Domain: Healthcare — Pharmaceutical Chemistry
Target concept: Stability Testing of Vitamin C Drugs (Oxidation Colorimetric Method)

Video generation prompt: Prepare a vitamin C solution in a beaker, add a few drops of indicator reagent, and gradually add the oxidizing agent using a pipette while gently shaking the solution. Use transparent glassware, clearly labeled reagents, and maintain steady hand movements. Only demonstrate the sequential addition of reagents and the mixing process of the solution.

Evaluation guide:
Target concept: Vitamin C stability is operationally defined by its ascorbic acid content's capacity to act as a reducing agent, probed with the DCPIP redox indicator, a dye that is blue when oxidized and colorless when reduced. Minimum mechanistic assumptions: (1) DCPIP exists in a stable blue (oxidized) form and a colorless (reduced, leuco) form; (2) a few drops of DCPIP added to a fresh vitamin C solution are reduced (decolorized) almost immediately because ascorbic acid is present in large excess relative to the small amount of dye; (3) the KIO₃ oxidant added afterward is consumed preferentially by that excess ascorbic acid, so the first few drops cause no visible color change; (4) once enough oxidant has been added to exhaust the ascorbic acid's reducing capacity, further oxidant instead re-oxidizes the colorless (leuco) DCPIP back to blue, so the reappearance and persistence of blue is what signals exhaustion of the vitamin C's stabilizing capacity; (5) color change is diffusion-limited and becomes visually uniform only once shaking mixes the reagents. Phased storyline: [t=0-2s] clear vitamin C solution → 3 DCPIP drops added → each drop shows a brief blue streak that disperses and fades to colorless within about 1 s, confirming ample reducing capacity at baseline; [t=2-4s] first KIO₃ drop + shake → solution stays colorless (reducing capacity still in excess); [t=4-6s] second KIO₃ drop + shake → solution stays colorless (capacity being drawn down but not yet exhausted); [t=6-8s] third KIO₃ drop + shake → a faint blue-green tint appears at the drop site and diffuses outward (capacity nearing exhaustion); [t=8-10s] fourth KIO₃ drop + shake → the tint deepens and stabilizes into a uniform, saturated blue that holds for the rest of the clip (endpoint: the vitamin C's reducing capacity is fully exhausted, demonstrating loss of stability). Visibility constraints: must capture hue transitions in sRGB-rec709 gamut; avoid backlighting that would wash out the faint early-stage tints; camera angle must show the beaker's cross-section (not top-down) to resolve the meniscus and color saturation; no occlusion of label text or the pipette tip during delivery.

Manual:
Stability Testing of Vitamin C Drugs (Oxidation Colorimetric Method) Demonstration Instructions
I. Basic Demonstration Information
Demonstration Title: Stability Testing of Vitamin C Drugs via Oxidation Colorimetric Method
Subject: Healthcare — Pharmaceutical Chemistry — Stability Testing
Demonstration Purpose: To visually demonstrate how the DCPIP redox indicator reveals the point at which an oxidizing agent has exhausted vitamin C's reducing (stabilizing) capacity in solution; to reinforce understanding of redox-based colorimetric assays in pharmaceutical quality control; and to train precise, sequential reagent handling under controlled visual observation conditions.

II. Demonstration Carrier/Equipment
Core Carrier: Freshly prepared 0.1% (w/v) aqueous vitamin C solution (ascorbic acid), 0.02% (w/v) 2,6-dichlorophenolindophenol (DCPIP) indicator solution (blue in oxidized form), and 0.01 M potassium iodate (KIO₃) oxidizing agent solution.
Auxiliary Equipment: 100 mL transparent glass beaker (clearly labeled "Vit C Solution"), 1 mL calibrated glass pipette with rubber bulb (labeled "Oxidant"), small dropper (labeled "DCPIP"), white laboratory bench background, neutral lighting (no UV or colored sources), steady hand support (optional wrist rest), timer (for reference only — not shown).

