Atomic Absorption Spectroscopy Analysis: Assessing Calcium Content in Commercial Supplements and Exploring pH Effects on Solubility

The primary aim of this experiment was to verify the claimed calcium content of 200mg per tablet in a commercial supplement by employing atomic absorption spectroscopy (AAS) analysis. To begin, standard solutions with varying calcium concentrations were prepared using a 50ppm stock solution of calcium in a 100mL flask. The stock solution was created by dissolving exactly 5mg of calcium (Ca) in the flask. Subsequently, standard solutions of 25ppm, 12.5ppm, and 6.25ppm were derived from the stock solution by diluting specific volumes in individual flasks.

In the second part of the experiment, the impact of pH on the solubility of calcium tablets and the efficiency of digestion was explored. Solutions of dissolved tablets with pH levels ranging from 0 to 5 were analyzed using AAS. The absorbance values for solutions with pH 0 to 5 were recorded as 0.1441, 0.1676, 0.3650, 1.4077C, 1.4018C, and 1.3923C, respectively. Using calibration curves, the calcium concentrations in these solutions were determined to be (4.62, 5.64, 14.28, 59.91, 59.65, and 59.23) ppm. The highest concentration was observed at pH 2, indicating optimal conditions for calcium tablet dissolution.

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However, the calculated amount of calcium in the tablets (59.91mg) contradicted the manufacturer's claim of 200mg per tablet.

The historical background of atomic absorption spectroscopy was briefly outlined. First observed in 1802 with the discovery of Fraunhofer lines, the concept of AAS was later demonstrated by Sir Alan Walsh in 1953. AAS is employed to measure the concentration of gas-phase atoms by analyzing the absorption of ultraviolet or visible light, leading to the excitation of electrons from lower to higher energy levels. The process involves atomizing the sample and subsequently absorbing radiation emitted by free atoms.

The key components of an AAS instrument include a hollow cathode lamp, an atomizer for sample atomization, a monochromator for selecting the analysis wavelength, and a detector for converting light into an electrical signal. The hollow cathode lamp emits light at the wavelength absorbed by the sample atoms, and the detector records the reduction in intensity caused by the absorption of light by the atoms.

Objective:

1. To analyze a commercial calcium supplement using Atomic Absorption Spectrophotometry.
2. To compare the concentration of the unknown sample (commercial calcium supplement) with the recommended calcium intake for humans.

Theory: Prior to conducting the experiment, it is essential to prepare a calcium stock solution to create a 100ml 50ppm solution and then dilute it to generate a series of standard solutions (25, 12.5, and 6.25ppm) for calibrating the spectrometer.

Atomic spectroscopy involves studying the absorption or emission of light by individual atoms, specifically free atoms or ions rather than compounds. Atomic Absorption Spectroscopy (AAS) is primarily employed for inorganic compounds. The absorption and emission of light correspond to the absorption and emission of energy by electrons within the atom, covering a broad range of energetic transitions observable across the electromagnetic spectrum.

The most common source for atomic absorption measurements is the Hollow Cathode Lamp (HCL). In this experiment, the cathode of the HCL is constructed of lead. Atomization, a crucial step in atomic spectroscopy, involves a nebulizer and a burner. The nebulizer transforms a liquid sample into a fine spray or aerosol, which is then introduced into the flame. The flame is generated using an air-acetylene combination, reaching a temperature range of 2400-2700K.
As mentioned earlier, only a small fraction of atoms in the flame exist in an excited state at any given moment. This means that a significant percentage of atoms remain in the ground state and are available for excitation. Atomic Absorption Spectroscopy (AAS) capitalizes on this by using a light beam to excite these ground state atoms in the flame. In this regard, AA is similar to molecular absorption spectrophotometry, where light absorption by ground state atoms is measured and correlated with concentration. The light source, known as a hollow cathode tube, is a lamp that emits precisely the wavelength needed for analysis, eliminating the need for a monochromator.

The emitted light from this lamp precisely matches the light required for the analysis, despite the absence of a monochromator. This is because the metal atoms being tested are present within the lamp, and when the lamp is activated, these atoms receive energy, causing them to transition to excited states. Upon returning to the ground state, the same wavelengths essential for the analysis are emitted since it is the analyzed metal with identical energy levels that undergoes excitation. The directed light passes through the flame containing the sample, which is typically wide, approximately 4-6 inches, providing a sufficiently long path length for detecting low concentrations of atoms in the flame.

