I gathered this Wood Sorel for tincture from mountain  specimens, in 1995.  
Generally tinctures have a "shelf life" of 2 years if un-contaminated.

The disclaimer:  While I am highly trained in herbalism, I am NOT an accredited medical Doctor.   I present this because I think it is important to "spread the word" and my own experience with it. If you chose to try this treatment, you do so at your own risk. That being said:

The Native American peoples have used Wood Sorrel, "shamrocks”,  masticated and the resultant mixture was placed on the cancerous skin.  I had a skin cancer removed from my face a few years ago, leaving me with an inch long scar on my face.  

As the kindred's Yarb Woman, I was well aware of the treatment, but as it was my first cancer, I opted for surgery.  Be that as it may, just beneath the scar, I've developed a new cancer.  It is now almost completely gone thanks to this herbal remedy.

Take two handfuls of Wood Sorrel and wash lightly.  (Whole plant)  Throw into a blender and puree it for a moment or two.  Do not drain the liquid that results from the puree job, and in the event there is more liquid than solids, one should still use the grain alcohol to preserve it, using (because we are going for strength) a third part of alcohol.  
Place in jar and pour pure grain alcohol over it to cover.  Normally you'd use a 20% Vodka water mixture.  Strain the finished product through paper coffee filters before bottling in large dark "eye dropper" bottles.   Cap and let sit for 3 weeks.  

I scrubbed the cancer (only once!) knocking off the flaky coating then applied this strong mixture 3 to 4 times daily. I'm on my third week of treatment.  The area has "puckered and dried" and last night it fell off!

Oxalis acetosella is toxic in LARGE amounts.  It is, after all, Oxalic Acid, and  folks with gout, gastritis or rheumatism, SHOULD NOT use it internally.  This tincture, used internally at the rate of 30 drops to a cup of hot tea (to evaporate the alcohol) is good to treat fevers.   Itis diuretic in its action and is also used in urinary problems where infection is not a big problem.  A better treatment for  urinary tract or kidney pain and infection is 2/3 decocted dandelion root to 1/3 stinging nettle infusion, blended and given at the rate of 1/2 cup as needed for pain and "scalding urine" OR 20 drops of this blend tinctured. BTW: The Dandelion/stinging nettle tincture is great to treat kidney stones.  

Wood Sorel tincture also treats boils and acne - a salve made of the tincture in vaseline and paraffin is good to treat rashes, but Curly Dock aka "Yellow Dock" is better for cases of rashes from poison Ivy or Stinging Nettle sting. I make a blended salve that can't be beat which contains these two plants (among others) to treat cases of puncture wounds and other slow healing external problems.

The Native Americans also used Wood Sorrel to treat mouth cancers, so, if you dip, chew, smoke tobacco, keep that in mind.