WERNER OXIDATION OF OXALIC ACID XLIV.-Oxidation of Oxalic Acid by Potassizcrn Dichromate. By EMIL A. WERNER F.I.C. Assistant in the Chemical Laboratory, Trinity College University of Dublin. IN a preliminary notlice on the above subject (Abstr. December 1887), I pointed out that t,he oxidation of oxalic acid by potassium dichro-mate was limited by the formation of a chromoxalate even in presence of sulphuric acid provided the latter was not presentl in a concen-trated form. In the present paper I propose to give the results of a study of the oxidation of oxalic acid by potassium dichromate per se and in vary-ing molecular proportions. A paper on the same subject has been quite recently communicated t o the Society by Mr. C. H. Bothamley (this vol. p. 159) but as the writer has overlooked the formation of a chromoxalate in t8he interactions I believe that he has been misled in t,he interpretation of his results.I n short for a complete and successful soliltion of the problems presented in the interactions of oxalic acid and potassium dichromate, a previous knowledge of the properties of the cliromoxalates is abso-lutely necessary more particularly of Croft’s red po tnssium compound, whose precise composition and relations I have recently studied (this Whether in the solid state or in aqueous solution 7 mols. of oxalic acid are required for the complete reduction of 1 mol. of potassium dichromate the sole products of the interaction being Croft’s salt car-bonic anhydride and water thus :-vol. p. 404). k’,Cr2O7 + 7(H2C20,2H,0) = KzH2C~*z( C,O,)d(OH) + 6C0, q- 19H20 BY POTASSIUM DICHROMATE.603 The experimental data proving the correctness of this equation are given under Experiment I further on. Bearing in mind this equation it seemed highly probable that d l cases of the interaction of potassium dichromate and oxalic acid, between 1 and 7 mols. might be represented by one general equation, and the outcome of the present investigation has been to give com-plete corroboration t,o this view and 10 prove that when dichromate on the one hand or oxalic acid on the other is used in excess beyond the proportions required by the foregoing equation that excess is found in the residue on completion of the reaction. The general method adopted in the experiments was the following : -An intimate mixture of the two substances both in fine powder was slowly heated in a dry weighed conical flask to the temperature of interaction after which the flask with its contents was heated in an air oven at 110-120° until the weight was constant and the loss determined.This does not represeut as Mr. Sothamley assumed, " the complete dehydration of the oxalic acid and its partial oxidation by the dichromate," but its partial oxidation together with its partial dehydration for the red potassium chromoxalate formed (not chromic oxalate) retains as I have shown (Trans. 1888 404) % mols. H,O, a t the temperature and under the conditions of the interaction. The residue was dissolved in water diluted to a definite volume and the chromium and oxalic acid determined in aliquot portions.The esti-mations of the carbonic anhydride by absorption in ammoniacal calcic chloridc solution were made in separate experiments carried out in presence of water ; the spontaneous nature of the reaction accompanied as it is by considerable development of heat when hhe substarices inter-act in the solid state rendering the correct estimation of the carbonic anhydride by any absorption method very difficult. The potassium dichromate and oxaiic acid used in these experiments were both purified by recrystallisation the latter carefully air-dried, whilst the former was heated to 150-160" and allowed to cool in a desiccator. A. Potassiuwb Dichromate (1 niol.) Hydrated Oxalic Acid (7 mols.) I n this case the dichromate is completely reduced and the inter-action takes place according to the equation already given.