All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ʻalayhi wa sallam, is His slave and Messenger.
According to the majority of the scholars, intoxicating alcohol is impure (najis), and this is the scholarly view adopted at Islamweb. It is impermissible to mix medicine with it. The fatwa of the Permanent Committee for Islamic Research and Fatwas reads, “It is not permissible to mix medicine with intoxicating alcohol. However, if it is mixed with alcohol, then it is permissible to use it if the percentage of alcohol is so small that its effect does not show in the color, taste or smell of the medicine; otherwise, it is impermissible to use what is mixed with it.”
The percentage mentioned in the question (70%) is definitely a high percentage and entails declaring this medicine mostly an intoxicant and not a tincture. Using this tincture as medicine is in fact using alcohol as a medicine. If the tincture that you produce contains 70% alcohol, then it is impermissible to use it as medication except in case of a compelling necessity, and it is prohibited to sell or buy it as well.
Verily, Allaah would not make our remedy in what He has prohibited for us. Umm Salamah narrated that the Prophet, sallallaahu ʻalayhi wa sallam, said, “Allaah did not make your cure in what He has deemed prohibited for you.” [Al-Bayhaqi - Ibn Hibbaan graded it saheeh (sound) as Al-Haafith Ibn Hajar said in Buloogh Al-Maraam]
This hadeeth was also narrated by Al-Bukhari, as a Mu’allaq report (i.e. its reporter omits the whole chain of narrators and quotes the Prophet, sallallaahu ‘alayhi wa sallam, directly) in an authoritative manner on the authority of Ibn Masʻood .
Also, Abu Hurayrah said, “The Messenger of Allaah, sallallaahu ʻalayhi wa sallam, forbade the evil medicine.” [At-Tirmithi and others - Al-Albaani graded it saheeh (sound)] Some versions of the hadeeth interpreted the meaning of “evil medicine” as poison. Al-Haakim interpreted it as alcohol (intoxicants). The preponderant view is that it includes all evil substances: poison, intoxicant, or anything else.
Allaah knows best.