The following is a short differential comparison of the acid remedies as they are found in the homeopathic books and materia medica of Boericke , Clark , and Lesser . Further information comes from the “Homöopathisches Repetitorium” by the DHU , and from personal notes that were collated over the years.
There are a number of commonalities that the acid remedies have. So for example all of the acids have a weakness, exhaustion or collapse about them. They also have the theme of body odour. This can be bad breath, sweat, the scent of excretions etc. Most acids have profuse sweating.
There are specific action radii of the acids, that is they have particular curative action on specific spheres. So for example they are indicated for rheumatism, gout, arthritis, haemorrhage, the uric diathesis and diabetes. There are three major body systems that these remedies act upon, but they are not restricted to these:
Gastro-intestinal tract: Acet. Ac., Carb. Ac., Hydrofluoric Ac. (Fl. Ac.), Hydochl. Ac. (Mur. Ac.), Lac. Ac., Ox. Ac., Nitr. Ac., Sulf. Ac..
Musculo-skeletal system: Benz.Ac., Fl. Ac., Form. Ac., Lac. Ac., Ox. Ac., Sal. Ac., Sulf. Ac.
Central nervous system: Carb. Ac., Hydr. Ac., Phos. Ac., Picr. Ac., Sal. Ac..
Some indications for the acids follow below:
Acet. Ac.: Acts primarily on the gastro-intestinal tract (gastroenteritis). The patient is usually lean and has a pale complexion, almost wax-like. There is emaciation and great weakness, great thirst, hyperacidity and profuse sweating. Patients may have anaemia. There may be a tendency to fainting and collapse, debility, wasting, and dyspnea. It is a remedy for diabetes. It may be indicated for nodule formation in the joints.
Benz. Ac.: A remedy for the rheumatic-uric acid diathesis, but also for musculo-skeletal afflictions. Indicated for gout, arthritis (deformans). Pains are tearing and stinging. A marked symptom of this remedy is pain in the achilles tendon. There may be swelling. The pains may move from top to bottom and from right to left. This remedy is not a primary therapeutic for rheumatism, it is rather indicated for rare cases where it is a precise match. Sweating does not improve any symptomatology. A main indication is the sharp, offensive odour of urine (like horse urine). The colour of the urine is dark brown or dark red. Worse for movement and cold.
Carb. Ac.: Its action is on the mucous membranes, the central nervous system and afflictions of the gastro-intestinal tract. As such it is indicated for septic, gangrenous processes, ulcers and carcinoma. Arthritis. Hyperacidity. It acts as an acute Kreosotum . There may be paraesthesisa, confusion, collapse. Frequently there are neuralgias that come and go suddenly. Headaches are experienced as if of a bandage tied around the head. There is itchy skin. The patient has a marked acuteness of smell. Secretions are fetid and acrid.
Fl. Ac. (Hydrofluoric Acid): Its radius of action ranges across the skeletal and muscular apparatus and the skin. Also affected are the vessel and lymph system. This remedy is for deep seated, destructive processes as are ulcerations, varicosis, caries. It is one for the complaints of old age . Indications are sour eructation, gastritis (symptoms are improved by tight clothing), arthrosis, alopecia, itching of the orifices, profuse offensive perspiration. There is restlessness and hurriedness in the patient. The patient may be emaciated despite good appetite. All is worse for heat, better for cold.
Form. Ac.: The musculo-skeletal apparatus lies in focus of this remedy, but there are also indications of the asthmatic, allergic diathesis (excema, asthma bronchiale). Rheumatic pains evolve from left to right and disappear from right to left. There are degenerative joints, gout and arthritis. Usually there is a sudden onset of symptoms on the right side. This remedy can ‘re-tune’; completely alter the patient state. There is sensitivity to cold and wet, and much urination. Worse: cold & wet. Better: warm & pressure.
Hydr. Ac.: This remedy is for afflictions of the central nervous system, such as for indications of paralysis and convulsions (hysteric, tetanic, epileptic). This remedy is for the sudden, severe attack and not the lasting state. There can be spasmodic constriction in the oesophagus with the audible sensation of drink passing down. The feeling of constriction in the larynx creates the feeling of suffocation. It is indicated for angina pectoris, apoplex, asthma attacks, and sunstroke with the sensation of a cloud over the brain. In collaps there is cyanosis, icy coldness, dyspnea and precordial anxiety.
