A Modified Procedure for the Determination of Phosphorus in Detergents

4.1 Weigh out 2.5 ± 0.1 g detergent to the nearest 0.001 g Record weight (w).

4.2 Transfer the sample to a 250 ml Pyrex 'Berzelius' type beaker. Place the beaker in a cold muffle furnace. Heat the sample to 550°C and leave at this temperature for 30 minutes. Cool to room temperature and cautiously add 10 ml of Hydrochloric Acid. Cover with a Fisher "Speedyvap" watch glass and evaporate to dryness. Cool and repeat the HCl addition and evaporation two additional times. After the third evaporation, continue to heat for a further 20 minutes to ensure complete dehydration of SiO₂. Cool and proceed with section 4.4 if it is known that the sample does not contain Borate or Perborate.

4.3 If Borate or Perborate are suspected in the sample, prepare a solution of 90 ml Methanol and 10 ml Hydrochloric acid. Add 10 ml of this solution to the sample, heat the sample on a shaker hot plate or steam bath and ignite the methanol vapour. If the vapour burns with a green flame, borate is present and must be removed before proceeding. If no green flame is observed, evaporate to dryness and proceed with section 4.4. To remove borate and perborate add to the sample 200 ml methanol, 10 ml Hydrochloric acid, and 3 or 4 hollow glass beads. Cover the beaker with a watch glass (Fisher "Speedyvap") and boil down to 20 ml on a steam bath. (The boiling time must be at least 30 minutes). Evaporate down to less than 10 ml on a steam bath under a stream of nitrogen or clean dry air.

4.4 Add 90 ml of deionized water and 10 ml Hydrochloric acid (by pipette) to the sample. Cover with a watch glass and boil gently for 30-60 minutes. All phosphate must be in the ortho form. Cool to room temperature.

4.5 Dilute to 200 ml, place on an electrometric titration stand and neutralize to pH of about 3.0 with 1 + 1 NaOH added from a burette, (a polyproplyene burette is found to be satisfactory). Record the volume of 1 + 1 NaOH used (V). As necessary cool the beaker during this neutralization to maintain temperature below 300 C.

4.6 Add the 0.25N NaOH in 0.25 ml increments, recording the pH and volume added after each addition. Plot the pH change (ΔpH) against volume of NaOH titrant added, and determine the titre between the two end points (T). Two typical curves are shown in Figures 7 and 8, which illustrate this procedure.

4.7 If a number of detergents are being run at the same time calculate the mean volume of 1 + 1 NaOH added to them (V mean).

4.8 To calibrate the 0.25N NaOH and other reagents; weigh out 0.24 ± 0.01 g KH₂PO₄ to the nearest 0.0001 g, place in a 250 ml beaker, add 200 ml water, add 8 ml Hydrochloric acid and boil gently for 30 minutes. Cool to room temperature and add the mean volume of 1 + 1 NaOH (V mean) used to neutralize the detergents. Cool and place on an electrometric titration stand and bring to a pH of about 3.0 with concentrated HCl. Carry out this procedure in duplicate.

4.9 Proceed as in 4.6 with each of the duplicates from 4.8 and calculate the mean titre for a weight of KH₂PO₄ of exactly 0.240 g. Record as T''.