358 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS DETERMINATION OF PHOSPHATE AND BORATE For the determination of phosphate it is necessary to remove or destroy all surface active matter, which would otherwise vitiate the experiment. A sample of the detergent, containing 0.7--1'0 g P205 and about 0.05 g B203 is ashed. The phosphate is hydrolysed by boiling with dilute nitric acid. The pH is then adjusted to 4.5 by careful neutrahsation with N sodium hydroxide, so that the phosphate is present as H2PO4-ions. 100 ml of 10•o silver nitrate solution is added. An equilibrium is set up: H2PO4--k3Ag+•Ag3P04 • q-2H + This is a true equilibrium, appreciable concentrations of all four com- ponents being present. If the liberated acid is titrated with N sodium hydroxide the equilibrium shifts continuously to the right, and at pH 6'0 neutralisation and precipitation are complete. Because the acid being titrated is a strong one (nitric acid), the pH at the end point is virtually independent of the concentration. Each ml of N alkali is equivalent to 0'0355 g P•Os. The borate, which is equivalent to the perborate used in manufacture, is then readily determined. The silver phosphate is filtered off. Excess sodium chloride is added and the precipitated silver chloride is also filtered off. The pH of the tiltrate is adjusted to 5.5 with 0.1 N alkali, glycerol equal to about half the volume of the solution is added, and the boric acid titrated to pH 8'5 with 0.1 N alkali. Each ml of alkali is equivalent to 0.01539 g of NaBOa.4H•O. The surviving perborate in the finished product is determined on a fresh sample, either by titration with permanganate or by treatment with acidified potassium iodide solution and titration of the liberated iodine with thiosulphate. DETERMINATION OF MEAN CHAIN LENGTH OF CONDENSED PHOSPHATES The examination described in the preceding section conveys to the analyst how much phosphate was present in the sample. It does not indicate what kinds of phosphate are present, nor what kind was used in manufacture. Indeed, because the manufacturing process causes some degradation, the kind put in can only be inferred from the kinds still present. There are several ion-exchange or paper chromatographic procedures which will give a precise analysis, both qualitative and quantitative, of the survi- ving phosphates. But it is usually adequate, and much quicker, to determine the mean chain length, i.e. the average number of phosphorus atoms per molecule. The principle of the method is this. If a sample of the condensed

SOME ASPECTS OF THE ANALYSIS OF HOUSEHOLD DETERGENTS 35•) phosphate is converted to the diacid form and precipitated with silver ions, it yields two hydrogen ions per molecule: H2PnO3n+•n--k (nq-2)Ag+•Agn+2P•O3n+1 • +2H +. If another sample is first hydrolysed and then treated similarly, it yields two hydrogen ions per phosphorus atom, or 2n hydrogen ions per molecule: Na•+•P•Oa•+•--•n NaH2PO4--•n AgaPO4 • +2nil +. If the two experiments are done on equal aliquots of the same solution, and the liberated acid is titrated, the ratio of the two titrations is equal to the mean chain length. The procedure is as follows: A sample of the detergent powder is rendered free from surface active matter by repeated extraction with boiling alcohol. After being dried, a sample containing about 1 g of P•O5 is dissolved in water and diluted to 250 mi. A 25 ml aliquot is boiled under reflux with 25 ml of 3 N nitric acid for at least 30 minutes. Meanwhile a second 25 ml aliquot is placed in a beaker and made slightly acid with dilute nitric acid. pH electrodes are inserted, the solution is slowly titrated with 0.1 N alkali, and the pH is plotted against volume. When a complete step has been plotted, the pH at the steepest point is determined, and the pH of the solution adjusted to the value found. The condensed phosphate is then present as the diacid salt, NanH•P•Oan+•. 10 ml of 10% silver nitrate solution is added, and the liberated acid is titrated to pH 6.0 with 0.1 N alkali. Let this titration be V• mi. The refluxing solution is then cooled and its pH is adjusted to 4.5, so that the phosphate is present as NaH•PO4. Again 10 ml of 10% silver nitrate solution is added, and the liberated acid titrated to pH 6.0 with 0.1 N alkali. Let this titration be V• mi. Then the mean chain length = V•/V•. It should be noted that although the two aliquots of the phosphate solution must be exactly equal, it is not necessary to know the exact weight of material used or the exact normality of the alkali. This method has been tested on a number of pure phosphates and on a phosphate recovered from a detergent and analysed by chromatography. The values obtained were: Tetrasodium pyrophosphate pentasodium triphosphate hexaguanidinium tetraphosphate recovered phosphate do. by chromatography 2.00 P atoms/mol 3.02 P atoms/mol 4.00 P atoms/mol 2.66 P atoms/mol 2.67 P atoms/mol If a calomel electrode is used as the reference electrode for this type of titration, it is desirable that it should be of the sleeve type, as the wick