91Ó°ÊÓ

Bonert and Pohl reported results for the atomic absorption analysis of several metals in the caustic suspensions produced during the manufacture of soda by the ammonia-soda process. \(^{31}\) (a) The concentration of Cu is determined by acidifying a \(200.0-\mathrm{mL}\) sample of the caustic solution with \(20 \mathrm{~mL}\) of concentrated \(\mathrm{HNO}_{3}\), adding \(1 \mathrm{~mL}\) of \(27 \% \mathrm{w} / \mathrm{v} \mathrm{H}_{2} \mathrm{O}_{2},\) and boiling for \(30 \mathrm{~min} .\) The resulting solution is diluted to \(500 \mathrm{~mL}\) in a volumetric flask, filtered, and analyzed by flame atomic absorption using matrix matched standards. The results for a typical analysis are shown in the following table. $$ \begin{array}{ccc} \text { solution } & \mathrm{mg} \mathrm{Cu} / \mathrm{L} & \text { absorbance } \\ \hline \text { blank } & 0.000 & 0.007 \\ \text { standard } 1 & 0.200 & 0.014 \\ \text { standard } 2 & 0.500 & 0.036 \\ \text { standard } 3 & 1.000 & 0.072 \\ \text { standard } 4 & 2.000 & 0.146 \\ \text { sample } & & 0.027 \end{array} $$ Determine the concentration of \(\mathrm{Cu}\) in the caustic suspension. (b) The determination of \(\mathrm{Cr}\) is accomplished by acidifying a \(200.0-\mathrm{mL}\) sample of the caustic solution with \(20 \mathrm{~mL}\) of concentrated \(\mathrm{HNO}_{3}\), adding \(0.2 \mathrm{~g}\) of \(\mathrm{Na}_{2} \mathrm{SO}_{3}\) and boiling for \(30 \mathrm{~min}\). The Cr is isolated from the sample by adding \(20 \mathrm{~mL}\) of \(\mathrm{NH}_{3}\), producing a precipitate that includes the chromium as well as other oxides. The precipitate is isolated by filtration, washed, and transferred to a beaker. After acidifying with \(10 \mathrm{~mL}\) of \(\mathrm{HNO}_{3}\), the solution is evaporated to dryness. The residue is redissolved in a combination of \(\mathrm{HNO}_{3}\) and \(\mathrm{HCl}\) and evaporated to dryness. Finally, the residue is dissolved in \(5 \mathrm{~mL}\) of \(\mathrm{HCl}\), filtered, diluted to volume in a 50 -mL volumetric flask, and analyzed by atomic absorption using the method of standard additions. The atomic absorption results are summarized in the following table. $$ \begin{array}{lcc} {\text { sample }} & \mathrm{mg} \mathrm{Cr}_{\text {added }} / \mathrm{L} & \text { absorbance } \\ \hline \text { blank } & & 0.001 \\ \text { sample } & & 0.045 \\ \text { standard addition } 1 & 0.200 & 0.083 \\ \text { standard addition } 2 & 0.500 & 0.118 \\ \text { standard addition } 3 & 1.000 & 0.192 \end{array} $$ Report the concentration of \(\mathrm{Cr}\) in the caustic suspension.

Step by step solution

01

Analyze Cu Data

First, we need to establish the relationship between absorbance and concentration for copper using the provided standards. The absorbance values for standards are as follows: (1) 0.014 for 0.200 mg/L, (2) 0.036 for 0.500 mg/L, (3) 0.072 for 1.000 mg/L, and (4) 0.146 for 2.000 mg/L. Plot these absorbance values against the concentration to find the calibration curve and use linear regression to find the equation of the line.

02

Determine Calibration Equation for Cu

Perform linear regression on the data points: (0.200, 0.014), (0.500, 0.036), (1.000, 0.072), (2.000, 0.146). This gives the equation: \[ y = 0.0725x - 0.0005 \]where \( y \) is absorbance and \( x \) is the concentration in mg/L.

