Analytical Chemistry: From Titration to Colorimetry

Introduction

In this investigation we are going to find the unknown concentration of sodium hydroxide and it was found by using titration, titration is the method to figure out the concentration of an unknown substance. Also we are trying to find the concentration of hydrochloric acid by titrating sodium carbonate. We chose specific equipment for this investigation because we wanted to be as accurate as possible for example, we used a top pan scale to three decimal place, a weighing boat, glass pipette and a burette.

We used a top pan scale so the measurements were as accurate as possible rather than two decimal place. We also used a weighing boat so the weight of the object can be accurate again. The glass pipette was used as it is more accurate due to the fact it has a meniscus line and the burette is very accurate also because of having a meniscus line.

Making sodium carbonate solution

The health and safety issues of sodium carbonate is that it is not suggested to inhale the substance as it could cause you to cough or it can give you sore throat.

It can also cause your eyes to go red and when prevented this in the investigation by wearing goggles to prevent this to happen to our eyes.

Calibrating the balance

Mass (g)	Test 1	Test 2	Test 3	Average
10	10.48	10.47	10.47	10.47
20	20.88	20.87	20.85	20.86
30	31.71	31.36	31.67	31.38
40	42.09	42.18	42.16	42.14
50	52.55	52.56	52.67	52.56

We have also investigated the degree of error of the balance and we have seen if it is very accurate. The degree of error was 0.482 and this shows that the balance wasn’t completely accurate.

We had to also rinse the equipment because it could get contaminated and could also affect the future results and could make it unfair test.

The method of making sodium carbonate solution is firstly you have to collect all of the equipment such as the graduated flask and a flask with some sodium solution. Then you make sure the top pan scale is on zero. After that you start by placing the weighing boat on to the scale while it’s still on zero. Thirdly you start pouring the sodium carbonate into the weighing boat until it reads 1.40 or as close to 1.40 as possible. Then you need to pour sodium carbonate into the graduated flask, after that you start to add water until it reaches the meniscus line and wait until the sodium carbonate has been dissolved.

We used 2 decimal place balance but it was not completely accurate and we could’ve used a three decimal place balance as we would get more accurate results which would’ve meant were closer to the average.

Another potential error is that we didn’t measure the meniscus to eye level every time we did and this clearly effected our results as our results were not that close to the average. This would’ve effected the results as the table might’ve not been level which meant the meniscus measurement was effected and we could repeat this investigation and do things differently which will impact the end result positively.

Also if we had used a magnetic stirrer it would’ve been an improvement because when we were dissolving the crystals in the liquid we was shaking it and having a rough estimate to when the crystals dissolved but with an magnetic stirrer it would be much more accurate as it will stir the substance for the correct time and would have made sure the crystals were dissolved.

Finally if we had used an electronic pipette it would have eliminated human error and there was a human error clearly as when we had to calibrate the pipette and also the calibration might have not been 100 % accurate and with an electronic pipette it would be completely accurate

The calculation of moles

Mr= Na2 Co3= (2×25) +12+ (3×16) =106

MOLES = Mass ÷Mr= 1.40÷106= 0.0013 mass in 250 cm3

Titration of Hydrochloric acid

The safety considerations of hydrochloric acids is that it can be corrosive to the skin so we have to be careful when we handle it and if it goes on to our skin you can wash it off with water. We had to also wear goggles in case the hydrochloric acid when into our eyes because it can cause pain and you can get a blurred vision. We calibrated the pipette by pouring water into a beaker and then using the pipette to suck up the water up to the meniscus line which was 25 cm. Once we did that we had to put a beaker on a scale so it will hold the water and then made sure it was reading zero, then we poured the 25 cm3 of water in to the beaker and it should read the same amount which was in the pipette. When calibrating the burette we put 50cm3 of water into the burette which was up to the meniscus line and then poured into the beaker which was on to the scale and then if it read 50cm3 or as close to that it will show the accuracy of the calibration.

