[CONTRIBUTIONS TO THE CHEMISTRYOF SUGAR ANALYSIS.]
I. POTASSIUM OXALATE A S A LEAD PRECIPITANT IN SUGAR ANALYSIS. BY H A R R I SE.
SAIVSER.
Received S c p t c m k r 29, ,904.
THEofficial methods of analysis of the Association of Official Agricultural Chemists direct that a solution which is to be tested gravimetrically lor reducing sugars shall first be freed of soluble impurities by means of lead subacetate, and that the excess of lead shall then be removed by sodium carbonate previous to the examination with Fehling’s so1ution.l Unfortunately, the use of carbonate for this purpose is attended with disadvantages. I t was shown some years ago? that when lead is removed as carbonate from solutions containing invert sugar or levulose, the use of an excess of alkaline precipitant tends to carry a portion of the precipitate back into solution so that the purpose of the separation is in part defeated. It was proposed, as a remedy for this evil, that the solution be saturated with carbon dioxide after the addition of sodium carbonate; and it was claimed that by this means the lead might be precipitated completely in a granular form well suited to f i l t r a t i ~ n . ~ The same end may be attained, according to the writer’s experience, by substituting sodium bicarbonate for the carbonate. Lead is precipitated quickly and completely, even in the presence of invert sugar, and the precipitate filters well without seeming to redissolve in an excess of the reagent. Unfortunately, however, the use of the bicarbonate has the undesirable effect of converting into carbonate a portion of the sodium hydroxide of the Fehling’s solution. So far as the writer is aware, no exact data have ever been published to show that the addition of a little sodium carbonate to a mixture of reducing sugar and Fehling’s solution has any effect upon the sugar-copper ratio, sa long a5 the amount of. sodium hydroxide in the solution remains ~ o n s t a n t . Indeed, ~ experiments 1 Bull 46 ; Bureau of Chemistry, C. S. Dept. Agr.. p. 33.
Stern a n d F r l n k e l : Zlschr. angnu. Chenr., 1893,p. 579; Borntrager : Ibl‘d., 1894,p. 521. Stern and Hirsch : Ibid., 1894,p. 116. 4 Borntrager (Zfschr.anal. Chem., 37, 160 (1898))a n d Bruhns (Ibid., 37, 244) claim that sugar solutions suffer changes of reducing power when a n excess of sodium carbonate is used for the removal of lead, hut they refer, presumably. to the use of a large excess. 2
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H A R R I S E . SA\VYER.
conducted i n this labo'ratory seein to contradict such a view.
T ~ L Ito S ,each of four j o cc. portions of freshly prepared Fehling's solution, 30 cc. of a certain sugar solution were added. To two of these, 20 cc. portions of water \\.ere added, bringing the total volumes to io0 cc. To the other tiyo, 20 cc. portions of decinormal sodium carbonate were similarly added. The first two portions gave respectively 0.5 I 55 and 0.5 1jo gram of cupric oxide; the second two, 0.51 j 1 and 0.5144 gram. T h e differences between the two sets of results are too slight to be attributed t o the influence of the added carbonate. O n the other hand, it is well establishetl that i f the arnount of sodium hydroxide present i n E;ehliiig's solutioa is yaried, the sugar-copper ratio may be affected.l It is true that the reducing powers ol' dextrose and lev~iloseare less influeliced by such a variation than those of certain other sugars, and it may be that a reductio'n in the amount of liytlroxide present, through its partial conversion into carhonatc, ~,voultlhave less effect upon the sugarcopper ratio than an actual diminution of total all~alinit);. But it seeins none the less desirable to maintain the proportion of hydroxide as nearly constant as possible. and to avoid the introduction of n bicarbonate or of carlion clioside i t 1 a sugar solution which subsequently is to be tested with F'ehling's soltitioil.' On this account it has been attempted to remove the lead in other forms than carbonate. Thus. Gill has recommended its precipitation as sulphite, claiming that an excess of sulphurous acid is without effect on Fehling's solution;3 and Pellet has employed both sodium sulphite and st1lphL1roL1j acid, obtaining, hawever. precipitates which passed readily through filter-paper.4 Eorntriger has stuciied the use of sodium phosphate as a prcxcipitant, finding that its action is rapid and complete, hiit that it lea\-es the filtrate acid ant1 increases the reducing power of the invert sugar contained therein.5 Many analJsts have used sodiiuii sulphate ; and Borntriiger considers it more generally satisfactory than either the carhonate or phosphate, since the reaction and reduring pon-er of the filtrate remain Rjeldahl : " Meddelelser f r a Carlsherg 1,al)oratoriet." 4 , pp Bruhns: Z t x h r . aital. C h o u . , 38, \z I I ~ J ~ . :!I, Chem. SOC., April, 1S71. p. 91. :+--'L,> 4 BuN. de I'..(ss'?r dpA Chr'nri~le~. 15, Zfjrli?',a u g t w Chint., 1S514. 1) i z : I * / h i d . . 1\94, pp. i 2 5 ,
