37.1.37 AOAC Official Method 942.15 Acidity (Titratable) of Fruit Products First Action 1942 A. Indicator Method —Final Action 1965
Titratable acidity can be expressed conventionally in g acid per 100 g or per 100 mL product, as appropriate, by using the factor appropriate to the acid; for malic acid use 0.067 as factor; oxalic acid, 0.045; citric acid monohydrate, 0.070; tartaric acid, 0.075; sulfuric acid, 0.049; acetic acid, 0.060; lactic acid, 0.090. (a) Colorless or slightly colored solutions.—Dilute to ca 250 mL, with neutralized neutralized or recen recently tly boiled H 2O, 10 g pre prepar pared ed jui juice, ce, 920.149(a) (see 37. 37.1.0 1.07),or 7),or 25 mL prep preparedsolut aredsolution ion,, 920.149(b)or(c) (see 37.1.07). Titrate with 0.1M alkali, using 0.3 mL phenolphthalein for each 10 100 0 mL sol solut utio ion n be bein ing g tit titrat rated ed,, to pi pink nk pe persi rsist stin ing g 30 s. Re Repo port rt as mL 0.1M alkali/100 g or 100 mL original material. solutions. s.—Dil (b) Highly colored solution —Dilute ute test samp sample le of kno known wn weight with neutralized H 2O and titrate to just before end point with 0.1M alkali, using 0.3 mL phenolphthalein for each 100 mL solution being bei ng tit titrat rated. ed. Tra Transf nsfer er mea measur sured ed vol volume ume (2 or 3 mL) of sol soluti ution on int into o ca 20mL ne neut utra rall H2O in sm smal alll be beak aker er.. (Inthis ex extr traa di dilu luti tion on,, co colo lorr of fruit juice becomes so pale that phenolphthalein color is easily seen.) If test shows that end point is not reached, pour extra diluted portion back into original solution, add more alkali, and continue titration to end point. By comparing dilutions in small beakers, differences produced by few drops 0.1M alkali can be easily observed. B. Glass Electrode Electrode Method Method —Final Action 1980
Before Bef ore use use,, che check ck app appara aratus tus wit with h sta standa ndard rd buf buffer fer sol soluti utions ons,, 964.24 (see A.1.04) and Table 964.24 (see A.1.04). Rinse glass electrode in H2O se seve vera rall ti time mess un unti till rea readi ding ng is ca pH 6. Imm Immer erse se el elec ectr trod odes es in te test st sample contained in beaker. (Test sample should titrate 10–50 mL 0.1M NaOH and be contained in initial volume of 100–200 mL.) Stir moderately. Add alkali quite rapidly until near pH 6. Then add alkali slow sl owly ly to pH 7. Af Afte terr pH 7 is re reac ache hed, d, fi fini nish sh ti titr trat atio ion n by ad addi ding ng 0. 0.1M 1M alkali 4 drops at time, and record total volume and pH reading after each eac h add additi ition. on. (Ad (Add d wh whole ole dro drops, ps, so tha thatt fracti fraction on of drop drop doe doess not remain mai n on bur buret et tip tip.) .) Con Contin tinue ue tit titrat ration ion ≥4 dr drop opss be beyo yond nd pH8.1,and in in-terpol ter polatedata atedata fortitra fortitratio tion n cor corresp respond ondingto ingto pH8.1. pHvalue pHvaluess use used d for interpolation should lie in range 8.10 ± 0.2. [ Note s: (1 ) Always keep glass electrode covered with H 2O when not in use. ( 2 ) If strongly acid cleaning solutions are used, electr ele ctroderequi oderequires res sev severa erall h to com comee to equ equili ilibri brium um on sta standi nding ng in H2O. (3 ) If ele electr ctrodeand odeand sti stirrerare rrerare wip wipedlight edlightly ly wit with h pie pieceof ceof fil fil-ter pap paper er bef beforeinsert oreinsertioninto ioninto sta standa ndard rd buf buffer fer,, sam samee sol soluti ution on may be used for several checks on instrument.] References: JAOAC 25 , 412(1942); 28, 507(1945); 71, 86(1988).
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