significant true or false

False ; more substance is needed

example of secondary standards

• NaOH is one example of a secondary standard. Commercially available NaOH contains impurities of NaCl, Na 2 CO3 , and Na 2 SO4 , and readily absorbs H2O from the atmosphere. To determine the concentration of NaOH in a solution, we titrate it against a primary standard weak acid, such as potassium hydrogen phthalate

• acetic, hydrochloric, and sulfuric acids may be standardized against a sodium hydroxide solution that recently has been standardized against a certified primary standard

Titer value

mass of a substance (in grams) that reacts with 1 mL of a standard
solution during a titration. It helps determine the exact concentration of an analyte in solution.

Normality factor

titration curve

plot of some function of the analyte or titrant
concentration on the y axis versus titrant volume on the x axis.

Two types of titration curve

1. sigmoidal titration curve

2. line segment curve

sigmoidal curve

A sigmoidal curve in which the p-function of analyte (or sometimes the titrant) is plotted as a function of titrant volume

linear segment curve vertical axis

instrument signal
that is proportional to the concentration of the analyte or titrant.

advantages of sigmoidal titration curve

speed and convenience

Linear Segment Curve

The linear segment type is advantageous for reactions that are complete only in the presence of a considerable excess of the reagent or analyte

In a sigmoidal titration curve, where is the equivalence point?

• At the inflection points:

  • point on a curve
    where the concavity
    changes

  • highest slope

  • the point in the curve
    where the second
    derivative changes
    sign.

drawing titration curve

check ppt