Equipment Test 1 Test 2 Test 3 Average

Burette 50cm3 5.3 5.4 5 5.23

Pipette 25cm3 0.8 1 1.2 1

There is a small degree of error as the results of the burette and pipette were not completely accurate meaning when we used the equipment in the experiment actual experiment the results of that will not be 100% accurate and eliminate any errors.

Calculations

Na2Co2+2HCl 2NaCl+H2O+CO2

Na2 CO3=0.013cm3 in 250cm3÷10= 0.0013cm in 25 cm3

Ratio=1:2

Moles of HCl= 0.0013×2=0.0026cm3

Concentration of HCl = moles ÷ (volume ÷1000)= 0.0026 ÷ (15.3 ÷1000) = 0.0026 ÷ 0.0153 = 0.169 cm3

There is a small degree of error as our average was 0.169 but we were meant to get 0.2 so we were off by 0.031. A possible reason why the result wasn’t 100% accurate was because when measuring the colour change we were using different people which meant that the results weren’t always accurate because people would see the colour change at different points. The percentage of our result was 84.5% and the difference was 15.5% to the actual percentage

Titration to identify the concentration of sodium hydroxide sample

Equation for reaction and calculation of sodium hydroxide concentration?

NaOH + HClNaCl + H2O

25cm of NaOHHCL in Burette

We used the same method for titration as the hydrochloric acid experiment but we used sodium hydroxide instead this time. We did a trial run to see the rough estimate for the results to help us get an idea for when we actually did it.

Trial 1 2 3 4

Initial cm3 0 0 0 0 0

Final cm3 26.5 25.9 26.2 25.5 26

Titre cm3 26.5 25.9 26.2 25.5 26

Indicator

25.9+.25.5+26=77.4

77.4÷3 = 25.8 average titre

Moles of HCl= 25.8 ÷ 1000 = 0.0258

0.0258 × 0.169 = 0.0043602 moles

Ratio = 1:1

Moles of NaOH = 0.0043602 moles

Concentration of NaOH = (NaOH = Moles ÷ volume)= 0.0043602 ÷ 0.025 = 0.174408cm3

There is a small degree of error as our reading was below the average and it could’ve been because we didn’t measure the 25 cm of the substance correctly and because of different people were reading the measurements. Another error could’ve been because the solution was not exactly filled to the meniscus line meaning it wasn’t accurate. We could of improved the result by having one person taking the results from the burette because then it would’ve been more accurate

0.2-0.174408=0.025592

0.025592/0.2= 0.12796

0.12796 ×100=12.796%

This shows the percentage difference to what the actual result should have been and we could’ve improved this by repeating the investigation again and hopefully get more accurate results.

Calibration of the pH metre-We had to get the pH metre and made sure it was on zero so that we could make it more accurate and then we had to put it into water so we could measure the pH metre and then take readings of it and then it was ready to use for finding the pH for different substances.

Substances 1 2 Averages

Ethanol 2.16 2.22 2.19

Ammonium 9.11 9.10 9.11

Nitric acid 2.24 2.31 2.28

HCl 1.79 1.62 1.71

Sodium hydroxide 0.1m 7.43 7.53 7.48

Sodium hydroxide 1m 12.74 12.85 12.79

Water 6.33 6.44 6.39

Second titration carried out taking pH readings at intervals of 1cm3 – repeat until concordant results are seen at an acidic value

