A l t h o u g h a n o r m a l drying oven can be used, a Spencer type oven is r e c o m m e n d e d . In order to reduce the time required for an analysis, a special type of oven in which heated air is blown t h r o u g h the sample placed on a tray provided with a metal gauze b o t t o m is used in some factories. Special care is required to ensure that fine particles are n o t blown o u t of the tray and t h a t the openings of the gauze are n o t blocked. W h e r e greater accuracy is required, samples of 1000 g m u s t be used for the analysis. A p p a r a t u s for this p u r p o s e is on the m a r k e t .
cylinder or a pipette with a large tip should be used for measuring o u t sulphited juice. Millilitres of iodine solution (N/32) used are multiplied by 100 (when 10 ml of the sample were taken) or by 20 (when 50 ml of the sample were taken) to obtain the concentration of sulphur dioxide in milligrams per litre. If the volume of iodine required is abnormally large, an N / 1 6 solution of iodine m a y be used. In this case the factor becomes 200 (when 10 ml of the sample were taken) for sulphited juice.
f. pH: See chapter VII, 11.
g. Calcium and magnesium: T h e following procedure m a y be used to p r e p a r e either mixed or clarified juices for the titration. A p p r o x i - mately 200 ml of juice are treated with 2 g of basic lead acetate a n d the precipitate filtered off. A fast, calcium-free filter p a p e r such as W h a t m a n N o . 4 should be used. T h e filtrate is collected in a dry 350 ml Erlenmeyer flask a n d heated to 60°C. T h e flask is fitted with a r u b b e r stopper t h r o u g h which pass a delivery and an exit t u b e . H y d r o g e n sulphide from a K i p p s a p p a r a t u s is passed t h r o u g h the solution for 30 seconds (in a fume-cupboard), then the exit tube is clamped a n d the flask left for 3-5 minutes with occasional shaking u n d e r pressure of hydrogen sulphide. After filtering t h r o u g h a fast, calcium-free filter paper, an aliquot is tested with potassium iodide solution to ensure complete removal of lead. If a yellow precipitate forms, the treatment with hydrogen sulphide m u s t be repeated. 20 ml of the cooled filtrate is measured o u t in duplicate from a burette or pipette into conical flasks. T h e titration is carried out as described in chapter VII, 9, a a n d 9, b.
5. Mixed juice
a. Brix: As described in chapter VII, 3.
b. Pol: T h e pol of mixed juice should normally be determined using the m e t h o d given in chapter VII, 4, a. Sometimes, for example, when juice contains m u c h suspended m a t t e r (clay) it is advisable to use the m e t h o d given u n d e r chapter VII, 4, b.
c. Clerget sucrose by the Jackson and Gillis method IV
i. The cooled sample of mixed juice is b r o u g h t into the l a b o r a t o r y and thoroughly mixed. T h e density of the hourly sample is determined in the usual m a n n e r with a brix h y d r o - meter. A sub-sample from each hourly sample is a d d e d to a wide-mouthed glass-stoppered bottle. T h e weight of each sub-sample must always be in the same p r o p o r t i o n to the weight of juice recorded by the factory mixed juice scales during the period in which the sample was collected. D r y basic lead acetate is a d d e d to the composite sample with every sub-sample at the rate of 1 g per 100 ml of juice. T h e a m o u n t of sub-sample a n d the weight of basic lead acetate
6 2 — 1962 Laboratory Manual for S. A f r . Sugar Factories
a d d e d m u s t be carefully controlled. At the end of four h o u r s the composite sample is shaken and filtered.
ii. T w o 50 ml portions are pipetted into 100 ml sugar flasks.
T h e sucrose dete rmina tion is carried o u t as described in chapter VII, 5, a.
iii. C A L C U L A T I O N : T h e formula given in chapter VII, 4, a shows h o w to convert the saccharimeter reading into the pol ( a p p a r e n t sucrose content) of the solution analysed. A similar formula (or Schmitz's table) should be used to convert t h e readings Dr and Ir into PD a n d PI, i.e., the readings which would have been obtained if a n o r m a l weight solution of the juice h a d been analysed. To convert the " n o r m a l " readings PD and Pj into the sucrose percentage of the juice, the following formula should be used when the J a c k s o n & Gillis m e t h o d IV has been followed:
where t = the t e m p e r a t u r e in °C of the invert solution at the time of polarization and m = the total weight in g r a m s of dry substance in 50 ml undiluted juice and m a y be calculated using the f o r m u l a :
F o r routine purposes it is r e c o m m e n d e d to calculate the divisor by the use of Tables Va a n d Y b . T a b l e Va gives the s u m of the first two terms of the divisor as a function of the corrected brix before dilution whilst Table Vb gives the t e m p e r a t u r e correction.
Alternatively Dr a n d Ir are converted i n t o S' using a formula similar to the one used to calculate S. T h e n S' is converted i n t o S (the sucrose percentage of the juice) either by applying the formula given in chapter V I I , 4, a or by using Schmitz's table (Table III).
T h e divisor of this formula can also be found by using Tables Va a n d V b .
This m e t h o d of calculating S' from Dr a n d Ir a n d correcting S' for n o r m a l i t y is generally used in the N a t a l sugar industry a n d shall be used for cane p a y m e n t p u r p o s e s . N . B . In the direct pol ( H o m e ' s dry lead) m e t h o d a 200 mm t u b e is used, b u t in the double polarization method 400 mm tubes must be used to take account of the dilution (50 to 100 ml) of the clarified juice.