= 1.2973 gram K,Cr,Ol = 3.8922 , I€2C,04,2H20. h'xpt. 1.-5.1895 grams Temperature of interaction 28-30", A t 110-120°-Loss . . . . Theory . . 2.6791 , = 31.63 , 7) 2.6445 grams = 50.95 per cent. of tots1 weight 604 WERNER OXIDATION OF OSALIC ACID H,C204,2H20 (unoxidised as chromoxalate in residue)-Acid Acid taken. unoxidised. Found 2.2570 grams. Ratio 100 57.98 or Theory 2.2241 , Ratio 100 57.14 or 7 4.05. 7 4. The residue when dissolved in water yields a rich purple-red solu-tion perfectly free from any unaltered dichromate. A series of experiments had previously shown that water in what-ever proportion it may be present is entirely without influence on the nature of the interaction.13. Potassium Dichroma te and Hydrated Oxalic Acid equal Mol ecu 1 es. I n this case as the experimental results show only one-sevenbh of the dichromate is reduced six-sevenths remaining unaltered. = 16789 gram H2Cz04,2H20. = 3.9176 grams K2Cr207. Expt. II.-5*5965 grams Temperature of interaction 30-32". At 110-120"-Loss . . . . . . Theory . . 1.153 , = 80.61 , ,, 1.184 gram = 21.15 per cent. of total weight. Cr,O formed-K2Cr207 KoCr,0, taken. reduced. 100 14.66 or 7 1.02. Found Theory. 0.289 , = 0.339 , 100 14.26 or T 1. 0.297 gram = 0.5744 K,Cr207 H,C204,2Hz0 (unoxidised as chromoxalate) -Acid taken. Acid unoxidised. Found 0.9610 gram. Ra.tio 100 57.23 or 7 4.006. Theory 0.9593 , Ratio 100 57.14 or 7 4.Bxpt. HI.-In presence of water = 1.78 gram H2C204,2H20. GO evolved-5.9335 grams . . . . { = 4.1535 , K,Cr,07. Found . . 0.553 gram = 0.7917 gram H2C204,2H20. Theory 0.5327 , = 0.7628 , ,) Crz03 formed-K2Cr20i taken. K2Cr30; reduced. 7 1. Found 0.;-3110 gram Theory 0.3067 ? BT POTASSIUJI DICHROMATE. 605 H2C204,2H,0 (unoxidised)-Found . 1.032 gram. Theory 1.0171 ,, C. Potassium Dichromate (1 mol.) Hydrated Oxalic d c i d (2 mols.). I n this case -; of the dichromate are reduced + remaining un-altered. = 2.4833 grams H2C204 2H20. = 2.8372 , K2Cr207. Expt. IV.-5*3805 grams Temperature of interaction 51-52". At 110-120'- Loss 1.726 gram = 3247 per cent. Theory 1.706 , = 31.71 ,, Cr203 formed- Found . . 0.432 gram. H,C2O4,2H20 (unoxidised)-Theory 0.429 ,, Found 1-42] gram.Theory . 1.419 gram. Ezpt. V.-5%6 grams in presence of water. CO evolved-Found . . Theory 0.7819 , = 1.1195 , 7, 0.7933 gram = 1,1358 gram HzCz04,3H20. D. Potassium Diclwomafe (1 rnol.) Hydi-ated Oardie Acid (9 mols.), corresponding t o K2Cr207 + 7 + 2 mols. HzC20t,2H20. I n this case the interaction takes place as in A the excess of oxalic acid over the 7 mols. remaining unaltered." Expt. VI.-5.64 grams in pre-CO evolved- Found . . 1.1 10 gram. = 4.479 grams H2C2O4,2Hz0. sence of water . . { = 1.161. , K2Cr207. Theory 1.042 ,, Theory 0.600 ,, Cr20s formed- Found 0.621 gram. H2C204,2H20 (unoxidised)-Acid taken. Acid unoxidised. 9 6-03 Found Theory 3403 2.986 grams I 9 6.00 f In this particular case the excess of osalic acid combiries wit,h the red chrom-oxalate foming the feeble compound K2H4Cr2(C204)6 of the blue eeriea which, however is decomposed by much water 606 WERNER OXIDATION OF OSALIC ACID Solutions prepared directly from potassium dichromate and red potassium chromoxalate in the proper proportions were found in each case to be identical in every respect with the solut,ions of the products of the respective interactions.With anhydrous oxalic acid the interactions are the same as with the hydrated acid with the exception that a?d~ydt-ous chromoxalate is formed as the following experiments show. Expt. VII.-Potassium Dichromate and A1271 ydrous Oxalic Acid equal mols. 4.9645 grams. Temperature of interaction 40-42". Loss . . 