I have been searching in books, fora and articles but I can't seem to find a clear cut answer to the following question:

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.