Volume of HCl (1 cm^3)	pH 1	pH 2	Average
0	12.38	12.39	12.385
1	12.32	12.26	12.29
2	12.21	12.13	12.17
3	12.14	12.15	12.14
4	12.13	12.02	12.075
5	12.11	12.01	12.06
6	12.01	11.97	11.99
7	12.04	11.87	11.955
8	11.79	11.80	11.795
9	11.77	11.78	11.775
10	11.73	11.72	11.725
11	8.99	8.97	8.98
12	8.46	8.49	8.475
13	7.95	7.91	7.93
14	7.50	7.42	7.46
15	7.31	7.27	7.29
16	7.12	6.98	7.05
17	6.73	6.61	6.67
18	6.55	6.50	6.525
19	6.37	6.27	6.32
20	6.36	6.15	6.255
21	5.90	5.76	5.83
22	5.67	5.56	5.615
23	5.42	5.11	5.265
24	3.26	3.18	3.22
25	2.14	2.17	2.155
26	1.71	1.64	1.675
27	1.65	1.57	1.61
28	1.51	1.48	1.49
29	1.46	1.42	1.44
30	1.38	1.37	1.375

Moles of HCL = volume × concentration

(16 ÷ 1000) × 0.169= 0.002704 moles

Ratio 1:1 moles of NaOH = 0.00270cm3

Concentration of NaOH = 0.002704 ÷ 0.025 = 0.10816 cm3

0.2-0.10816=0.09184

0.09184 ÷ 0.2= 0.4592

0.4592×100= 45.92%

45.92% was how far our results was off from the actual result which was 0.2. We could’ve improved this by having one person watching the pH change.

There was a degree of error in the measurements as it so it was harder to measure the pH level because the reading was keep on changing and it never stayed on one number which made it harder to get a very accurate result especially when we had to find the pH level twice c for the volume of HCl.

Finding the average measurement for NaOH was more accurate than taking pH readings at intervals of 1cm3 as we got a closer result to 0.2 than the second titration because we got 0.174408cm3 and in the second titration we had got 0.10816 cm3. This could’ve been for many reasons however an example is that we might’ve not let out exactly 1 cm of the solution when doing the investigation which meant the volume of HCl was lower than it had to be. Also we didn’t have one person taking the readings of the moles of the volume of HCl. This would affect the results because people will not see the same things as others so this will not make it as accurate, as if you were to do it with the same person.

We compared our results with another students and the pH readings investigation and the average was 0.19 and this shows that they were more accurate in this experiment as they got really close 0.20. They did this by not having as many errors as what we had and they might have done one little error in the investigation to not get 0.2 which could’ve been not going down by 1 cm exactly in the volume of the HCl. Also for the final results for the indicator their results was 0.18 which was again more accurate and it was closer to 0.2 which means when they did the practical they had very little mistakes. The mistake they could’ve made for that investigation is that they might have had different people taking the results when they colour change occurred and this would’ve made it less accurate.

Colorimetry introduction

We used Colorimetry to find the unknown substance of different chemicals.

Find unknown concentration of copper sulphate

CuSO4 - Mr= 63.5 + 32 + (4 ×16) =159.5

CuSO4. SH2O Mr= 159.5 + (5 ×18) = 249.5

In 1000cm3, 249.5g= 0.1 × 0.1 = 24.95/ 10

100cm3, 2.495g= 0.1 m

The potential errors which could’ve occurred is that there was different groups which making up the standard solution and it might’ve not been accurate as it was different people making up the solution.

Also another error could’ve been when we used a measuring cylinder instead of a pipette which means it might’ve not been as accurate because we can see the measurement more easily and we know how much to pour out.

Making dilutions

Concentration (M) Volume of copper sulphate solution (cm3) Volume of water (cm3)

Method of making dilutions

You make the dilution by adding a 2.495g amount of copper sulphate solution into the measuring cylinder and the pour it into the beaker and then you wash the measuring cylinder. Then you put the volume of water into a measuring cylinder and then put it into the beaker. There is still alternatives to Colorimetry and the other method is to look at the substance by eye but this is the most effective way and the most accurate way.

How does Colorimetry work?

Colorimetry is an analytical tool for determining the concentration of a substance based on absorbance of light with the amount which can get through a sample of pure solvent. For Colorimetry firstly there needs to be passing light the sample and then you have to measure the intensity of light passing through and the more concentrated a solution the less light passed through. The Colorimetry gives a quantitative value of absorbance.

