Because the black current juice is darkly coloured, therefore pH indicator would not be the most suitable method for this experiment. I’m going to use a pH meter for this experiment.
The Science
Many of the soft drinks contain acids. It is use to balance the sweetness of the sugar present in the drinks. Citric Acid is one of the most common acid uses in soft drinks.
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Soft drink Blackcurrant juice contains:
Water, Sugar, Glucose Fructose Syrup (from wheat and maize), Blackcurrant Juice from Concentrate (4%), Citric Acid, Vitamin C, Preservatives (Sodium Benzoate, Sodium Bisulphate) and Colour (Anthocyanins)
Citric Acid
Citric Acid is an organic acid. We can find it from citrus fruits such as lemon and orange. It is added to soft drinks to create an acidic taste (sourness).
Citric Acid also helps preserve drinks from micro-organism because of its acidity. Micro-organism can only grow and multiply in neutral condition.
The reaction between Sodium Hydroxide and Citric acid:
The following equation shows the reaction of what’s happening during the neutralisation reaction between Citric Acid and Sodium Hydroxide:
Citric Acid + Sodium Hydroxide → Sodium citrate + Water
C6H8O7 + 3NaOH → C6H8O7Na3+ 3H2O
From the equation above we have worked out 1 mole of citric acid reacts with 3 moles of sodium hydroxide (1:3 ratios).
Graph Demonstration
Strong and Weak Acids
Acids are vary in strength. The strength of an acid is based on the number of Hydrogen ions in the atom.
Strong acids:
Strong acids have a strong proclivity to donate Hydrogen+ ions. All of the strong acids such as hydrochloric acid, Sulphric acid and Nitric acid are all 100% ionised in water. This mean all the strong acid would dissolve the reaction is fully complete and no unreacted substance remains in the solution.
Weak acids:
Weak acids have a weak tendency to donate Hydrogen+ ions. The donation of hydrogen+ ions does not dissociate completely.
If you dissolved a segment of weak acid in water, only a small amount of weak acid would dissolve into Hydrogen+ ions and negative- ions and the rest of the weak acid would remain un-dissolved in the solution.
Experiment Begin:
- Measure out 25 ml of black current juice by using a pipette
- Pour it into a 10 cm3 beaker
- Fill 50 cm3 burette with sodium hydroxide (make sure the tap is tight before pouring anything into it and use a funnel to avoid spillage)
- Add 1.0cm3 of sodium hydroxide into the solution and shake it
- Measure the pH and take a reading
- Continue procedure 4 & 5
- Until the pH gradually changes slowly add 0.1cm3
- Take a reading
Equipments needed
Average Results
Analysis
1dm3 = 1000cm3
NaOH
Calculation
- No. moles (Acid) = Concentration (NaOH) x Volume of titre required/ 1000
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Concentration of acid = moles x 1000/ volume (dm-3)
Concentration of NaOH to use:
No mol NaOH = No. mol citric acid /3 = 0.001
Conc. NaOH= No. of mol Citric acid x 1000/Vol = 0.001 x 1000/10 =0.1
Formula: C6H8O7+3NaO4 → C6H8O7Na3+3H2O
3:1
0.1 mol of NaOH = 0.068 mol of C6H8O7
Use the No. moles and put it into the Formula 2
Concentration of acid = 0.0006 x 1000/ 25
= 0.024 mol dm-3
Investigation 2: Determine the concentrations of blackcurrant juice in the diluted samples
In order to find out the concentration of blackcurrant juice in the diluted samples, I’ll use the following two methods
- Colour matching
- Colourimetry
The technique /science
The techniques for this experiment are Colour matching, Colourimetry and Calibration graph.
Colour matching
Colour matching is a method relies on a person’s eyes to determine the intensity/ the wavelength of light of a coloured solution with the nearest colour from a set of coloured standard solution. This method however is not accurate as human errors can be made because it relies on the judgment of the person’s eyes.
Colorimetry:
Colorimetry involves using light to work out the concentration of an unknown coloured solution.
Colourimetry uses a device called the colourimeter. It is more accurate than Colorimetry as it is measured by machinery. The colour is measured using a spectrophotometer in the machine. Colourimeter obtains 3 wideband of readings in within the visible spectrum boundary to obtain an estimate of a colour sample. The reading determines how concentrated the solution is. The greater the concentration, the lower the reading. The lower the concentration, the greater the reading.
Calibration graph: By plotting a Calibration graph using the Colourimetry results, we can easily compare the data and determine the absorbance of both known and unknown concentration.
Calibration curve:
Calibration curve is a method to identify the concentration of a substance in an unknown sample solution. This is done by comparing the unknown to a set of standard samples of known concentration.
Colours:
Different colours have different wavelength and frequency. Colours are refractions of light. By using a prism, we can split the colours from the light into seven different colours to create a colour spectrum. The colourimeter uses the similar method to find out the concentration of a solution.
These are the following equipments needed to begin colourimetry experiment:
- Colourimeter
- Test tubes
- Samples of different concentrated solutions
The experiment begins:
- Prepare 2 test tube racks
- Place 11 test tube into the racks
- Label them from 0% to 100% concentration (up in tens)
- Follow the table below and measure out the amount of solution and substance for each test tube
Results
Evaluation:
Through out the experiment, I’ve completed most of the tasks. However, I think my experiment it’s not as good as I expected to be. The results I’ve got for my first experiment have some slight anomalous values, which will impact on my final results. These are the possible reasons:
- Volume control: extra solution (NaOH) can be accidentally added into the substance during the titration. A small amount of extra solution can have a big impact on the pH
- Room temperature: Room temperature rise can speed up the collision between particles and therefore increase the chances of reactions between citric acid and sodium hydroxide
- Time limit: If we have given more time to complete our experiments more repeated of experiments can be done. This will increase the accuracy for my final result and more chance of avoiding biases
- Equipments: equipments can be improve therefore the results would be xmore accurate
