Experimentation
Firstly safety is very important and a lab coat must be worn at all times whilst in the laboratory and long hair should be tied back. Safety goggles should also be worn to protect eyes and no food or drink should be consumed in the lab. Bags should be left outside as they are an obstruction and a safety hazard.
Risk Assessment
Apparatus
Here is a list of some of the apparatus used in the experiment:
White tile
Funnel- for pouring liquids into instruments with small openings
2 x flask
Distilled water- water in which all minerals and impurities have been removed by distillation
Clamp stand
Sodium hydroxide- strong alkaline
Phenolphthalein- pH indicator
Burette- measuring glass tube instrument with a tap that can be turn on and off at the bottom
Hydrochloric acid- strong acid
Gastric juice- acid made in the stomach that works with enzymes to break proteins down
Pipette- glass tube used to measure and transfer a measured a certain amount of liquid.
Method
Check all the right equipment need is to hand then fill the 25cm3 pipette up with hydrochloric acid just past the 25cm3 marking line and slowly release some of the acid until the discus is sitting on top of the line. Then add this hydrochloric acid to a conical flask.
Add 2 drops of phenolphthalein indicator to the conical flask.
Place the burette and clamp onto a stall so its eye level and a more accurate reading can be achieved and make sure the burette nozzle is closed.
Place the small funnel at the top of the burette so it can be filled with sodium hydroxide whilst checking for leaks or air bubbles.
Take the initial reading of the burette and recording it on a table. The readings from the burette should be read to the nearest 0.05cm3.
Then titrate the hydrochloric acid in the conical flask against the sodium hydroxide solution from the burette making sure the white tile has been placed underneath the conical flask as it will make it easier to see the end point.
When the hydrochloric acid starts turning pink and changing back to colourless again slow the flow from the burette down and rinse the sides of the conical flask with distilled water incase sodium hydroxide is on the sides.
When the solution turns a slight pink stop the titration and see if the colour stays, if so the end point has been reached and if not add a couple more drops of the sodium hydroxide from the burette.
Do not put too much sodium hydroxide as the solution will turn a dark pink and it needs to be a light pink.
Take a reading off the burette once the end point is reached and try to read it to the nearest 0.01cm3. Record the reading on to a table.
Rinse out the conical flask with tap water and then with distilled water, then repeat this procedure. Do this at least twice.
After repeating the experiment the pipette and conical flask need to be thoroughly rinsed out with tap water and then distilled water.
Repeat the whole titration but now instead of having 25cm3 measured into the pipette and transferred into the conical flask, have 25cm3 of the gastric juice provided.
Repeat this at least twice and record all result in a table. Tidy your work area and then carry out the necessary calculations.
Lastly the pH of the gastric juice used needs to be measured using a pH meter that has been correctly calibrated with a standard pH 4.0 buffer. Record your results.
The only problem found whilst carrying out the experiment was an air bubble that was found in the burette, so a new burette was used instead. Other than that no problems were encountered and the titration was carried out very smoothly.
Results
Titration data for the standardisation of sodium hydroxide with 25 cm3 0.01 mol dm-3 HCL
Average Volume of sodium hydroxide solution used = 25.30cm3
(25.20 + 25.20 + 25.50 / 3 = 25.30cm3)
Number of moles of NaOH present in this volume = 2.5 x 10-4
(25cm3 / 1000 x 0.01 = 2.5 x 10-4)
Therefore: Molar concentration of NaOH = 9.8814 x 10-6
(2.5 x 10-4 / 25.30 = 9.8814 x 10-6)
Titration data for 25cm3 gastric juice with the standardized solution of NaOH
Average volume of sodium hydroxide solution used = 30.57 cm3
(30.15 + 30.40 + 31.15 + 30.40 / 4 = 30.5667)
Number of moles of NaOH present in this volume = 3.1 x 10-4 moles
(30.57cm3 / 1000 x 0.01 = 3.1 x 10-4)
Therefore number of moles of HCL in 25cm3 of gastric juice = 3.1 x 10-4 moles
Therefore molar concentration of HCL in gastric juice = 0.008mol dm-3
Concentration of hydrogen ions in gastric juice = 0.008mol dm-3
The measured pH value of the gastric juice = 2.24pH
Using the - Log10 on the calculator the pH level can be calculated =
pH = - Log10 (molar concentration of H+)
= -Log (0.008) = 2.1pH
Discussion
The measured pH value of the gastric juice was 2.24pH using the calibrated pH meter and the pH value that I calculated using my results was 2.1pH. Therefore, my pH calculated was 0.14 lower than the value found using the pH meter. This should that a few minor errors came up throughout my experiment but overall my results are quite close and accurate.