Lac.Ac.: It impacts mainly on the gastro-intestinal sphere and the musculo-skeletal system and is frequently used for acute rheumatic affections and for diabetes. In rheumatism there is excessive sweating. One marked symptom may be excessive, non-offensively foot sweat. In diabetes there is increased frequent urination particularly at night. This remedy may also be indicated in morning sickness, nausea with acid eructations. Eating improves these symptoms. Hyperacidity occurs with increased salivation.
Mur. Ac. (Hydrochloric acid): There is focus on the gastro-intestinal tract. Further indications are for the haemorrhagic diathesis, in haemorrhoids, gastritis and affections of the skin. The patient expresses extreme weakness, there is numbness and a tendency to collapse. The patient may be sensitive to sunlight, may experience a dry mouth with sticky saliva, there is offensive bad breath. Diarrhoea will be sour and fetid. The patient may complain of stitching about the heart.
Nit. Ac.: Acts on the gastro-intestinal sphere and the urinary organs. There is inflammation at the mucous membranes to skin with tendency for ulceration. Pains reported are sting-like. There may be gastritis or haemorrhoids. There is great general weakness during which the patient is sensitive to sound/noise and physical contact. Urine is cold and offensive (like horses urine). The patient feels worse at night and during hot weather and feels better when driving or horse riding.
Ox. Ac.: This remedy acts mainly on the gastro-intestinal system but also has a marked effect on the spine. It is indicated in motor paralysis, collapse, stupor , neuralgia and rheumatism of the left side. There are violent cutting pains in small circumscribed areas. Symptoms are of periodic recurrence. The symptoms worsen if the patient thinks about them and in movement. The patient may experience palpitations when lying down.
Phos. Ac.: focus is on the central nervous system. There is debility and mental exhaustion that is followed by physical exhaustion. The patient expresses weakness of memory, apathy and lethargy. This may come after acute disease, grief, excess. Other indications are alopecia, gastroenteritis and diabetes. It relieves the pains of cancer. The patient is worse for mental and physical exertion and better for warmth.
Picr. Ac.: The main action radius of this remedy is the central nervous system. Mental exhaustion comes with headaches that have a bursting character that is comparable to Glonoine. The nervous exhaustion is severe almost to a complete loss of energy (prostration) and mental burn out (Brain fag). The patient is extremely tired is unable to concentrate, may suffer of sleeplessness and vertigo. The patients mood may be depressive. Further indication is spinal degeneration with paralysis. Pains have a burning character. There is a sensation of ‘pins and needles’ in arms and legs. In uraemia there may be complete anuria. Mental effort worsens, as does suns heat. Improvement comes with cold air, firm pressure on the head and lying down.
Sal. Ac.: The central nervous system and the musculo-skeletal apparatus are in focus of this remedy. There are indications for acute joint rheumatism as well as muscular rheumatism, for neuralgia, vasomotor syndrome and Meniere-syndrome. There is a blood rush to the head, vertigo, ringing and buzzing in the ear (tinnitus), deafness. The state is worsened at night, by movement, touch, cold, from suppressed foot sweats, and is improved by warm applications and dry heat.
Sulf Ac.: Action is on the musculo-skeletal system and gastro-intestinal tract. Indications are: Gastritis, rheumatic pains, respiratory complaints, the haemorrhagic diathesis. There is bad breath, sour eructations, and increased salivation. In the weakness and exhaustion there is not visible trembling in the patient. The patient has exaggerated hurriedness. Sweats are sour and sticky. The patient feels worse in cold and heat and in the mornings and better for warmth.
Tart. Ac.: There is a gastro-intestinal focus with intense burning in the throat and stomach. Vomit may have a green colour. The patients tongue may be dry and brown. Diarrhoea is associated with weakness. There is marked pain in the heels with a tearing sensation in the soles of the feet.. Weakness is particularly prevalent in the evening, with dullness and lassitude. There may be paralytic debility.
Tan. Ac. (Tannic acid): This remedy has excessive secretion from the mucous membranes. Pain in the abdomen is sensitive to pressure. There may be obstinate nervous coughs or constipation. Indicated also for haemorrhages.
Gall. Ac. (Gallic Acid): Indicated for phthisis , passive haemorrhages, itching skin.
 Boericke, W. (2004) Pocket manual of homeopathic materia medica and repertory New Delhi: B.Jain publishers Ltd.
 Clarke, J. (1994) A Dictionary of practical materia medica New Delhi: B.Jain publishers Ltd.
 Lesser. O. (1983) Textbook of homeopathic materia medica New Delhi: B.Jain publishers Ltd.
 DHU (2011) Homöopathisches Repetitorium Karlsruhe: Deutsche Homöopathie Union.