III. Demonstration Content Description
Layout: Place the labeled beaker centrally on the white background under even lighting. Ensure all reagents are visibly labeled and placed within easy reach but outside frame during active demonstration.
State Control: Vitamin C solution must be freshly prepared (<5 min prior) to prevent pre-oxidation; DCPIP must be deep blue (oxidized state); KIO₃ solution must be colorless and clear. No heating, stirring rods, or secondary mixing tools used — only pipette/dropper addition and gentle hand-shaking.
Sequential Protocol (strictly followed and visible):
1. Start with ~50 mL colorless vitamin C solution in the beaker.
2. Add 3 drops of DCPIP indicator using the dropper → each drop shows a brief blue streak at the point of contact that disperses and decolorizes within about 1 second as it is reduced by the excess ascorbic acid → bulk solution remains colorless (confirms ample reducing/stabilizing capacity at baseline).
3. Using the labeled pipette, add KIO₃ oxidant dropwise (1 drop ≈ 0.05 mL), while simultaneously performing gentle, circular wrist-shaking motion (≤2 Hz, amplitude <3 cm) — each addition followed by ~3 s of shaking. The first two drops are consumed by the excess ascorbic acid and produce no visible color change.
4. Observe delayed, then progressive and stable re-coloration: after the third drop, a faint blue-green tint appears at the drop site and spreads on shaking; after the fourth drop the tint deepens and stabilizes into a uniform, saturated blue (endpoint = ascorbic acid's reducing capacity fully exhausted; the persisting blue confirms DCPIP is no longer being reduced, i.e., the sample has lost its stabilizing capacity).

IV. Observation of Core Phenomena
Initial State (Pre-addition): Beaker contains ~50 mL clear, colorless vitamin C solution; DCPIP dropper and KIO₃ pipette clearly labeled; hands steady, no splashing.
After DCPIP Addition: Each of the 3 drops shows a brief blue streak that disperses and decolorizes within about 1 second, leaving the bulk solution colorless — confirms ample reducing capacity and proper solubility/mixing.
During Oxidant Addition & Shaking:
– The first two KIO₃ drops produce no visible color change; the ascorbic acid still present is consumed without depleting the reducing capacity.
– After the third drop, a localized blue-green tint appears at the point of impact and diffuses outward once shaking begins.
– After the fourth drop and its shake, the tint deepens further and the solution stabilizes at a uniform, saturated blue for the remainder of the clip.
– Final solution is a uniform, saturated blue, with no turbidity or precipitate, indicating the ascorbic acid's reducing capacity has been fully and irreversibly consumed.
Visual Feature Recognition Baseline:
– The delay before blue reappears reflects how much reducing capacity the sample started with: more oxidant drops are needed before the tint appears when more intact vitamin C is present, fewer when less is present (drop count itself is not scored here — only the visual trajectory matters).
– Endpoint is defined by a persistent, saturated blue sustained for ≥5 s of continued shaking — no reversion to colorless.
– All glassware remains transparent; labels remain legible and unmoved; hand motion is continuous, gentle, and controlled — no jerking, spilling, or abrupt stops.

V. Summary of Key Knowledge Points and Principles
Core Concept Definition: Stability testing of vitamin C relies on its ascorbic acid content's capacity to act as a reducing agent. DCPIP is reduced to colorless leuco-DCPIP by ascorbic acid; when DCPIP is added to a solution with excess reducing capacity, it decolorizes almost immediately. As KIO₃ oxidant is then introduced, it progressively consumes that reducing capacity; once exhausted, further oxidant re-oxidizes the colorless DCPIP back to blue. The delay before blue reappears — and its subsequent persistence — therefore indicates how much reducing (stabilizing) capacity the sample originally had.
Pharmaceutical Significance:
– The point at which blue permanently reappears signals exhaustion of the sample's reducing/stabilizing capacity — critical for shelf-life determination.
– Visual clarity of the reversal enables rapid, equipment-free field assessment in resource-limited settings.
– Strict adherence to sequence (Vit C → DCPIP → gradual oxidant + shaking) ensures a reproducible readout and avoids false endpoints (e.g., premature or missed color reversal).

