Both ascorbic acid and oxalic acid (active ingredient in Pool Stain Treat) are reducing agents. The first thing they will reduce is the chlorine in the pool to chloride which is why the level should be low when you add them so you do not have to add additional product to reduce the metals. If your FC is 1.5 ppm you should be ok but you might need to add a bit extra product in 48 hours if all the stains have not disappeared. I would add no more than an additional half pound and some extra seqesterant.
FWIW, both ascorbic acid and oxalic acid (and citric acid for that matter) are pretty much interchangeable in terms of effectiveness at reducing metal stains (converting them to a colorless, soluable form that the sequesterant can then 'deactivate'.)
Ascorbic acid is the least harmful of the three which is a big plus in my opinion. It's vitamin C and the LD50 (median lethal dose) for a rat is 11,900 mg/kg; citric acid LD50 (rat) is 3000 mg/kg ; and for oxalic acid (rat) it's 375 mg/kg
From a MSDS for oxalic acid (Mallinckriot Baker, Inc):
3. Hazards Identification
Emergency Overview
--------------------------
POISON! DANGER! MAY BE FATAL IF SWALLOWED. CORROSIVE. CAUSES SEVERE IRRITATION AND BURNS TO SKIN, EYES, AND RESPIRATORY TRACT. HARMFUL IF INHALED OR ABSORBED THROUGH SKIN. MAY CAUSE KIDNEY DAMAGE.
SAF-T-DATA(tm) Ratings (Provided here for your convenience)
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Health Rating: 4 - Extreme (Poison)
Flammability Rating: 1 - Slight
Reactivity Rating: 1 - Slight
Contact Rating: 3 - Severe (Corrosive)
Lab Protective Equip: GOGGLES & SHIELD; LAB COAT & APRON; VENT HOOD; PROPER GLOVES
Storage Color Code: White (Corrosive)
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Potential Health Effects
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Oxalic acid is corrosive to tissue. When ingested, oxalic acid removes calcium from the blood. Kidney damage can be expected as the calcium is removed from the blood in the form of calcium oxalate. The calcium oxalate then obstructs the kidney tubules.
Inhalation:
Harmful if inhaled. Can cause severe irritation and burns of nose, throat, and respiratory tract.
Ingestion:
Toxic! May cause burns, nausea, severe gastroenteritis and vomiting, shock and convulsions. May cause renal damage, as evidenced by bloody urine. Estimate fatal dose is 5 to 15 grams.
Skin Contact:
Can cause severe irritation, possible skin burns. May be absorbed through the skin.
Eye Contact:
Oxalic acid is an eye irritant. It may produce corrosive effects.
Chronic Exposure:
May cause inflammation of the upper respiratory tract. Prolonged skin contact can cause dermatitis, cyanosis of the fingers and possible ulceration. May affect kidneys.
Aggravation of Pre-existing Conditions:
Persons with pre-existing skin disorders or eye problems, or impaired kidney or respiratory function may be more susceptible to the effects of the substance
From a MSDS for citric acid (Mallinckeodt Baker, Inc):
3. Hazards Identification
Emergency Overview
--------------------------
WARNING! CAUSES SEVERE EYE IRRITATION. CAUSES IRRITATION TO SKIN AND RESPIRATORY TRACT.
SAF-T-DATA(tm) Ratings (Provided here for your convenience)
-----------------------------------------------------------------------------------------------------------
Health Rating: 1 - Slight
Flammability Rating: 1 - Slight
Reactivity Rating: 2 - Moderate
Contact Rating: 3 - Severe
Lab Protective Equip: GOGGLES & SHIELD; LAB COAT & APRON; VENT HOOD; PROPER GLOVES
Storage Color Code: Green (General Storage)
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Potential Health Effects
----------------------------------
Inhalation:
Causes irritation to the respiratory tract. Symptoms may include coughing, shortness of breath.
Ingestion:
Causes irritation to the gastrointestinal tract. Symptoms may include nausea, vomiting and diarrhea. Extremely large oral dosages may produce gastrointestinal disturbances. Calcium deficiency in blood may result in severe cases of ingestion.
Skin Contact:
Causes irritation to skin. Symptoms include redness, itching, and pain.
Eye Contact:
Highly irritating; may also be abrasive.
Chronic Exposure:
Chronic or heavy acute ingestion may cause tooth enamel erosion.
Aggravation of Pre-existing Conditions:
No adverse health effects expected.
From a MSDS for ascorbic acid (Mallinckrodt Baker, Inc):
3.Hazards Identification
Emergency Overview
--------------------------
As part of good industrial and personal hygiene and safety procedure, avoid all unnecessary exposure to the chemical substance and ensure prompt removal from skin, eyes and clothing.
SAF-T-DATA(tm) Ratings (Provided here for your convenience)
-----------------------------------------------------------------------------------------------------------
Health Rating: 1 - Slight
Flammability Rating: 1 - Slight
Reactivity Rating: 1 - Slight
Contact Rating: 1 - Slight
Lab Protective Equip: GOGGLES; LAB COAT; VENT HOOD; PROPER GLOVES; CLASS A EXTINGUISHER
Storage Color Code: Green (General Storage)
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Potential Health Effects
----------------------------------
Ascorbic acid is relatively non-hazardous in routine industrial situations. It is not expected to present significant health risks to the workers who use it.
Inhalation:
May cause mild irritation to the respiratory tract.
Ingestion:
Large oral doses may cause gastrointestinal disturbances.
Skin Contact:
May cause mild irritation.
Eye Contact:
May cause mild irritation.
Chronic Exposure:
No information found.
Aggravation of Pre-existing Conditions:
No information found.
As you can see ascorbic acid is the safest, then citric acid, then oxalic acid.
