Protocol Online logo
Top : New Forum Archives (2009-): : General Biology Discussion

Finding GLUCOSE concentration ...Values for dilutions to make standard graph? - Not understanding the logic here! Sure, its simple thing to you (Mar/13/2011 )

Lab exercise on Finding Glucose Concentration:

We are given a 20 mM(micro-molar) solution of Glucose. Our tutor asked to chose several dilutions to make a standard graph of concentrations(x-axis) versus absorbance (y-axis) so we could compare this with another calibration graph when we try to find concentration of glucose in a sample of blood.

For standard curve, what we did was we got dilutions like this and measured the absorbance:

I ABSOLUTELY DON'T UNDERSTAND THE LOGIC HERE...WHY IS THERE 0.1, 0.2 and why is this value going up by 0.1 and why until 1.0?? And why is a value of 0.4ml glucose corresponding to 8mM Glucose Solution Concentration?

How is this working...? (i know the whole thing to calculate the concentrate from a sample using absorbance from graph and calculating the concentration...but I just don't get the whole idea of this dilution and how they link with glucose concentration...i just don't get these figures for glucose and water concentration and why they need to make up 1ml and why not 2ml or 3ml...?!?!?!

Any help in explaining this would be a massive favour...thanks a lot in advance.

Tube no. Glucose Solution Conc. (mM) Glucose (ml) Water (ml) Absorbance
1 0 0.0 1.0 0.020
2 2 0.1 0.9 0.035
3 4 0.2 0.8 0.053
4 6 0.3 0.7 0.067
5 8 0.4 0.7 0.088
6 10 0.5 0.6 0.096
7 12 0.6 0.5 0.112
8 14 0.7 0.4 0.143
9 16 0.8 0.3 0.164
10 18 0.9 0.2 0.167
11 20 1.0 0.1 0.182



First thing to keep in mind is that concentration in this case is an amount of substance in volume of the solution.

Let's say - if you start with 20 mM and add same volume of water you have same amount of substance but twice the volume = half of the starting concentration.

Making a standard curve you take different volumes of standard solution (ie. amount of sample) and add different volumes of water to get different result concentrations.

The final volume doesn't have to be 1 mL (and btw - it isn't for some of the tubes...) but it is done this way for practical reasons (ie. volume of cuvette) and because it is easier to understand the principle this way.


K.B. on Mon Mar 14 08:35:22 2011 said:

The final volume doesn't have to be 1 mL (and btw - it isn't for some of the tubes...)...

i think that was caused by a typo.

and mM=millimolar (not micromolar)

(from your friendly neighborhood nitpicker)