The Sensitivity of Benedict's Test-Investigation
In this investigation, we will be attempting to find the lowest
concentration of glucose that Benedict's can detect.
I predict that that the 10% glucose solution will have the darkest
colour dark after testing for reducing sugars whereas the 0.001%
glucose solution will have the lightest colour. This is because as the
percentage of glucose is increased the darker the colour will become
meaning that there is more glucose present. I have predicted this
after finding out some information about the Benedict
Benedict's solution contains a weak solution of copper II sulphate.
Reducing sugars can reduce this to a precipitate of brick red copper I
10% glucose solution Benedict's solution
Distilled water Wax pencil
10cm3 graduated pipette 10cm3 measuring cylinder
5cm3 syringe Teat pipettes x2
5 boiling tubes and rack 5 cuvettes and holder
Colorimeter Boiling water
1) Label 5 boiling tubes 1 to 5 with a wax pencil near the top of each
2) Pipette 10cm3 of 10% glucose solution into tube 1.
3) Transfer 1cm3 of this solution from tube 1 to tube 2. Using the
measuring cylinder add distilled water s as to prepare, in tube 2, a
1% glucose solution. Mix well.
4) Using firstly 1cm3 of the solution from tube 2, go on to prepare
three more glucose solutions systematically: 0.1% (tube 4) and 0.001%
5) Finally, adjust so that all five tubes contain an equal volume of
6) Using the syringe add 5cm3 of Benedict's solution to each tube.
7) Transfer all five tubes to the boiling water bath provided for
exactly three minutes.
8) Remove and return to the rack.
9) Make a subjective comparison of the colours in each tube by
observation. You can use a scale of '+' to '+++++' to record the depth
of colour, from lightest to darkest.
10) Use the colorimeter to measure the percentage transmission of each
of your samples.
11) Tabulate all your readings and present them in a suitable graph.
Table of results
Results of groups
Concentrations of glucose
Concentrations of glucose (%)
Colour present after heating with Benedict's solution
From this table you can see that the 10% glucose solution had the
darkest colour dark orange after testing for reducing sugars whereas
the 0.001% glucose solution remained blue. This suggests that as the
percentage of glucose was increased the darker the colour became
meaning that there was more glucose present.
From the graph of average percentage transmissions of glucose in each
sample, we can see that the graph has formed a smooth curve indicating
as the concentration of the glucose increases so does the percentage
transmission of the colour. The results of the experiment did show the
correct results as predicted. However, there was an anomaly in the
results. In group three after testing the percentage transmission of
glucose sample 0.01% using a colorimeter, the result was 0.07. After
testing this for the 0.001% glucose sample, the result was 0.08. This
was not supposed to happen and in actual fact the 0.01% sample of
glucose should have had a higher percentage transmission than the
0.001% sample of glucose not the other way round. This suggests that,
there was an error in the experiment. This may have occurred because
of a reading error on the colorimeter or it may have been because the
glucose samples had been mixed up (s explained in the evaluation).
I believe the experiment was accurate as the same volume of liquid was
used in each sample. This was also true for the benedict's solution.
Therefore, it was a fair test. However, it was not a complete fair
test as the experiment was only carried out once, therefore the
results lacked reliability. To make this experiment more accurate we
should carry it out at least three times, so the results can be
There were many small errors that occurred throughout the experiment
affecting its results. Firstly when measuring the distilled water
there may have been inaccuracy reading the measuring cylinder posing
inaccurate results. Also, when we were using the pipette to put the
10% of glucose solution into the boiling tube and transferring a
sample into other boiling tubes there were air bubbles in it, so the
amount of glucose may not have been accurate. Therefore, there may
have been different volumes of liquid in each boiling tube.
Another problem aroused by the anomaly of the experiment as described
in the conclusion. This could have occurred because when we put the
samples of glucose into cuvettes after heating them with benedict's
solution present in them the two end samples of 0.01% and 0.001% were
similar colour. As some groups, when putting them in the colorimeter
may not have labelled the cuvettes, they may have been mixed up
producing these results. There was also the problem of the percentage
transmissions on the colorimeter fluctuating, therefore there were two
or three readings, and so a 100% accurate reading was not taken.
Finally another problem was time keeping. When we placed the boiling
tubes in the boiling water bath it was impossible to put them all in
at the same time and the same situation occurred when taking them out.
Therefore, the time when each boiling tube went in and came out was
different, maybe causing some glucose samples having more time to
This test was quantitative as we measured the cloudiness (turbidity)
of the solution using a colorimeter. To extend this investigation
making it quantitative we could also filter and weigh the precipitate
produced. This would give us an indication of the measurement of the
amount of sugar present in the sample.
-Make sure a lab coat is worn, so that if any chemicals spill on you,
they will not ruin your clothes and even more importantly will not
harm you, or irritate your skin.
-If a substance touches your hands or skin, wash it immediately as it
may be irritant or corrosive in turn harming your skin. If it does
irritate tell you teacher.
-Be careful of fragile equipment, especially glass. Keep equipment
away from the edge of surfaces to prevent glass from breaking and any
chemicals from spilling.
-Tuck in stool underneath tables and make sure bags and coats are hung
up so pupils do not trip up or get hurt.
-Follow instructions carefully and if you are not sure ask the teacher
to help you to prevent you from doing anything wrong.
-Be careful of the boiling water bath, as it will be hot. Do not
directly put your finger in there, as it will burn them.
-Tidy away equipment making sure used equipment is not mixed in with
clean equipment, as this will affect other pupil's results when they
carry out the experiment.