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At the beginning, the solution had been filtered and the white precipitate was collected. The precipitate is known to be calcium oxalate. The filtrate was then obtained and used for the determination of the absorbance by using the atomic absorption spectroscopy (AAS). The absorbance of the calcium filtrate is increasing gradually as the concentration increase gradually. As more metal ion is present (concentration increase gradually), they tend to collide with each other easily and cause the absorbance to increase when the concentration increase. Hence as concentration of the filtrate increase, the absorbance also increase. For general, the wavelength of calcium is 422.7 nm in the atomic absorption spectroscopy (AAS).
Chloride is an anion (oxidation state of -1) which means that it consist of a negatively charge. Whereas calcium is a cation (oxidation state of +2) which means that it consisted of a positive charge. As calcium has the charge of +2 and chloride has the charge of -1 which is unbalanced, hence they are not stable.
In order to achieve the stability, 2 chloride ions are needed to bind with 1 calcium ion. As this happen, it will cause chloride ion to absorb strongly as this can be seen from the high absorption value of chloride compare to others ion. The depression of chloride is less than phosphate, this may due to the charge of these 2 ions.
Chloride is an anion and it will bind with calcium to form a stable and neutral compound but as general, chloride have the oxidation state of -1 and thus it will have less energy that will generally cause it to have less absorption value. Whereas phosphate is a cation and it will bind with calcium to form a stable and neutral compound but as general, phosphate have the oxidation state of -2 and thus it will have more energy that will generally cause it to have more absorption value.
The phosphate ions interfere with the determination of calcium. In the presence of phosphate ions, the absorption due to the calcium is weaker as compared to that in which phosphate ions were eliminated. This may due to the formation of stable phosphate of calcium that does not easily break up into atoms in the flame. The suppression by phosphate was proportional up to a phosphate/calcium ratio of about 2:3, after which it became constant. Hence, it is proportional decrease in absorption with increasing phosphate concentration.
High concentration of phosphate will have constant interference. This is because phosphate inhibited the calcium absorption strongly. As phosphate can react with calcium ion and in the flame to produce calcium pyrophosphate, Ca2P2O7 that removes large numbers of calcium atoms from the flame. This cause a reduction in the absorbance since the calcium must be in the atomic form to absorb its resonance line. This type of solution interfere can be reduced or eliminated chemically. Hence, it will cause the absorption value to depress.
Absorbance of magnesium and aluminium
For magnesium, there are large number of ions was found that the calcium interference were less serious in magnesium at lower concentration but more serious at the higher concentration. The depression of aluminium was nonlinear and depended on the ratio of calcium to aluminium.
From the result above, there is suppression of calcium by magnesium to a Mg/Ca ratio of 1and the absorption is constant. This shows that there is an enhancement of magnesium atoms at the free calcium atoms. The wavelength of magnesium is 285.2 nm in the atomic absorption spectroscopy (AAS).
Calcium absorption is subject to a number of interference when aluminium is added to it. This will cause its signal to suppress. The suppression by aluminium was chemical in nature. This element will react as anions with calcium to give the calcium salt which is calcium aluminate. The wavelength of aluminium is 309.3 nm in the atomic absorption spectroscopy (AAS). But in theoretical, Al can enhance the effect of interference. This condition may be caused by ionization of the aluminium itself as it contained high charge. The obtained result is totally different from the theoretical result may be due to some technical error and human error.
Comparison between Magnesium and Aluminium
The interfere with the determination of calcium from the aluminium is more than magnesium. This may due to the different charge and shape of aluminium and magnesium. From the result obtained, aluminium depress more compared to magnesium because aluminium have 1 extra positive charge compared to aluminium. As it has more charge it will cause the aluminium ion to be ionized more compared with the magnesium that is lack of 1 positive.
On other hand, the size of the aluminium is generally bigger than magnesium. Bigger size of aluminium will cause it to have less energy compared to the magnesium that has a smaller size. This cause aluminium to has less absorption value. As the wavelength of magnesium and aluminium in the atomic absorption spectroscopy (AAS) are almost the same, so it is sometimes difficult to differentiate the absorption band of the magnesium and aluminium in the atomic absorption spectroscopy (AAS).
Absorbance of phosphate + 1% EDTA
The absorbance of phosphate and 1% EDTA is slightly lower than the absorbance of phosphate without 1% EDTA. This is because the suppression interference have been overcome and minimized for phosphate in the present of EDTA. When there is a high concentration of EDTA was added to the solution in order to form a chelate is dissociated is the flame to give free calcium vapor.
EDTA was added for the determination of calcium in order to prevent interference from phosphate because EDTA will protect calcium from binding with phosphate. Hence, it will have a lower absorption value than the absorption value of phosphate alone.
Absorbance of phosphate + 2000 ppm strontium
Strontium released calcium completely from the foreign refractory metal, which is then complexed with chloride. When there is a presence of strontium in the phosphate, it cause the absorbance of the solution to increase, this is because strontium can overcome all the anionic interferences.
Strontium can act as a releasing agent that means that it can combine with the phosphate and prevent its reaction with the calcium. Hence, it will have a higher absorption value compared to the phosphate alone and those that contain EDTA.
Actually for the ions, the absorption value that obtained should be all positive, but for the result obtained experimentally for the 2nd, 3rd, 4th and 5th are all negative. This may due to some factor. The 1st factor is human error made during the experiment such as do not add the solution follow the concentration correctly, do not take some precaution and the solution may be contaminated.
2nd factor that caused the negative value of the result obtained may be due to the technical error such as the atomic absorption spectroscopy (AAS) that is used to determine the absorbance of the ions do not calibrate well. Besides that, that machine used may not be maintain or repair for a long time, o it will caused some inaccurate in the result that obtained.
Percentage (%) of precipitation of calcium oxalate is 99.97%.
The amount of calcium present in water can be determined by flame atomic absorption spectroscopy.
The sequence for the depression in the absorbance of the element is arranged in the descending order as followed:
(more) Phosphate + 1% EDTA > Phosphate > Magnesium and Aluminium > Chloride > Phosphate + 2000 ppm Strontium (less)