03

Calculate Concentration of Sample Cu

Now, use the equation from Step 2. The sample's absorbance is 0.027. Substitute this value into the equation to solve for x:\[ 0.027 = 0.0725x - 0.0005 \0.027 + 0.0005 = 0.0725x \0.0275 = 0.0725x \x \approx 0.379 \text{ mg/L} \]This is the concentration of Cu in the diluted solution. Multiply by the dilution factor \( \frac{500}{200} \) to find original concentration:\[ 0.379 \times \frac{500}{200} = 0.9475 \text{ mg/L} \]

04

Analyze Cr Data

For Cr, we use standard additions. Following provided data, we have a blank absorbance of 0.001 and sample absorbance of 0.045. The absorbance of solutions with added Cr are: 0.083 for 0.200 mg/L, 0.118 for 0.500 mg/L, 0.192 for 1.000 mg/L. Plot these points to construct the graph of absorbance versus concentration of added Cr.

05

Calculate Slope for Cr Standard Additions

Using the standard addition data points and the equation: absorbance ≈ slope * mg Cr added + intercept. Using the method of calculation by plotting or by regression, the slope is found to be about 0.180.

06

Calculate Original Concentration of Cr

The original concentration of Cr can be calculated by considering the extrapolation to zero absorbance plus the intercept with the graph. Using the method of standard additions, the amount to drop to 0 absorbance helps find original concentration:\[ 0.045 = 0.180x + 0.001 \0.044 = 0.180x \x \approx 0.244 \text{ mg/L} \]Again, considering dilution factors from previous processing, calculate to find the concentration in the original 200 mL solution converted to a 50 mL volume:\[ 0.244 \times \frac{50}{200} = 0.061 \text{ mg/L} \]

07

Determine Original Concentration of Cr in Caustic Suspension

Adjust for the initial volume conversion, the added concentration in terms provided initially:\[ 0.061 \times \frac{50}{5} = 0.61 \text{ mg/L} \]reflecting the transformation into the final analysis volume.

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Key Concepts

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

Concentration Determination

Determining the concentration of a substance like copper in a solution is a key step in chemical analysis. In this exercise, we use Atomic Absorption Spectroscopy (AAS) to find out how much copper is present. The process begins by preparing the caustic solution. We acidify it, add hydrogen peroxide, and then boil it. This treatment helps to break down any interfering substances and makes the metal easier to measure.

Once we have a clean solution, we dilute it to a known volume. This step ensures the sample's concentration falls within the range that the AAS instrument can accurately measure. We then analyze the prepared sample using AAS. The instrument measures how much light is absorbed by the sample, which correlates to the concentration of copper.

However, to get accurate results, we need a calibration curve. This curve is made by measuring the absorbance of standard solutions of known copper concentrations. With this curve, we can find the concentration of copper in our unknown sample by matching its absorbance to the curve. This method transforms raw absorbance data into meaningful concentration information.

Chemical Analysis

Chemical analysis involves breaking down and studying the composition of substances. In this case, we want to know the amounts of copper and chromium in a caustic solution from the soda production process. This analysis is essential to ensure the final product's quality and the efficiency of the production process.

We use a sensitive method known as Atomic Absorption Spectroscopy (AAS) for this purpose. AAS is ideal because it can detect even small amounts of metals in solutions. The process involves volatilizing the sample and exposing it to a light beam of a specific wavelength. Each metal absorbs light at a unique wavelength, helping us distinguish and measure them accurately.

By analyzing the signals received from the instrument, we can determine the concentration of specific metals in a complex chemical mixture. This analysis also relies heavily on creating a suitable environment in which the metal atoms can absorb light freely. This is why sample preparation, as seen in the caustic solution case, is so important.

Standard Additions Method

The method of standard additions is a powerful technique used in chemical analysis for samples with complex matrices that could interfere with results. It helps us accurately measure a specific component. In this exercise, we used it to determine the concentration of chromium in our caustic solution.

The process begins by taking an aliquot of the sample and measuring its absorbance. Then, known quantities of a standard solution of the analyte (in this case, chromium) are added, each time measuring the absorbance. The idea is to observe how the absorbance changes with added amounts, as it should increase in a predictable way.

By plotting these absorbance values against the concentration of the added standard, we get a line. The concentration of the sample before any additions can be found by extrapolating this line back to where it would intercept the concentration axis, known as zero absorbance. This intercept reveals the original concentration of the analyte in the sample without interference effects.

Using the standard additions method corrects for any matrix effects present by maintaining the sample's native conditions throughout the analysis process. This makes it particularly useful for complex samples like those in industrial processes.