How do you take the natural logs of the concentrations (in excel) ; this should produce a straight line.

time(hours)	concentration(mg/l)
0	13.08430802
1	12.17320516
2	11.04200922
3	12.34268145
4	10.73857835
5	10.63192624
6	10.2781822
7	11.17999925
8	9.775810352
9	9.109435305
10	11.15556364
11	8.708439334
12	8.06769449
13	8.989223484
14	7.839470492
15	8.673972237
16	8.073653842
17	7.457722515
18	6.756976344
19	7.104036733
20	7.279477751
21	7.032597867
22	6.843017934
23	6.077613303
24	5.748752021
36	4.252647751
48	2.776772022
60	1.928790001
72	1.366973892
84	0.975944097