91Ó°ÊÓ

One method for the analysis of \(\mathrm{Fe}^{3+}\), which is used with a variety of sample matrices, is to form the highly colored \(\mathrm{Fe}^{3+}\) -thioglycolic acid complex. The complex absorbs strongly at \(535 \mathrm{nm}\). Standardizing the method is accomplished using external standards. A 10.00 -ppm \(\mathrm{Fe}^{3+}\) working standard is prepared by transferring a 10 -mL aliquot of a 100.0 ppm stock solution of \(\mathrm{Fe}^{3+}\) to a 100 -mL volumetric flask and diluting to volume. Calibration standards of 1.00,2.00,3.00,4.00 , and 5.00 ppm are prepared by transferring appropriate amounts of the 10.0 ppm working solution into separate 50 -mL volumetric flasks, each of which contains \(5 \mathrm{~mL}\) of thioglycolic acid, \(2 \mathrm{~mL}\) of \(20 \% \mathrm{w} / \mathrm{v}\) ammonium citrate, and \(5 \mathrm{~mL}\) of \(0.22 \mathrm{M} \mathrm{NH}_{3}\). After diluting to volume and mixing, the absorbances of the external standards are measured against an appropriate blank. Samples are prepared for analysis by taking a portion known to contain approximately \(0.1 \mathrm{~g}\) of \(\mathrm{Fe}^{3+},\) dissolving it in a minimum amount of \(\mathrm{HNO}_{3}\), and diluting to volume in a \(1-\mathrm{L}\) volumetric flask. A 1.00 -mL aliquot of this solution is transferred to a \(50-\mathrm{mL}\) volumetric flask, along with \(5 \mathrm{~mL}\) of thioglycolic acid, \(2 \mathrm{~mL}\) of \(20 \% \mathrm{w} / \mathrm{v}\) ammonium citrate, and \(5 \mathrm{~mL}\) of \(0.22 \mathrm{M} \mathrm{NH}_{3}\) and diluted to volume. The absorbance of this solution is used to determine the concentration of \(\mathrm{Fe}^{3+}\) in the sample. (a) What is an appropriate blank for this procedure? (b) Ammonium citrate is added to prevent the precipitation of \(\mathrm{Al}^{3+}\). What is the effect on the reported concentration of iron in the sample if there is a trace impurity of \(\mathrm{Fe}^{3+}\) in the ammonium citrate? (c) Why does the procedure specify that the sample contain approximately \(0.1 \mathrm{~g}\) of \(\mathrm{Fe}^{3+}\) ? (d) Unbeknownst to the analyst, the \(100-\mathrm{mL}\) volumetric flask used to prepare the 10.00 ppm working standard of \(\mathrm{Fe}^{3+}\) has a volume that is significantly smaller than \(100.0 \mathrm{~mL}\). What effect will this have on the reported concentration of iron in the sample?

Step by step solution

01

Identify the Appropriate Blank

The blank should contain all the reagents except the analyte of interest (Fe^{3+}). Thus, the blank will consist of thioglycolic acid, ammonium citrate, NH_{3}, and water, but without any Fe^{3+}. The absorbance reading from the blank is used to correct for any absorbance due to the reagents.

02

Determine the Effect of Impurity on Reported Concentration

If there is a trace impurity of Fe^{3+} in the ammonium citrate, this would mean an additional amount of Fe^{3+} is present in the sample. Consequently, the absorbance measured would be higher than it should be, leading to an overestimation of the Fe^{3+} concentration in the sample.

03

Reason for Specified Amount of Sample

The procedure specifies approximately 0.1 g of Fe^{3+} to ensure that the concentration of Fe^{3+} falls within the calibration range of the experiment. This ensures accurate and reliable measurements since the concentrations prepared (1.00-5.00 ppm) suggest a particular expected range for the analysis.

