Chemical engineering chemistry
MB Distillation 21 - This document contains material balance questions for one to practice for tests.
Chemical engineering chemistry
Practice materials
Chemical engineering chemistry
Practice materials
Chemical engineering chemistry
Mandatory assignmentsLab title: Spectrophotometry
Date: 14 May 2023
In crustal rocks on Earth, phosphorus ranks as the eleventh abundant element, and all of its known terrestrial minerals are orthophosphates. Phosphate ion which is found in lakes and streams due to agricultural operations is a polyatomic ion with the chemical symbol 𝑃𝑂 4 3−. A salt or an ester of phosphoric acid is known as phosphate, and it contains the phosphate ion. Phosphate is useful in multiple ways. In addition to being used in detergents, phosphate is also added to cola drinks in the acid form, H3PO4, to give them a sour flavour. Plankton and aquatic plants that produce food for fish are stimulated to grow by phosphate, but excessive use of fertilizers and detergents causes aquatic plants to develop too quickly, depleting the water's dissolved oxygen content and harming aquatic life. Since phosphate is found in detergents, fertilizers, soil, meat drinks, and detergents, there are multiple techniques for determining the amount of phosphate in such samples two being atomic absorption spectrometry and spectrophotometry.
Spectrophotometry is a quantitative analytical technique that measures the intensity of light as it travels through a sample solution in order to determine the sample's absorbance. The absorbance of a sample is determined by the concentration of the absorbing molecules and the path length at which the absorption occurs. This law of absorption is known as the Beer-Lambert Law. This law is denoted by the formula below:
Where l is the path length along which the sample absorption occurs (cm), c is the concentration of the analyte mol. 𝐿−1), and 𝜀 is the molar absorptivity (L 𝑚𝑜𝑙−1 𝑐𝑚−1). The most widely used techniques are spectrophotometric techniques using ammonium molybdate and molybdovanadate. Ammonium molybdate spectrophotometric are utilized for different reducing agents such as ascorbic acid, and stannous chloride. Ammonium metavanadate is a crystalline (sand-like), transparent, white, or yellow powder. It serves as a photographic developer and reagent as well as an ingredient in dyes, inks, varnishes, and paints. In the presence of appropriate reducing agents, ammonium molybdate combines with phosphate to create ammonium phosphate molybdate. Molybdate and vanadate forms a complex that absorbs UV light so that the absorbance can be determined. Adding molybdate to an acidic solution which contains orthophosphate and vanadate a molybdovanadophosphoric acid is formed. This acid is yellow orange in colour. The intensity of this colour is dependent on the concentration of reagents and the acidity of the solution.
Glass cuvettes are used to measure absorption in the visible region. The wavelength at which absorptions are measured is 400nm. This surpasses the absorption maximum UV range which is 314nm, at 400nm there is still adequate sensitivity
Table 1: Concentration, mass and absorbance of phosphate
Standard phosphate added (mL)
Concentration of 𝑃𝑂 4 2−
in Mass in μg of 𝑃𝑂 4 2− in standard
Measured absorbance @400nm
Subtracted reagent blank absorbance 0ml 0 0 0 0
Sample Replicate 1 0 0. Replicate 2 0 0. Replicate 3 0120 0.
Graph 1: Absorbance vs mass graph
The equation for the line was derived from figure 1 and applied to calculate the mass of phosphate in the duplicate cola sample. In order to determine the mass of , the absorbance of each sample of cola was entered into the equation's y intercept.
y = 0,0003x - 0, R≤ = 0,
0 100 200 300 400 500 600 700 800 900
Graph 2: excel worksheet for absorbance vs mass graph
concerntration (ppm) corrected absorbance
8 0 y = 0,0003x - 0, R≤ = 0,
Concentration of PO42-solution calculation
(20ppm)(16) = C 2 (100ml) (20ppm)(24) = C 2 (100ml) C 2 = 3 C 2 = 4
(20ppm)(32) = C 2 (100ml) (20ppm)(40ml) = C 2 (100ml) C 2 = 6 C 2 = 8ppm Mean=4. SD= 2.
Mass calculations( in ug) 1. m = c × v 2. m = c × v 3. m = c × v = 0 ×100=0ug = 1×100ml =160ug = 3×100ml =360ug
Solution 2 : same calculation was used to calculate solution 3. Solution 3=4
To find the phosphate in the 10ml cola sample, the concentration of the aliquot(5ml) is determined
Caliquot=Cdiluted cola sample
The mass of phosphate in the sample was determined as follows:
Mass of phosphorus in sample:
30 is molar mass of phosphorus , 90 is molar mass of phosphate
Mass in 10ml 8. Mass in100ml 82. Mass of phosphorus in 10ml 2. Mass of phosphorus in 100ml 27
The findings indicate that the cola sample contains 27 mg of phosphorus per 100 ml. Given that the cola sample's actual concentration of phosphorus is 17 mg per 100 ml, these results indicate that there is more phosphorus present in the cola sample in comparison to the true value.
The reason for this result could be that there were errors that occurred when the lab experiment was being done. The errors could have occurred in the process of transferring all the materials that were used for the lab. Using more nitric acid, molybdate and metavanadate than what was required could
have led to the phosphorus value being too high. The readings on the burette could have also been misread if they were not read at eye level with the bottom of the meniscus. This would lead to a parallax error. Improper use of a pipette could have also led to errors. Not closing the spectrophotometer multiple times when taking the absorbance readings as well as not properly wiping the cuvette before placing it in the spectrophotometer could have also led to the incorrect result. Incorrect placement of the cuvette in the spectrophotometer could have also played a role. The frothy side of the cuvette needed to be facing the front side. The solutions in the 9 flasks were not left to react for 20 minutes but 10 minutes due to time constraints. This could have played a role as the absorbance readings taken from the solutions would be too little.
The precision of the experiment was however acceptable, 0,014 which is the standard deviation for the concentration of the cola samples (the smaller the standard deviation the more precision the data set is.) The RSD value for the concentration of phosphate in the replicate cola samples 0,00338 (0%) which is lower than the required 2%.
In the future instruments should be used properly. Accuracy should be accounted for, and time should be spent wisely so that there is enough time to complete the lab properly.
The aim of the experiment was to determine the amount of phosphate in cola using spectrophotometry. This was done by preparing 6 standards of increasing known concentration of phosphate and measuring the absorbance of each sample by plotting a calibration curve. The results of the experiment determined the amount of phosphate in the cola sample to be 82 per 100ml which is equivalent to 27 mg of phosphorus per 100ml. The actual amount in the cola was given to be 17mg of phosphorus per 100ml. According to the RSD and standard deviation of the concentration of cola samples there was precision in the data set values, although this was the case there was lack of accuracy when experiment was being performed.
Shyla, B. and Nagendrappa, G., 2011. A simple spectrophotometric method for the determination of phosphate in soil, detergents, water, bone and food samples through the formation of phosphomolybdate complex followed by its reduction with thiourea. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 78 (1), pp-502.
Fernando Guerrero-Romero, M. R., 1995. Consumption of soft drinks with phosphoric acid as a risk factor for the development of hypocalcemia in children. The journal of Pediatrics, 126(6), pp. 940-
Brown, Theodore L, et al. Chemistry: the Central Science. 14th. s. : Pearson Publishing, 2017.
Anjum, M.A. and Lee, J., 2017. Sulfur and nitrogen dual-doped molybdenum phosphide nanocrystallites as an active and stable hydrogen evolution reaction electrocatalyst in acidic and alkaline media. ACS Catalysis, 7 (4), pp-3038.
