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Pembuatan larutan Fehling- Fehling I26,28 g cuSo4.5H2O dilarutkan di dalam air suling sampai I liter- Fehling IILarutkan 346 g Na-K-tartrat ditambah 100 g NaOH dilarutkan dalam air sulingsampai I liter- Pada penggunaan larutan Fehling, campurkan larutan Fehling I dan Fetrling IIdeaganperbandingan I : l.

Fehling's TestIn this test the presence of aldehydes but not ketones is detected by reductionof the deep blue solution of copper(II) to a red precipitate of insoluble copperoxide. The test is commonly used for reducing sugars but is known to be NOT spe

cific for aldehydes. For example, fructose gives a positive test with Fehling'ssolution as does acetoin.Two solutions are required:A positive test is indicated by a green suspension and a red precipitate.The test is sensitive enough that even 1 mg of glucose will produce the characteristic red colour of the compound.

Benedict's testBenedict's test determines whether a monosaccharide or disaccharide is a reducing sugar, and is hence similar in purpose to the Tollens and Fehling's test. Thismakes use of a single solution of copper(II) citrate which does not deteriorate

as quickly on standing. Again Benedicts solution contains copper sulphate. Reducing sugars reduce soluble bluer copper sulphate, containing copper(II) ions to insoluble red-brown copper oxide containing copper(I). The latter is seen as a precipitate.To give a positive test, the carbohydrate must contain a hemiacetal which will hydrolyse in aqueous solution to the aldehyde form. Benedict's reagent is an alkaline solution containing Cu(II) ions, which oxidize the aldehyde to a carboxylicacid. In turn, the cupric ions are reduced to cuprous oxide, which forms a red

precipitate.RCHO + 2Cu2+ + 4OH- ? RCOOH + Cu2O + 2H2OThe colour ranges from green to yellow to orange to brick-red depending on the amount of reducing sugar in the sample; with a sample containing 1% glucose, theprecipitate is usually brick-red. Benedicts's test will give a color change for

any mono- or disaccharide containing a hemiacetal or hemiketal group. Since sucrose or table sugar does not contain these groups, it will not give a positive test.

Clinitest tablet are used to test sugar levels in urine. The tablets contain various chemicals (copper sulfate, sodium hydroxide, sodium carbonate) and eating them can cause serious symptoms.

Pada prinsipnya baik fehling, tollens maupun benedict digunakan untuk mengetahuiapakah suatu gula merupakan gula pereduksi atau bukan (mempunyai gugus aldehidabebas).

Fehling terdiri dari campuran CuSO4 + Asam tartat + Basa. Jika gula tersebut mer

upakan gula pereduksi (glukosa, galaktosa, dll) Cu akan berubah menjadi Cu2O yang berwarna merah bata.

Benedict terdiri dari campuran Na2Co3 + CuSO4 + Natrium sitrat. Reaksi Benedictakan menyebabkan larutan yang berwarna biru akan berubah menjadi orange atau kuning. Untuk mengetahui gula pereduksi yang mempunyai sifat reduksi lebih kuat, reaksi Fehling lebih jelas perubahan warnanya.

Sedangkan Tollen terdiri dari Ag2SO4 yang bila ada gula pereduksi Ag akan direduksi menjadi Ag+ yang akan membentuk cinci perak. Kelemahan dari reaksi Tollen ad

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alah dia bukan cuma bereaksi dengan gula pereduksi tetapi juga bereaksi dengan senyawa keton yang mempunyai gugus metil.