VI. Demonstration Precautions
– Use only fresh vitamin C solution: aged (already partly oxidized) solutions would cause the blue tint to reappear after fewer oxidant drops than expected, or the initial DCPIP drops to decolorize more slowly → invalid baseline.
– Avoid direct sunlight or fluorescent glare: blue DCPIP is sensitive to UV; ambient light must be neutral (5000K CCT) to ensure accurate hue perception, especially for the faint blue-green tint stage.
– Pipette tip must not touch beaker interior or solution surface before dispensing — no contamination or premature reaction.
– Shaking must be gentle and circular, not vigorous — vigorous shaking introduces air bubbles or splashing, obscuring color judgment.
– Demonstration duration: ≤10 s total video time — accelerated but causally faithful: DCPIP addition and decolorization (2 s), then 4 oxidant drops with synchronized shaking, a silent colorless interval, and the real-time color reversal to a stable blue (8 s).

Prompt Grounding rubric:
level_1: No beaker present, or it is opaque rather than transparent; none of the three labels ("Vit C Solution" beaker, "DCPIP" dropper, "Oxidant" pipette) are legible; the solution already shows a blue or other tint before any reagent is dispensed, instead of starting perfectly colorless; the dropper or pipette never appears on screen; the hand is absent throughout, or is performing an unrelated action (e.g., writing, holding a phone) rather than dispensing reagents or shaking the beaker.
level_2: Beaker is present and transparent, but the DCPIP dropper or the oxidant pipette is unlabeled or mislabeled; DCPIP is added but produces no visible color response at all; the oxidant is added but shaking never follows any of the drops; hand motion is absent, jerky, or clearly inconsistent with a gentle circular shaking motion.
level_3: Beaker and all three labeled items (Vit C beaker, DCPIP dropper, oxidant pipette) are present; DCPIP is added and produces a visible color response, but it disperses unevenly (streaks or pooling at the drop site rather than blending into the bulk liquid); 1-2 oxidant drops are added with only approximate shaking, and the shaking clearly starts well before or well after the drop lands; minor spillage occurs, or a label is briefly obstructed by the hand or pipette.
level_4: All core elements are present and correctly sequenced: labeled Vit C beaker → 3 DCPIP drops added via the labeled dropper → 4 oxidant drops added one at a time via the labeled pipette, each drop followed promptly by gentle circular shaking; all three labels stay legible throughout; no spillage; hand motion is steady and confined to the beaker; only a minor flaw remains (e.g., one transient air bubble, a pause of roughly half a second between a drop and the start of shaking, or slight blurring of a label during a shake).
level_5: Perfect fidelity to the prompt: (1) the transparent, labeled Vit C beaker is centered in frame from the first shot, holding ~50 mL of colorless solution; (2) exactly 3 DCPIP drops are added from the labeled dropper in a visible, controlled sequence; (3) exactly 4 oxidant drops are then added one at a time from the labeled pipette, each immediately followed by continuous, smooth, low-amplitude circular shaking with no finger tremor; (4) reagent order (Vit C solution present first, DCPIP added next, oxidant added last and gradually) is followed exactly as specified with nothing skipped or reordered; (5) all three labels remain sharp and unobscured for the entire clip, with zero spilling, bubbling, or abrupt motion breaks.

