I ran two experiments with two times the same reaction conditions.

(It are chemical reactions, with same temperature, concentration, sampling times,...)

From both experiments I take samples at the same time:

TIME EXP1 EXP2

T1 x11 x21

T2 x12 x22

T3 x13 x23

T4 x14 x24

T5 x15 x25

T6 x16 x26

T7 x17 x27

T8 x18 x28

Now I measure from each sample a property (concentration), 3 times

So for example for x11 I measure:

0.9 0.6 0.8 (mol/L)

Now I can calculate the average of these measurements and the standard deviation.

**But how do I calculate the standard deviation of the value at T1 given both experiments?**

What I do now is:

1) Take averages of 3 measurements of x11

2) Take averages of 3 measurements of x21

3) Calculate standard deviation of averages

So I have deviation caused by an error on measuring the concentration (repeated 3 times) and by replicating the experiment (repeated 2 times). I have a feeling that the 3 steps above do not represent the deviation caused by the error on measuring the concentration. Especially if there is a large spread on the measurements of one of the samples.