How to use it?

Use the correct filter for the substance

Put a sample of pure water in the cuvette and set it a zero

Put in a variety of standard reference solution and use their values to plot a calibration graph

Put in the test solution and its absorbance

Use the graph to find the concentration.

Calibration of Colorimetry

Water is used as a reference sample and water is very pure so we used water as it will give 100% transmission.

The health and safety issues we to be careful with the copper sulphate solution as it can cause redness to your skin. If you do get the solution on your skin you can put water on to the affected area to prevent itching and redness.

Results of absorbance

Concentration (Mol/dm3) Absorbance nm Transmission %

1	2	Average
0.00	0	0	0
0.02	0.52	0.53	0.525
0.04	0.7	0.71	0.705
0.06	0.58	0.59	0.585
0.08	0.92	0.93	0.925
0.10	0.90	0.91	0.905

Results of unknown samples A, +-B, C and D

Unknown substance Absorbance Concentration Transmission Concentration

Value	Error	Confidence Level	Uncertainty
A	0.14	0.004	95%	0.002
B	0.28	0.008	45%	0.014
C	0.49	0.018	30%	0.022
D	0.82	0.062	16%	0.06

The results for transmission is more accurate than absorbance shown by the unknown substances A and D as they were both 0.02 away from each other in the absorption and transmission concentration. However B and C’s concentration in both absorption and transmission graphs were not that accurate as the differences for both of them were quite big for example B’s concentration difference was 0.06 and C’s was 0.04. We could improve this by drawing a better best line of fit or we could draw the graph again.

Carrying out titration

It was better using the pH metre as it was more accurate and it was less hard to put more of the sodium hydroxide solution and if we had let out too much of the solution the pH wouldn’t be accurate and it wold be harder to measure the exact accuracy. It was easier to use the pH metre as there wasn’t much errors we could’ve done as we had to put the pH metre into the sodium hydroxide.

Our technique when filling the burette was that we put a funnel at the top of the to put the solution into easily so there was no spillage of the solution and we had to measure it up to 25 cm. when we were filling the pipette with the liquid we had to make sure it also up to 25 cm and made sure we filled it up to the meniscus line and no more as it could affect the results. Clearly shown by our results the errors which could’ve occurred were possibly not measuring the meniscus line at eye maybe cause an error. Also, another error could’ve been putting too many drops into the solution which could’ve changed the results.

When we were mixing the solution, it might’ve not been mixed in well as we were swirling it with hand by using a conical flask and there could’ve been a potential error as we might’ve not mixed it in properly. The magnetic stirrer would’ve been much more accurate as the stirrer will stir the solution for the right amount of time and will make the solution mixed properly which will be more accurate.

The pH could’ve been affected due to the poor stirring as the mixture might have not been mixed n properly meaning the times we were taking the pH readings was shortened or not consistent.

A rough titration makes it more accurate as the trial run helps us get an estimate to what the readings will as it will narrow the results. Also it helps us in a positive way because if we did something wrong in a the trial run we will be able to correct that when we actually do the real investigation and there will also be less errors which means there will be more accurate results.

Hand drawn graphs were easy to draw as it was easy to read and easy to find the results from the investigation. However the line of best fit was harder to draw as it wasn’t completely accurate. If I had used a computer service it would’ve been much more accurate as there will no human errors and it will be more accurate with a better line of best fit where you will be to see the anomaly results.

When we were finding the point of neutralisation I think it was hard to find when we were using the pH metre because the number never stayed on 7 which was the neutral number but it was moving up and down so had to take an average. But when we were finding neutralisation for

The electric pipette filler would be much more accurate than a normal pipette as the electronic pipette can measure exactly how much it can hold and it is much easier to use than a normal pipette as you will only have to press a button for how much liquid you want and how much to release. This means there is less room for error meaning there will be more accurate results.