Replacing Phosphates in Detergents

From the Sydney Morning Herald (Australia) July 17, 2011, "IN THE latest round of supermarket ''me-too-ism'', Coles and Woolworths have backed a plan to rid shelves of environmentally damaging detergents.

The supermarkets have pledged to make their home brand laundry detergents phosphate-free by next year, a year ahead of a pledge this month by discount supermarket chain Aldi to ban the chemicals, which have been linked with damage to waterways and marine life."

Sodium phosphates have often been added to detergents as a builder, or water softener. Builders are chemical compounds that remove calcium ions from solution by complexation or precipitation.

The sodium phosphates that have been used as builders include orthophosphates and complex phosphates:

1. Orthophosphates which precipitate out metallic ions such as calcium:
• trisodium phosphate, Na₃PO₄
• disodium phosphate, Na₂PO₄
2. Complex phosphates which produce metallic complexes with metallic ions that do not necessarily precipitate out of solution:
   
• tetrasodium pyrophosphate Na₄P₂O₇
• sodium tripolyphosphate (STPP) Na₅P₃O₁₀
• sodium tetraphosphate Na₆P₄O₁₃
• sodium hexametaphosphate (NaPO₃)₆

Sodium tripolyphosphate (STPP), the main detergent phosphate, is something of a wonder ingredient for detergents, helping to maintain pH, remove food and grease, inhibit corrosion, and suspend insoluble dirt. For the consumer, its main visible benefit was to reduce spotting and filming by sequestering calcium and magnesium ions in the wash water.

Sodium phosphates, which can make up to 50% of the weight of a detergent, can lead to problems with eutrophication of lakes and streams, resulting in the growth of algal blooms, killing fish and plants.

On July 1, 2010, major cleaning product manufacturers finished removing phosphates from all home automatic dishwasher detergents sold in the U.S. a result of new laws in 16 states, but consumers living in areas with hard water were not happy with many of the new phosphate-free products. Phosphate alternatives such as zeolites leave residue on dishware while citrates are expensive and don’t work as well.

In order to replace the effectiveness of phosphates, a long list of additives is used which can include:

• sodium citrate which helps maintain the proper pH level of the detergent helps immobilize soils that have been removed during the wash
  
• polyacrylates which are polymers designed to bind with calcium and magnesium ions, allowing the detergent to better perform
  
• tetrasodium etidronate which is also used as a water softening agent
  
• phosphonates which are also a water softening agents, but, although they contain phosphorus, the toxicity of phosphonates to aquatic organisms is low.
  

Reference:

http://www.smh.com.au/environment/conservation/phosphates-are-all-washed-up-20110416-1dijs.html#ixzz1JkEcsejm

Further Reading

Detergents

Naming Ionic Compounds

Writing Ionic Formula

Molecular Mass Calculations

Percentage Composition

Concentration (molarity)

Concentration (ppm)

Study Questions

1. Calculate the molecular mass (formula weight) of each of the following compounds:
• trisodium phosphate, Na₃PO₄
• disodium phosphate, Na₂PO₄
• tetrasodium pyrophosphate Na₄P₂O₇
• sodium tripolyphosphate (STPP) Na₅P₃O₁₀
• sodium tetraphosphate Na₆P₄O₁₃
• sodium hexametaphosphate (NaPO₃)₆
2. Calculate the percent by mass of phosphorus present _{in each of the following compounds:}
   
• trisodium phosphate, Na₃PO₄
• disodium phosphate, Na₂PO₄
• tetrasodium pyrophosphate Na₄P₂O₇
• sodium tripolyphosphate (STPP) Na₅P₃O₁₀
• sodium tetraphosphate Na₆P₄O₁₃
• sodium hexametaphosphate (NaPO₃)₆
3. Write the formula for each of the following
   
• tripotassium phosphate
• dipotassium phosphate
• tetrapotassium pyrophosphate
• potassium tripolyphosphate
• potassium tetraphosphate
• potassium hexametaphosphate
  
4. What do each of the following prefixes mean?
   • di
   • tri
   • tetra
   • hexa
   • poly
   
   
5. Assuming a particular brand of detergent, ABC, contains 35% by mass of sodium tripolyphosphate (STPP) Na₅P₃O_{10, how much phosphorus would be present in 10 grams?}
6. _{If 10g of ABC were placed in a dishwasher which used 60L of water to wash the dishwashers, what would be the concentration of ABC detergent in mol/L?}
7. _{Using the information in question 4, calculate the concentration of phosphorus in the wash water, in parts per million.}