Chemical Analysis of Data & Scientific American Report Writing - Chemistry Assignment Help

Assignment Task

Overview

The scientific report assignment will require you to present chemical analysis of data acquired from the pH titrations experiment you have done in the class and its interpretation. This should be written for a scientifically literate, but non specialist scientific audience, e.g. someone who would read Scientific American. It must therefore be at sufficient scientific depth, but accessible to the non-specialist as would Scientific American.

Practical: pH Titrations

Objective

The change in pH during a series of acid-base titrations, using a variety of acids, will be followed with a pH meter.

Background

At the low concentrations of hydrogen ions which are typically found in the environment, a pH meter may be used to monitor changes. A pH meter is essentially a potentiometer for measuring the electromotive force (e.m.f.) of the cell - 

The Calomel electrode retains constant potential', while the glass electrode varies its potential with the hydrogen ion concentration of the solution. Hence this specialised potentiometer may be calibrated directly in pH units.

The biological effect of changes in pH can most easily be seen by the sensitivity of freshwater species to acid conditions. Populations of salmon start to decrease below pH 6.5, perch below pH 6.0 and eels below pH 5.5. The eradication of life can result from a change of little more than 1 pH unit.

There are also chemical effects. A decrease in pH increases the solubilities of metals, so e.g. the use of lead piping for domestic water supplies becomes of greater concern as the water becomes more acidic. The weathering of minerals such as limestone or dolomite by water becomes more rapid with a decrease in pH.

A typical procedure for measurement of pH involves calibration with two buffer solutions spanning the expected pH range of the sample. The procedures for acidity or alkalinity' measure, by titration, the quantity of acid or base needed to change the pH of a sample to a fixed value (usually 4.5 or 8.3), corresponding to methyl orange or phenolphthalein end-points, respectively. From a chemical point of view this gives a measurement of the buffer capacity (resistance to change in pH) of the water. A high buffer capacity is a useful feature if an acidic or basic pollutant is being added to the water.

Procedure

1. Calibrate your pH meter using standard buffers as follows. Rinse the electrode assembly in pure water and then dip it into one of the two standard buffers. Adjust the meter so that its pH reading agrees with the stated pH of the buffer solution. Rinse again, repeat with the second of the two standard buffers and give a final rinse.

2. To carry out your first titration, place 20 cm3 of 0.1 mol dm-3 hydrochloric acid solution in a 200 cm3 beaker and dip in the electrode assembly, adding distilled water until it is properly immersed. Stir very gently and take a pH reading. (If magnetic stirrers are available it might be better to use these throughout the titration). Run in 4cm3 of 0.1 mol dm-3 sodium hydroxide from a burette, again stir well (or use the magnetic stirrer continuously) and take another pH reading. Take pH readings after each 4 cm3 addition of sodium hydroxide until about 5 cm3 from the approximate end-point, when 1 cm3 additions should be made and finally, 0.2 cm3 additions when close to the end point. Make further additions until about 10cm3 beyond the end point.

Scientific Report

You must focus scientific report under following sections, which includes interpretation of your experimental data in section (a), further enhanced calculations and plotting graph in section (b) followed by section (c) where it is required your applied knowledge of buffer:

1. Plot graphs of pH against volume of sodium hydroxide added, note the pH at each end-point and relate it to the pH range of the indicator normally recommended for that titration.

Use your pH titration curves for hydrochloric acid, ethanoic acid and phosphoric acid:

(i) Over which region(s) is the system behaving as a buffer? Explain your answer.

(ii) Estimate the pKa values for each acid, explaining how you have reached your conclusion.

A 25.0 cm3 sample of 0.25 mol dm-3 nitric acid solution is titrated with 0.10 mol dm-3 sodium hydroxide. Calculate the pH of the solution - 

(i) Before any sodium hydroxide has been added

(ii) After the addition of 10.0 cm3 sodium hydroxide

(iii) After the addition of 25.0 cm3 sodium hydroxide

(iv) After the addition of 50.0 cm3 sodium hydroxide

(v) After the addition of 62.5 cm3 sodium hydroxide

(vi) After the addition of 75.0 cm3 sodium hydroxide

(vii) After the addition of 100.0 cm3 sodium hydroxide

Plot a graph of pH vs volume of base added, state the pH at the end point and estimate the pKa of the acid, explaining your reasoning

Several biological and chemical systems require buffering. Research the literature to explain how and why this occurs. For biological systems your information will largely be in standard textbooks, but for the chemical system both texts and journals will supply useful information, particularly environmental sources.

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