04

Impact of an Incorrect Volumetric Flask on Reported Concentration

If the 100-mL volumetric flask used for the working standard actually has a smaller volume, the resulting concentration of the working solution would be higher than intended. This means that the calibration curve will suggest a higher concentration than actually present, causing the reported Fe^{3+} concentrations in samples to be lower than their true values.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Fe3+ analysis

Analyzing \(\mathrm{Fe}^{3+}\) ions accurately is crucial in various chemical and environmental studies, as this metal is widely present in different matrices and influences both biological and industrial processes. The goal is to measure the concentration of \(\mathrm{Fe}^{3+}\) in a sample with high precision.
In the described procedure, the analysis is based on the formation of a colored complex with thioglycolic acid. Thioglycolic acid reacts with \(\mathrm{Fe}^{3+}\) ions to create a complex that absorbs light at 535 nm, a wavelength measured using spectrophotometry. The intensity of the absorbed light directly correlates to the concentration of \(\mathrm{Fe}^{3+}\) present.
Understanding this relationship is essential because it allows scientists to convert absorbance values into concentrations using a calibration curve derived from external standards. It is vital to ensure that all reagents and apparatus do not introduce impurities that could affect the measurement, such as any trace \(\mathrm{Fe}^{3+}\) as a contaminant in ammonium citrate, which would lead to falsely elevated measurements.

External standards

External standards are pivotal in accurate quantitative analysis, serving as references to determine the concentration of an analyte in unknown samples. In the analysis of \(\mathrm{Fe}^{3+}\), calibration standards ranging from 1.00 to 5.00 ppm are prepared using a working solution. Each standard is treated with reagents like thioglycolic acid and ammonium citrate to mimic the matrix of actual samples.
The absorbance of each standard is measured to create a calibration curve. This curve maps absorbance against known concentrations, allowing researchers to infer concentrations of \(\mathrm{Fe}^{3+}\) in unknown samples from their absorbance. This method is reliable only if the measurement conditions are consistent across standards and samples.
A critical aspect is the preparation of a suitable blank solution. The blank contains all reagents except \(\mathrm{Fe}^{3+}\), ensuring any absorbance is due to reagents alone. This reading is crucial to correct the absorbance readings of the samples.

Volumetric flask calibration

The accuracy of volumetric flask calibration is crucial in quantitative analysis, especially when preparing standard solutions. In the analysis of \(\mathrm{Fe}^{3+}\), preparing a 10.00 ppm working standard involves diluting a known quantity of a stock solution into a 100-mL volumetric flask.
An error in the flask's calibration, such as a smaller actual volume, leads to a higher concentration than intended. This discrepancy results in a calibration curve that inaccurately suggests higher concentrations. Consequently, reported \(\mathrm{Fe}^{3+}\) levels in samples may appear lower than they truly are.
Ensuring precise measurements means verifying the calibration of all volumetric equipment, like flasks, to prevent systematic errors that can affect the overall reliability of the measurement. Consistent labeling and handling procedures further minimize errors in reagent preparation and sample analysis.

Thioglycolic acid complex

Thioglycolic acid plays a crucial role in \(\mathrm{Fe}^{3+}\) analysis by forming a highly colored complex. This complexation is vital as it allows for the spectrophotometric detection of \(\mathrm{Fe}^{3+}\) at 535 nm. The formation of a stable colorimetric complex enhances the sensitivity and specificity of the method, making it reliable for detecting iron in various matrices.
The complex formation involves chemical interactions between thioglycolic acid and \(\mathrm{Fe}^{3+}\) ions, leading to a pronounced color change. This reaction is critical because the resulting complex's color intensity is directly related to the \(\mathrm{Fe}^{3+}\) concentration in a sample.
Besides thioglycolic acid, other reagents like ammonium citrate are added to the solution to maintain a stable pH and prevent potential interference from other ions, such as \(\mathrm{Al}^{3+}\). Understanding these conditions ensures accurate and selective \(\mathrm{Fe}^{3+}\) measurement by minimizing potential confounding factors.