The rule of thumb for STPP is to use about 1/3 STPP to 2/3 detergent or soap. By weight or volume, it's not that critical, but 1/2 cup sounds a bit excessive. I would add the STPP before a soap, so as to tie up any hard water minerals that might bind with the soap and cause soap scum. However I've added both simultaneously in moderately soft water with no problems.

Phosphates may darken aluminum. As long as you don't see any pitting, it should be ok.

The use of phosphates in residential laundry may be illegal in your area. Check first.

Maryland Environment Code 9-1501 et al.
1. Restrictions: A person may not use, sell, manufacture, or distribute for use or sale any cleaning
agent that contains more than 0.0% phosphorus by weight expressed as elemental phosphorus
except for an amount not exceeding 0.5% phosphorus that is incidental to manufacturing.
 

Definitions: "Cleaning agent" means a laundry detergent, dishwashing compound, household
cleaner, metal cleaner, degreasing compound, commercial cleaner, industrial cleaner, phosphate
compound or other substance that is used or intended to be used for cleaning purposes.

I don't know how one could conclude that STPP is not a cleaning agent. Since it is a cleaning agent when used to help clean laundry or dishes, usually combination with another cleaning agent. And please note the Maryland law specifically mentions "phosphate compound" as a cleaning agent. STPP is without a doubt a phosphate compound.

As for states, most that regulate phosphate mention only the mfg, sale, distribution. However the following states also regulate the use of phosphates: Illinois, Maryland, Rhode Island, South Carolina, and Vermont. Check the link out for most information.

However, I doubt that anyone is going to get arrested for adding an ounce or two of STPP to one's laundry or dishwasher. But the laws appear to be on the books in these states. And the Maryland environmental code provides for up to $25,000 civil penalty for every day a violation occurs.

Well, if you read the link I provided, you'll see that Pennsylvania is one of those states that doesn't specifically mention "use" in its regulation of phosphates. Thus I'd say that as long as you're not manufacturing, selling, or distributing, it would be legal there to add some STPP to your wash.

However I am not a lawyer nor do I portray one on TV. This should not be considered to be legal advice.

That STPP is considered by the FDA to be a Food Additive. The Same goes for TSP as well. 

STPP is used heavily in Fish Packing & Processing. And TSP can even be found in Cheerios. 

The ingredient itself isn't banned or restricted. It's the way it's "mixed" and made into products that is restricted. So... Companies like SC Johnson or Procter & Gamble can't just throw a dash of the stuff into their products. 

Now. If STPP was being used on a massive level... perhaps knowingly by a dry cleaning chain or in a hospitality environment.  Fines would probably be imposed & perhaps, even legal action against a certain individual.  

However..

You've got to keep in mind. Farmers & Companies purchase these chemicals on massive scales, for use in meat packing, and industrial fertilization.  It's just not a industry with any oversight. 

I personally wouldn't worry or let it keep you up at night. No Police Department is going to investigate you.. for adding some phosphate into your laundry. They simply don't have the manpower or resources to even look into such a case. And that's assuming they wouldn't burst out laughing, being informed of the situation ;)

No idea what "low density" means vis a vis STPP. Usually there's technical grade (OK for laundry) or food grade (better than needed for laundry). Maybe the low density is puffed up like rice krispies?

Like most chemical powders, STPP is available in various particle densities, such as light, medium, and heavy. The light density powder occupies the most volume per unit weight. It is also the fastest dissolving. So, while it may be more bulky than the heavier particle density grades, it's probably the best choice for use in a washer where fast dissolution is desirable.

It's important to remember that despite the density differences, these different grades are essentially the same chemical: Na5P3O10. It's just that a pound of the light will contain more air spaces than pound of the heavy.

I hope this answers your question.

Again, the light density STPP is probably the best for use in laundry.

I hope this answers your question.

As a rule of thumb, if you mix a standard non-phosphated powdered laundry detergent with STPP, a good ratio to use is two parts detergent to one part STPP. This works out to 33% STPP by weight. It also works out to about 8% phosphorus element by weight, which is typical for older phosphated detergent formulations.