0.7675 gram = 15.45 per cent, Theory for hydrated chromoxalate = 13-16 ,, , anhydrous , = 14.50 ,, Expt.VIII.-Potassium Dichrornate (1 rnol.) Adzydrous Ozalic Acid (2 nzols.). 5.28 grams. Temperature of interaction 54-56'. Loss . . 1.291 gram = 24.45 per cent. Theory for hydrated chromoxalate = 21.33 ,, , anhydrous , = 23.50 ,, The rather high r e d t obtained in each case is due to a loss of a small quantity of the oxalic acid by sublimation. The residue from the interaction presents the appearance of a light bulky porous, pale-brown mass which develops heat when moistened with water. From a study of t,he interaction of equal weights of potassium dichromate and hydrated oxalic acid Mr. Bothamley deduced the following equation as representing the change which takes place a t 110-120" viz.:-2K2Cr207 -!- 6H&z04 = Cr2(C20,)3 + 6C02 + 6H,O + 2K2CrO4. As I have already mentioned chromic oxalate is not formed in any of these decompositions and neutral potassium chromate is never present as a product! of the interaction of potassium dichromate and oxnlic acid below 200" under any conditions. The equation adopted by Mr. Bothamley agrees only approximately with his experimental results and moreover it does not represent, the molecular ratios with which he worked. Equal weights of potassium dichromate and hydrated oxalic acid, correspond exactly with the molecular ratios :-3K2Cr,07 . . 882 7(H,C,0,2H,O) . . . . 882, therefore 2 mols. of dichromate will be left unchanged according to the equation BY POTASSIUM DICHROJIBTE.C O i 3z(,cr20 + i(H,C204,2H,0) = K2H2Cr,(C204)4(OH) + 2KzCra0 + 6C02 + 19Hz0, and this is proved by the following experiments. E. Potassium Dichyomate and H!ydrated Oxalic Acid equuZ weights. E.apt. 1X.-5*73 grams (= 2.865 grams H,C,0,,2K20). Tempera-ture of interaction ;32-35". Loss . . . . . . . . . . Theory . . 1.9684 , = 34-25 ,, 1.9395 grams = 3i3.84 per cent. Cr203 formed-H2C204 2H20 (unoxi di sed) -Found . . 0.503 gram. Theory . . 0.4937 gram. Found 1.662 gram. Theory . . 1.63i gram. Expt. X-Same as above in presence of water. CO evolved-5.875 grams. Found 0.9022 gram. Theory 0.8792 gram. Mr. Bothamley's own results agree fairly well with the above equation thus he obtained in two experiments the numbers 100 54 and 100 53 for the ratios of oxalic acid taken to oxalic acid un-oxidised from which he concluded that half of the acid was oxidised, but the true ratio is 100 57 or simply 7 4.The absence of neutral potassium chromate in the products from any one of the preceding interactions is readily proTed by the absence of any immediate pre-cipitate on the addition of barium chloride o r nitrate solutions to the product. Mr. Bothamley states (referring to the solution of the product from the interaction of the dichromate and oxalic acid in equal weights) that when mixed with ammonia a brown precipitate of chromium chromate is formed ; in my experiments ammonia did n o t produce a trace of precipitate but simply a change in colour due to its action on the red potassium chromoxalate present.The interactions of potassium dichromate and oxalic acid at a low red heat vary considerably with the proportions of dichromate and red chromoxalate formiug the mixture and though the decompositions are very simple yet they require rather complex equations for their representation. The fact is that the changes which occur under this condition are the result of the ordinary decomposition of the pot,assiunl chromoxalate complicated by the exceptional oxidising action of the dichromate on the oxalic radicle of the latter. In the second part of my paper on the chromoxnlates (Trans. 