Laboratory Manual for S. Afr. Sugar Factories 1963 — 63
Example: It is assumed that the four hourly samples are of equal weight.
Otherwise a weighted average must be used when calculating the brix of the sample.
Sample 1st h o u r 2nd „ 3rd „ 4th „ Average
Observed Brix 18.40 19.25 17.50 17.95
Temperature 17°C 19°C 21°C 21°C
Corrected Brix 18.24 19.20 17.56 18.01 18.25 1 i Calculation of S ' :
Temperature at time of Polarization: 22°C D r : 60.2°
I r : - 1 8 . 3 ° D r - I r : 78.5°
Divisor corrected for m (from Table V a ) : 132.37 Correction for t (from Ta le V b ) : — 1.00
Corrected divisor: 131.37
d. Clerget sucrose by the invertase method
i. Catch samples should be clarified by adding dry lead sub- acetate at the rate of 1 g per 100 ml a n d filtering. C o m p o s i t e samples should be prepared as discussed in 5, c, i above. The filtrates are de-leaded by adding a m i n i m u m quantity of a n h y d r o u s potassium oxalate and filtering and are analysed as described in chapter VII, 5, b, iii.
ii. Since 50 ml of the undiluted solution is diluted to 100 ml, readings should be taken in 400 mm tubes, provided that the solutions are sufficiently light in colour to permit accurate readings. If 200 mm tubes are used, readings should be multiplied by 2. T h e reading of the invert solution should be corrected for the optical activity of the invertase solution.
iii. Calculation: T h e formula used to calculate S' from Dr and Ir is:
S'= 100 ( D r — Ir)
132.1 + 0.0833 (m — 13) — 0.5 (t — 2 0 )
where t = the temperature of the invert solution at the time of polarization a n d m = the total weight in grams of dry sub- stance in 50 ml undiluted juice.
S' is converted to S (the sucrose percentage of the original solution) by the formula given in the example above or by referring to Schmitz's table (Table III).
Laboratory Manual for S. A f r . Sugar Factories 64 — 1963
e. Reducing sugars: Reducing sugars should be determined on a four-hourly composite sample of mixed juice which has been preserved with pulverised mercuric chloride (250 mg/litre). This composite sample should be p r e p a r e d in accordance with t h e principles given u n d e r 5, c, i above but substituting pulverised mercuric chloride for basic lead acetate. T h e sample which h a s been preserved with basic lead acetate for sucrose analysis should n o t be used since this practice often leads to low results.
It is n o t necessary to clarify the sample but where it c o n t a i n s sand or other suspended m a t t e r which is liable to block t h e delivery jet of the burette, it is advisable to filter or screen the sample before
proceeding with the analysis.
T h e L a n e & E y n o n m e t h o d of analysis is followed. A p o r t i o n of the sample is pipetted off a n d suitably diluted so t h a t 15-50 ml will be used in the titration. (With an average mixed juice containing 0 . 6 - 0 . 7 % reducing sugars, 50 ml m a y be pipetted into a 200 ml volumetric flask a n d m a d e to volume.)
Titrate against 10 ml Fehling's solution in t h e m a n n e r described in chapter VII, 6, a. T h e percentage of reducing sugars is calculated as described above for juices generally—see u n d e r 4, c above.
f. Calcium and magnesium: T h e same procedure is used for mixed juice as for other juices—see u n d e r 4, g above.
g. A vailable phosphate
i. Sufficient mixed juice is filtered to o b t a i n a b o u t 20 ml of fairly clear filtrate. 10 ml of this filtrate is pipetted into a 200 ml volumetric flask a n d m a d e up to volume. An aliquot of this (usually a b o u t 20 ml depending on the a m o u n t of P2O5
expected in the juice) is pipetted into a 100 ml flask. T h e solution is diluted to approximately 70 ml with distilled water a n d then 20.0 ml of a m m o n i u m m o l y b d a t e solution is pipetted into the flask, followed by 10.0 ml of the reducing solution, after which it is m a d e to the m a r k .
ii. T h e intensity of the blue colour developed is measured on a suitable absorptiometer or s p e c t r o p h o t o m e t e r 15 minutes after addition of the reducing solution. T h e equivalent a m o u n t of P205 is determined from a s t a n d a r d graph d r a w n beforehand, using solutions of accurately k n o w n P2O5 content.
W h e n the degree of accuracy required is n o t great, the colour developed m a y be c o m p a r e d visually with s t a n d a r d s of k n o w n P2O5 c o n t e n t using various volumes of diluted s t a n d a r d instead of juice for colour development. W h e n the equivalent a m o u n t of P2Os in the u n k n o w n has been ascertained, either from the s t a n d a r d g r a p h or by visual matching, it is multiplied by the dilution factor to obtain the P2O5 content of the original juice.
Laboratory Manual for S. A f r . Sugar Factories 1962 — 65
Example: 10 ml of filtered juice was diluted to 200 ml a n d 10 ml of this solution was used for development of the blue colour the intensity of which was found to be equivalent to 0.2 mg P2O5
T h e P2O5 content of the original mixed juice = 0.2 x 20 mg/10 ml juice
= 0.2 x 20 x 100 mg/litre juice
= 400 mg/litre of juice.