The light beam then enters the monochromator, tuned to a wavelength absorbed by the sample. The detector measures the light intensity, which, after adjusting for the blank, is output to the readout, akin to a single-beam molecular instrument. The absorption behavior adheres to Beer's Law, and concentrations of unknowns are determined using the same principles. Double-beam instruments are also utilized in AAS. However, in this case, the second beam does not traverse a second sample container. Achieving two closely matched flames is challenging, so the second beam bypasses the flame and is directly relayed to the detector. This design addresses variations in source intensity but doesn't eliminate effects from the flame (cuvette) or other sample components, which still need adjustment by separately measuring the blank.
Results

Sample calculation
We need to dilute 5 mg of CaCl2.2H2O solid in 100 mL solution to obtained 100 ppm of Ca stock solution. From the solids of CaCl­2.2H2O, we know that Ca = 40.08 g/mol, Cl = 45.45 g/mol, H= 1.008 g/mol and O= 16g/mol.
Percentage of Ca in CaCl2.2H2O:
RMM CaCl2.2H2O = (40.08 + 1.008 x 4 + 16 x 2 + 45.45 x 2) g/mol = 167.01 g /mol
Zn% = x 100% = 27.2 %
Total mass of CaCl2.2H2O to obtain 5mg of Ca:
=
W = 18.38 mg Ca
Thus we need to dissolve 18.38 mg of CaCl2.H2O in 50 mL solution.
To get series of standard solution of 25 ppm, 12.5 ppm and 6.25 ppm:
M1V1 = M2V2 ; let M1 = 50 ppm V2 = 50 mL
To get 25 ppm in a 50 mL flask from 100 ppm:
M1V1 = M2V2
50 ppm (V1) = 25 ppm (50mL)
V1 = 25 mL
thus, we take 25 mL of 100 ppm Ca solution and diluted it in a 50 mL flask with distilled water.
If, M2 = 12.5 ppm V1 = 12.5 mL
M2 = 6.25 ppm V1 = 6.25 mL
To calculate Ca dissolved in pH solution:
At pH 2, the concentration is 59.91 ppm,
Mass of Ca present = 59.91 ppm x 1 L = 59.91 mg
The primary objective of this experiment was to determine the calcium content in commercial supplement tablets, aiming to validate the stated calcium quantity of 200mg per tablet as mentioned on the supplement bottle. The analytical technique employed for this investigation was atomic absorption spectroscopy (AAS).

Before conducting the AAS analysis, a series of standard solutions based on calcium concentration were prepared. A 50ppm stock solution of calcium was created in a 100mL flask using CaCl2.2H2O salts. To achieve this concentration, precisely 5mg of calcium needed to be dissolved in the 100mL flask. Considering the composition of CaCl2.2H2O, it was determined that approximately 18.4mg of CaCl2.2H2O was required to obtain 5mg of calcium. Due to the irregular nature of the CaCl2.H2O salts, the exact mass measured was 19.3mg. The weighed CaCl2.2H2O was then diluted in a small beaker before being placed in the 100mL flask to ensure complete dissolution of all solids.

Subsequently, a series of standard solutions with concentrations of 25ppm, 12.5ppm, and 6.25ppm were prepared by diluting specific volumes from the stock solution in three individual flasks.

The AAS analysis involved using a Ca hollow cathode lamp in the spectrometer to analyze the calcium content. The flame atomization process was employed, where the sample containing calcium was atomized using flame atomizers with a temperature around 2700K. The atomization of calcium led to the excitation of electrons, causing the emission of specific wavelengths corresponding to the orange color for calcium. The emitted light was directed at the flame containing the sample, and the absorbance values were obtained.

The calibration line generated from the absorbance readings of standard solutions exhibited a linear relationship, with a correlation coefficient (r value) close to 1 (r = 0.9964), indicating a strong correlation between concentration and absorbance.

The experiment also investigated the effect of pH on the solubility of calcium tablets and their efficiency in digestion. In the second set of measurements, five solutions of dissolved tablets with varying pH levels (0 to 5) were analyzed using AAS. The absorbance values for these solutions were recorded, and concentrations were determined through interpolation and extrapolation of the calibration curve.

The results showed that the highest concentration of calcium (59.91 ppm) was achieved at pH 2, suggesting that pH 2 is the optimum level for dissolving calcium tablets, akin to the acidic nature of the human stomach. However, this concentration contradicted the claim on the commercial tablet box of 200mg per tablet. It was acknowledged that the "lab's stomach" simulation may not replicate all factors affecting the human digestive system, such as temperature, mechanical movement, and pressure, and hence, the claim of 200mg of calcium per tablet could still be considered plausible.

In conclusion, the experiment successfully utilized AAS to determine the calcium content in commercial supplement tablets, but the discrepancy between the experimental results and the manufacturer's claim warrants further consideration of the dissolving environment and conditions.