As you can see form my results table it shows the readings that I had taken were constant and have a trend. During the experiment human error may have been made, for example not reading the value on the burette accurately as the burette may not have been eye level which can make it hard to see what the reading was. Also filling up the pipette to where the mark may not have been very accurate which can cause inaccurate results.
On a whole I am very pleased with my results and think I carried out the experiment accurately, carefully and safely and this shows in my results. Accuracy is how close a measured value is to the actual value, and precision is how close the measured values are to each other, both of which I think are quite high as they are all in close range of each other. I believe I achieved very accurate and precise figures in my titration because I was very careful not to over shot the mixture and go over the end point and as soon as I started seeing the solution going pink I slowed the burette right down just adding drops until the light pink colour stayed.
If I was carrying out this experiment again the one change I would make to improve my results would be to make sure that all glassware is thoroughly cleaned in between each repeat so there is no contamination and my results do not vary. Also i would make sure that I stir the hydrochloric acid in the flask constantly as sometimes as I was adding the sodium hydroxide I stopped stirring it.
Conclusions
Aims:
Determine the number of moles of sodium hydroxide in hydrochloric acid.
Determine the number of moles of sodium hydroxide present in the sample of gastric juice.
Use the results to calculate the pH of the sample and compare it to the pH found using a pH meter.
Aim 1: To determine the number of moles of sodium hydroxide in hydrochloric acid.
I believe this was met and completed successfully and my results for this aim were as follow:
Average Volume of sodium hydroxide solution used = 25.30cm3
(25.20 + 25.20 + 25.50 / 3 = 25.30cm3)
Number of moles of NaOH present in this volume = 2.5 x 10-4
(25cm3 / 1000 x 0.01 = 2.5 x 10-4)
Therefore: Molar concentration of NaOH = 9.8814 x 10-6
(2.5 x 10-4 / 25.30 = 9.8814 x 10-6)
Aim 2: To determine the number of moles of sodium hydroxide present in the sample of gastric juice.
Once again I think I was successful and my results were very close to the expected results, and my results were:
Average volume of sodium hydroxide solution used = 30.57 cm3
(30.15 + 30.40 + 31.15 + 30.40 / 4 = 30.5667)
Number of moles of NaOH present in this volume = 3.1 x 10-4 moles
(30.57cm3 / 1000 x 0.01 = 3.1 x 10-4)
Therefore number of moles of HCL in 25cm3 of gastric juice = 3.1 x 10-4 moles
Therefore molar concentration of HCL in gastric juice = 0.008mol dm-3
Concentration of hydrogen ions in gastric juice = 0.008mol dm-3
Aim 3: Use the results to calculate the pH of the sample and compare it to the pH found using a pH meter.
I think this was quite successful as my value found was quite close the pH that was recorded from the calibrated pH machine, however, there have been a few errors carrying out this experiment which is why my calculated pH is slightly lower.
-Therefore molar concentration of HCL in gastric juice = 0.008mol dm-3
- Concentration of hydrogen ions in gastric juice = 0.008mol dm-3
The measured pH value of the gastric juice = 2.24pH
-Using the - Log10 on the calculator the pH level can be calculated =
- pH = - Log10 (molar concentration of H+)
= -Log (0.008) = 2.1pH
by
jamari (student)