You can get a large plastic lidded food storage container at many food service stores like Smart & Final, or at hardware/variety stores. These can be perfect for storing larger quantities of pre-mixed detergent formulations.

The larger plastic bins that some powdered detergents come in often can accommodate another 33% by weight of STPP.

I use a thorough mixing technique.

Once thoroughly blended, it's very convenient to add the mixture to the washer, with one measure, and not have to worry about proportions.

Even vintage detergents often contained both washing soda and STPP. The benefit of the washing soda is that it increases the pH more than STPP by itself. The washing soda became important as a way to hold the phosphate level to no more than 8% elemental phosphorus by weight.  Older formulations often also contained copious amounts of sodium sulfate, which has little to no function other than to bulk up the product and perhaps make it pour easier. "Ultra" formulations simply eliminate the sodium sulfate, with little to no functional impact, other than requiring ounces rather than cups for proper wash load dosing.

For HE or front loader machines, a potential advantage of "cutting" the detergent with STPP is that it can lower the amount of surfactant, which in turn lowers the sudsing, while maintaining maximum build/break/water softening capability. If one is concerned about the suds level being too low, one can always add more of the mix, say, 1/3 more than one usually might add. That way the proportions would be no different than if one were to add, say, 1 oz of original detergent and 1/2 oz of STPP, by weight. Do you have a suitable scale in your laundry room for that?

None of this, however, is all that critical. If one adds 20% or 40% STPP I doubt it will make much difference in the wash results. The important thing is to add enough of the total mix to loosen and suspend all the dirt and allow it to be rinsed away.

I have not noticed that STPP makes synthetic detergent suds up more. That is probably because synthetic detergent surfactants are not susceptible to inactivation for hard water/soil minerals.

It might aid in suds level of natural soaps because it can prevent creation of soap scum which inactivates soap. But then washing soda would do much the same, except with a precipitate as the product.

In my experience home laundry is a trial and error process. The amounts listed on the products are just a guideline, and even those say add more or less depending. Which is a reason why I don't care much for gel packs and tablets for laundry purposes. Although they do seem to work OK for dishwashers, especially those with multiple pre-rinses to remove as much food/grease/etc as possible before the addition of the detergent.

I remember back inthe 50's my Mom made her own soap (from bacon drippings, mostly, and it was evil smelling brown soap) and then used to in the Bendix. That was OK until my brother attempted to help out by adding even more soap. Cue in the footage of Mom running to the washer on its big cement block in the basement, trying to stem the tide of suds emerging from every washer orifice.... LOL...

In any case, I've grated Ivory Snow, added some STPP first to the washer, and then the grated Snow, and had relatively good results with manageable sudsing in the Neptune 7500 as long as the dosage was sparing.

Likely the brown is from oxidized fatty chains in the soap. It probably has reduced cleaning ability.

As for the glycerine, I was under the impression that Ivory Soap doesn't have any. I might be wrong, though. As for the NaOH, the STPP should provide sufficient alkalinity to for wash purposes.

If I'm not mistaken, vintage Calgon that contained STPP was not 100% STPP. It was more like 50/50 STPP plus washing soda.

For a top load washer I'd probably add 1-2 oz of STPP. More, or less, depending on water hardness and laundry soil level.

All this talk of STPP has got my curiosity piqued, so I ordered a couple pounds to play with and see if it lives up to everyone's comments. It should arrive this week. I've got some shirts with light stains that I'm going to try it out on. Fingers crossed.

Might have been said before, but older Ivory bar soap didn't contain glycerin. It was rinsed away during the manufacturing process and consequently I often heard complaints about Ivory being "too drying".

I haven't bought any for some years now, but the stash I have seems to be glycerin free. I can't speak for what's now on the shelves, but it may be the company decided it needed to add back in glycerin and/or other moisturizers in order to compete with the likes of Lever 2000 and other popular bar soaps.

The older Ivory bar soap, when grated, worked fairly well on whites, esp when boosted with STPP.