Scientific and Causal Correctness rubric:
level_1: The color response contradicts the corrected redox mechanism: the blue streak from a DCPIP drop fades away and never reappears even after all four oxidant drops are shown (the reverse of what consuming the reducing agent should do); or the solution turns yellow/brown (a decomposition color, not a DCPIP redox color); or the solution is already blue at t=0, before any DCPIP drop is shown; or oxidant is added repeatedly with continued shaking and no color ever appears, well past the point where the manual's protocol places the reversal.
level_2: The final color is right but disconnected from the oxidant dosing: blue appears with no oxidant ever shown being added (spontaneous reversal); or blue reappears immediately after only the very first oxidant drop, with no preceding colorless interval (implying the vitamin C solution had essentially no reducing capacity from the start, contradicting the fresh, concentrated solution shown at t=0); or the reappeared blue flickers back toward colorless within about 2 seconds, showing no stable endpoint at all.
level_3: The overall direction is right (DCPIP decolorizes promptly when added, the solution stays colorless through several oxidant drops, and blue eventually reappears), but the reappeared blue lingers only about 2-10 seconds before fading back toward colorless before the clip ends, so no stable endpoint is reached on screen; or the reversal begins noticeably too early (after just one drop) or too late (only in the last instant of the clip) relative to the roughly even spacing of the four oxidant drops.
level_4: Chemically plausible progression: each DCPIP drop decolorizes within about a second while ascorbic acid is still in excess; the solution stays colorless through the first two oxidant drops; a blue-green tint appears after the third drop and deepens with the fourth drop and its shake; the final blue is sustained for at least 10 seconds through the end of the clip; no turbidity, gas bubbles, or precipitate appear as side-reaction artifacts.
level_5: Full scientific fidelity: (1) each of the 3 DCPIP drops shows a brief blue streak that disperses and decolorizes within about a second, confirming ample reducing capacity at baseline; (2) the solution stays fully colorless through the first two oxidant drops, showing the reducing capacity being drawn down without yet being exhausted; (3) a faint blue-green tint appears after the third oxidant drop, exactly where the manual's protocol places near-exhaustion, and diffuses to uniformity within one shake cycle; (4) the color deepens with the fourth drop and stabilizes into a uniform, saturated blue that persists unchanged for the rest of the clip (at least 10 seconds), with no reversion to colorless, no flickering, and no turbidity or precipitate; (5) no unrelated color artifacts (yellowing, browning, cloudiness) appear at any point.

Spatiotemporal Consistency rubric:
level_1: Severe spatiotemporal breaks: the labeled Vit C beaker jumps to a different position or scale between consecutive shots with no continuous camera or hand motion bridging them; the hand holding the pipette vanishes from frame and reappears already mid-motion; a color change appears uniformly across the entire beaker within a single frame instead of spreading outward from the drop site; the benchtop lighting swaps from warm to cool white (or vice versa) partway through the clip with no visible light source to explain it; the labeled dropper or pipette disappears from frame between drops and reappears without any hand carrying it into view.
level_2: Moderate discontinuities: the beaker's position shifts noticeably between shots with no corresponding camera or hand movement to explain it; the hand's shaking motion stutters or freezes for more than half a second at a time; a color change appears to jump in visible steps rather than spreading continuously outward from the drop site; shaking begins more than a second after a drop lands; label text on the beaker, dropper, or pipette rotates or blurs unreadably during hand motion; the benchtop lighting visibly flickers.
level_3: Minor but noticeable inconsistencies: the beaker wobbles more than the hand's shaking motion would cause; the shaking hand's amplitude varies noticeably from one shake to the next; one stage of the color response (the colorless interval, or the blue-green tint) is on screen for under a second, too brief to register clearly; a label edge blurs briefly during fast hand motion; the beaker's shadow on the white benchtop shifts position between otherwise similar frames.
level_4: High spatiotemporal coherence: the beaker stays fixed in the center of the frame throughout; the hand's shaking motion is smooth, with a consistent rhythm and circular amplitude from shake to shake; each stage of the color response (colorless interval, tint onset, final blue) is visible for at least a couple of seconds; all three labels stay in sharp focus throughout; the benchtop lighting shows no visible shift in brightness or color temperature; no object appears to occlude or jump behind another unexpectedly.
level_5: Near-perfect continuity: (1) Spatial: the beaker's position and size stay effectively constant across the whole clip; the pipette tip stays at a consistent height above the liquid surface between drops; label text on the beaker, dropper, and pipette stays legible in every frame; (2) Temporal: the hand's shaking motion is fluid with no stutter or freeze at any point; each drop is followed by shaking with no perceptible delay between the two; the color spreads outward from each drop's impact point smoothly rather than in visible jumps; (3) Consistency: the benchtop lighting brightness and the position/sharpness of the beaker's cast shadow stay visually unchanged for the whole clip; there is no motion blur, focus racking, or reflective glare obscuring the color or the labels at any point.