1888 608 MTRNER OXIDATION OF OXALIC ACID 404) I have shown that the red potassium salt* decomposes at a red heat in accordance with the equation-2KzH,Crz(C204)4(0H) + 110 = 2K-Cr04 + Crz03 + In the presens cases the oxygen necesssry for the decomposition is wholly derived from the dichrorrinte (which is reduced correspond-ingly) when the latter is in excess whilst if the chromoxalate is in excess the whole of the dichromate is reduced (to Cr,O and K,Cr04), the remainder of the oxygen being derived from the air.A s this interaction is of secondary interest as compared with the more im-portant primary change I merely give the following two cases as examples :-16C0 + 4H,O. F. Product from Interaction qf R2Cr20 and Hydrated Oxalic Acid, equal mols. C 1K2H,Cr2(Cz0a)4(OH)z. 1 6KzCrz07. (See Ezpt. I& B). Ratio = Equation : 36KzCrz07 + 6KzHzCrz(CzO,),( 0H)z = 14Cr2O3 -!- 14KzCr20 + 28K2Cr04 + 48C0 + 12Ha0. Expt. XI.-0*944 gram.Hested to a low red heat. Deconiposi-Residue- Found . . 0,7900 gram = 83.68 per cent. Cr,03 formed-tion without violence. Theory 0.787 , = 83.37 ,, Found . . 0.1485 gram. Theory . . 0.1434 gram. G. Product from Interaction of KzCr207 and H,C,04,2H,0 equul weigh is. 1K,H,Cr2(C,04),(OH),. (See Ezpt. I X E) 2KzCra07. Ratio = Equation : 4KzCr207 + 2KzH&rz(C204)4(0H)2 + 5 0 = 3Crz0 + 6K2CrO* + 16C02 -t 4H,O. E q t . XII.-O.6175 gram. Heated t o low red heat. Decomposi-Residue- Found . . 0.4300 gram = 69.63 per cent. tion violent. Theory 0.4318 , = 69.94 ,, Omitting the water of crystallisation BY POTASSIUM DICHROMATE. 609 Cr203 formed-Found . . 0.1155 gram. Theory . . 0.1215 gram. Comparative experiments made with an intimate mixture pre-pared directly from finely-powdered dichromate and red potassium chromoxalate in the proper proportions led t o tbe same results.It is notewort,hy that the mixed solution of potassium dichromate and red chromoxalate which results from the preceding interactions, or a directly prepared solution of the two salts though exhibiting in every respect the properties of its constituents refuses to crystallise under the most favourable conditions. The absorption spectrum of the mixed solution was found from a preliminary examination to be the sum of the absorption-spectra of the solutions of the separate con-stituents. The two compounds appear t o exist in a feeble state of molecular combioation just sufficient t o prevent either one or the other from crystallising out.I hope to examine the solution further, later ou. The results of the present investigation may be summed up in the following conclusions :-1. The red potassium chromoxalate K2H2Cr2( C,O,),(OH) (Croft's salt) is in all cases without exceptiow a product of the inter-action of potassium dichromate and hydrated oxalic acid, below 200". 2. Neutral potassium chromate is never present as a product of the interaction of potassium dichromate and oxalic acid under any conditions below 200". 3. When hhe two substances interact in the solid state the initial temperature of the interaction which lies between 30" and 60 O varies with the molecular proportions employed. 4. The dehydration of the oxalic acid does not affect the nature of the interaction the anhydrous chromoxalate K,Cr2( C204),, being formed in this case. 5. Water by its solvent action facilitates the interaction that is, reduces the initial temperature but is otherwise without influence on the nature of the change. 6. Seven mols. oE oxalic acid is the minimum quantity necessary for the complete reduction of 1 mol. of potassium dichromate, and any excess of either above this ratio remains unchanged. 7. When the proportion of potassium dichromate to oxalic acid exceeds 1 to 7 mols. and the temperature of the mixture is raised t o low redness a secondary reaction occurs between the excess of dichromate and the red chromoxalate first formed. University Laboratory, Trinity College Dublin. VOL. LIII. 2